cloning, expression and functional ch aracterization of...
TRANSCRIPT
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Cloning, Expression and Functional Characterization of Secondary Amino 1
Acid Transporters of Lactococcus lactis 2
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Hein Trip1 , Niels L. Mulder2 and Juke S. Lolkema 4
Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, 5
University of Groningen, Groningen, The Netherlands. 6
7
* correspondence 8
J. S. Lolkema, Centre for Life Sciences, University of Groningen, Nijenborgh 7, 9747AG 9
Groningen, The Netherlands 10
tel. +31503632155; fax. +31503632154; email. [email protected] 11
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1Present address: Department of Earth, Environmental, and Life Sciences, TNO, Lange 13
Kleiweg 137, Rijswijk, P.O Box 45, The Netherlands 14
2Present address: Department of Pediatrics, Center for Liver, Digestive and Metabolic 15
Diseases, University Medical Center Groningen, University of Groningen, Hanzeplein 1, 16
9713 GZ Groningen, The Netherlands. 17
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running title: Amino acid transporters of L. lactis 19
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Copyright © 2012, American Society for Microbiology. All Rights Reserved.J. Bacteriol. doi:10.1128/JB.01948-12 JB Accepts, published online ahead of print on 9 November 2012
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Abstract 24
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Fourteen genes encoding putative secondary amino acid transporters were identified in the 26
genomes of Lactococcus lactis subsp. cremoris strains MG1363 and SK11 and subsp. lactis 27
strains IL1403 and KF147, twelve of which were common to all four strains. Amino acid 28
uptake in L. lactis cells overexpressing the genes revealed transporters specific for histidine, 29
lysine, arginine, agmatine, putrescine, aromatic amino acids, acidic amino acids, serine and 30
branched chain amino acids. Substrate specificities were demonstrated by inhibition profiles 31
in the presence of excess of the other amino acids. 32
Four knock-out mutants were constructed, lacking the lysine transporter LysP, the histidine 33
transporter HisP (formerly LysQ), the acidic amino acid transporter AcaP (YlcA) or the 34
aromatic amino acid transporter FywP (YsjA). The LysP, HisP and FywP deletion mutants 35
showed drastically decreased rates of uptake of the corresponding substrates at low 36
concentrations. The same was observed for the AcaP mutant with aspartate, but not with 37
glutamate. In rich M17 medium, deletion of none of the transporters affected growth. In 38
contrast, deletion of the HisP, AcaP and FywP transporters did effect growth in a defined 39
media with free amino acids as the sole amino acid source. HisP was essential at low 40
histidine concentration, AcaP in the absence of glutamine. FywP appeared to play a role in 41
retaining intracellularly synthesized aromatic amino acids when these were not added to the 42
medium. Finally, HisP, AcaP and FywP did not play a role in the excretion of accumulated 43
histidine, glutamate or phenylalanine, respectively, indicating the involvement of other 44
transporters. 45
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Introduction 49
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The lactic acid bacterium Lactococcus lactis is widely used for the production of cheese and 51
buttermilk and is therefore of great commercial importance. Lactic acid bacteria have adapted 52
to nutrient rich environments and lack various biosynthetic pathways. Most dairy and 53
laboratory strains of L. lactis are auxotrophic for a number of amino acids, for example 54
branched chain amino acids and histidine. It is believed that the capacity to synthesize these 55
amino acids was lost during adaptation to milk (1,2), which provides all essential amino acids 56
in the form of its major protein constituent casein (3). Casein is degraded by an efficient 57
proteolytic system that involves protein hydrolysis into a range of peptides of different 58
lengths, transport of some of these peptides via the oligopeptide uptake system Opp and 59
further degradation to free amino acids by intracellular peptidases (4,5). Nevertheless, dairy 60
strains of L. lactis are still able to grow in media containing free amino acids as the sole 61
source of amino acid (6), implying that transport systems for essential, free amino acids have 62
been conserved. A range of transport systems for essential as well as non-essential amino 63
acids has been described that use the proton motive force (by proton symport), the 64
electrochemical gradients of substrate and counter substrate (exchange) or ATP hydrolysis as 65
driving force (7). The exchange systems are in fact not used to supply amino acids for protein 66
biosynthesis, but are part of simple metabolic pathways that generate energy in the form of 67
ATP or proton motive force (pmf) by deimination or decarboxylation of the precursor amino 68
acid combined with precursor/product exchange across the membrane (8,9,10). 69
Most of the knowledge about amino acid transport in L. lactis and other lactic acid bacteria 70
was obtained from studies performed in the years around 1990. Transport studies using whole 71
cells or membrane vesicles revealed secondary uptake systems specific for branched amino 72
acids and methionine (11,12), alanine and glycine (13), lysine (14), serine and threonine, 73
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histidine, cysteine (15) and tyrosine and phenylalanine (16). ATP hydrolysis driven transport 74
systems were found for glutamate and glutamine (17) and possibly proline (18). Only few 75
amino acid transporters have since been cloned and characterized. From L. lactis, only bcaP 76
and glnPQ were cloned and experimentally shown to encode a branched chain amino acid 77
transporter (19) and an ATP driven glutamate/glutamine transporter (20), respectively. Other 78
genes have been annotated as amino acid transporters, but mainly on basis of their homology 79
with other known transporters or their genetic context. 80
Most described bacterial and fungal amino acid transporters that use proton symport or 81
precursor/product exchange as energy coupling mechanism are members of the Amino acid-82
Polyamine-Organocation (APC) superfamily (TC 2.A.3 in the classification of Saier et al. 83
(21)). Here we set out to clone and functionally express the APC family members of L. lactis 84
and to determine their substrate specificity. Four mutants, lacking one of the identified 85
transporter genes, were constructed and used to demonstrate the role of the transporters in the 86
physiology of the cell. 87
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Material and Methods 91
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Media and growth conditions. 93
L. lactis NZ9000 (22) and JP9000 (23), strains derived from strain MG1363 carrying the 94
nisRK genes in the pepN and pseudo_10 loci, respectively, were used for nisin inducible 95
expression of amino acid transporter genes and as parental strains of transporter deletion 96
mutants. L. lactis cells were grown at 30 °C in M17 media supplemented with 25 mM 97
glucose (hereafter referred to as GM17) and containing 5 μg/ml chloramphenicol when 98
appropriate, or in SA media, a chemically defined medium (6) containing free amino acids 99
unless stated otherwise. E. coli, used as a cloning host for deletion constructs, was grown in 100
LB media containing 150 μg/ml erythromycin. 101
102
Plasmid and strain construction. 103
Genes encoding putative amino acid transporters were amplified from chromosomal DNA of 104
L. lactis MG1363 and IL1403 (in case of aguD) using primers listed in Supplementary Table 105
S1, and cloned behind the nisin inducible promoter PnisA in pNZ8048 (24), using NcoI or PagI 106
restriction sites at the translational start of the genes and a XbaI restriction site approximately 107
200 bp downstream the genes. The resulting plasmids were transformed to L. lactis NZ9000 108
for nisin inducible expression. 109
Markerless deletion of lysP, lysQ, llmg_1452 (ylcA in strain IL1403; see supplementary Table 110
S2) and llmg_2011 (ysjA in IL1403) was performed with a two-step integration and excision 111
system using plasmid pCS1966 (25), which is based on positive selection of an erythromycin 112
resistance marker and negative selection against a 5’-fluoroorotic acid transporter encoded by 113
oroP. For deletion of lysQ, up- and downstream flanking regions of approximately 700 bp, 114
overlapping with the first 50 bp and last 100 bp of the gene, were amplified from 115
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chromosomal DNA using primers lysQ-fl1-fw, lysQ-fl2-rv, lysQ-fl2-fw and lysQ-fl2-rv 116
(Supplementary Table S1) and cloned adjacently into pCS1966 using E. coli DH5α as a 117
cloning host. The resulting plasmid was transformed into L. lactis NZ9000. Erythromycin 118
resistant colonies were tested for integration at the lysQ site using PCR. Correct clones were 119
grown overnight in non-selective medium to allow for recombination and excision of the 120
plasmid backbone including either the wild type or the disrupted lysQ gene, resulting in lysQ 121
deletion or return to the wild type situation, respectively. Cells were spread on SA medium 122
agar plates containing 10 μg/ml 5’-fluoroorotic acid for selection against the oroP gene and 123
colonies were tested by colony PCR for deletion of lysQ. For deletion of lysP, llmg_2011 and 124
llmg_1452, approximately 2.6 kb fragments of the respective genes including approximately 125
700 bp up- and downstream regions, were amplified using primers lysP-/2011-/1452-fl1-fw 126
and lysP-/2011-/1452-fl2-rv (Table S1) and cloned into pCS1966. From the resulting 127
constructs, approximately 1.2 kb internal fragments of the transporter genes were deleted by 128
amplifying the whole constructs except the internal fragments using primers lysP-/2011-129
/1452-fl1-rv and lysP-/2011-/1452-fl2-fw (Table S1). The PCR products were self-ligated 130
after digestion with restriction enzymes, resulting in the deletion constructs, which were 131
transformed into E. coli DH5α for propagation. The plasmids were transformed to L. lactis 132
JP9000, a MG1363 derivative containing the nisRK genes in the pseudo_10 locus (23). The 133
procedure was continued as described for deletion of lysQ. 134
135
Amino acid transport assays. 136
L. lactis strains harbouring expression constructs or the empty vector pNZ8048 were grown 137
in GM17 to mid-exponential phase (OD600 = 0.6) and induced with 5 ng/ml nisin. After 1 138
hour cells were harvested, washed and resuspended to an OD600 of 2 in ice-cold 100 mM 139
potassium phosphate buffer, pH 6.0, containing 0.2 % glucose and kept on ice until use. 140
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Samples of 100 μl of the cell suspension were incubated for 5 min at 30 °C under continuous 141
stirring followed by the addition of 14C-labelled amino acids (Ala, Arg, Asn, Asp, Glu, His, 142
Ile, Leu, Lys, Phe, Pro, Ser, Tyr and Val) or 14C-labelled putrescine (Perkin Elmer) to final 143
concentrations ranging from 1 to 5 μM, depending on the amino acid. In case of putrescine 144
uptake by cells expressing aguD, 50 μM (final concentration) of unlabelled agmatine was 145
added 1 minute after allowing cells to accumulate 14C-putrescine (present at 4.7 μM). Uptake 146
was stopped by addition of 2 ml ice cold 0.1 M LiCl and filtration through a 0.45-μm-pore-147
size nitrocellulose filter (BA85; Schleicher & Schuell GmbH). The filter was washed once 148
with 2 ml 0.1 mM LiCl and submerged in Emulsifier Scintillator Plus scintillation fluid 149
(Packard Bioscience). Radioactivity was measured by scintillation counting in a Tri-Carb 150
2000CA liquid scintillation analyser (Packard Instruments). 151
152
Measurement of amino acid and dipeptide concentrations using RP-HPLC 153
Samples were run on a Shimadzu high-speed HPLC Prominence UFLC and later analysed 154
using the LC Solutions 1.24 SP1 software from Shimadzu (Kyoto, Japan). Samples taken at 155
different time points from cell suspensions in 100 mM potassium phosphate buffer, pH 6, 156
containing 10 mM glucose and histidine-leucine, glycine-glutamate or phenylalanine-valine 157
at 5 mM concentrations were centrifuged and supernatants were subjected to derivatization 158
by DEEMM as described by Pudlik and Lolkema (26). Detection of aminoenone derivatives 159
was performed in an Alltech Platinum EPS C18 column with dimensions of 250 x 4.6 mm, 160
operated at 25 °C through a binary gradient using eluent A (25 mM acetate pH 5.8, 0.02 % 161
sodium azide) and eluent B (a 80:20 mixture of acetonitrile and methanol) as described (39). 162
External standards were prepared by mixing dipeptide and the two corresponding free amino 163
acids at concentrations of 4 mM and 0.2 mM or 1 mM and 50 μM, respectively. 164
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Results 166
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Secondary amino acid transporters of Lactococcus lactis 168
The genomes of L. lactis strains IL1403, MG1363, KF147, and SK11 were screened for 169
homologues of amino acid transporters by BLAST searches using different members of the 170
Amino Acid-Polyamine-Organocation (APC) superfamily (21) as query sequences. Fourteen 171
proteins were identified that showed significant homology (more than 20 % amino acid 172
sequence identity) with known APC family members (Table 1 and Supplementary Table S2). 173
Twelve of the proteins were found in all four strains, whereas YlcA (IL1403 nomenclature) 174
was not present in strain SK11 and YrfD was present only in the subsp. lactis strains IL1403 175
and KF147. Sequence identities between the putative transporter proteins varied from 64 % 176
(LysQ and LysP) down to insignificant values below 20% (e.g. ArcD1 and YlcA; see the 177
phylogenetic tree in Fig. 1). All fourteen protein sequences shared the hydropathy profile 178
typical of the APC family members, indicating the same fold containing 12 transmembrane 179
segments and with the N- and C-termini located in the cytoplasm (27,28). The ArcD2 and 180
ArcD1 proteins have one and two additional transmembrane segments, respectively. The 181
arcD1, arcD2, gadC and yrfD genes are located in clusters encoding a putative arginine 182
deiminase (ADI) pathway (arcD1 and arcD2) (29), a glutamate decarboxylation pathway 183
(gadC) (30) and an agmatine deiminase (AgDI) pathway (yrfD) (9), respectively, and, 184
although not demonstrated experimentally, their functions were annotated as 185
arginine/ornithine, glutamate/γ-aminobutyric acid and agmatine/putrescine exchangers, 186
respectively. In contrast, 8 of the other putative amino acid transporter genes appear to be 187
monocistronic, with neighbouring genes of unknown function or functions that could not be 188
directly related to amino acid metabolism. The exception is formed by the adjacent ydgB and 189
ydgC genes that are separated by a 71 bp non-coding fragment which does not contain a 190
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potential terminator or clear promoter sequence. These genes may be transcribed as one 191
mRNA. 192
The brnQ gene was identified in strains MG1363, SK11 and KF147 but missing in IL1403. 193
BrnQ has been proposed to be a branched chain amino acid transporter (19,31) but is not 194
related in amino acid sequence to the APC family transporters. Instead, BrnQ belongs to the 195
LIVCS family of branched chain amino acid/cation symporters (TC 2.A.26) (21). 196
Nevertheless, hydropathy profile alignment suggested that the two families are distantly 197
related and belong to the same structural class (MemGen class ST[2]) (27,28). No other 198
homologues of the LIVCS family were present in any of the four L. lactis strains. Members 199
of the DAACS family (TC 2.A.23), another family that contains many amino acid 200
transporters, were completely absent from the L. lactis genomes. 201
202
Cloning and characterization of the amino acid transporters 203
The putative amino acid transporter genes except for ctrA (aka bcaP), which was well 204
characterized as a branched chain amino acid transporter before (19), were expressed in the 205
MG1363 derived strain NZ9000 using the NICE expression system (24). Cells expressing the 206
genes were screened for enhanced uptake by comparing the uptake of fourteen amino acids 207
and putrescine (only in case of yrfD) to the uptake by control cells harbouring the empty 208
vector pNZ8048. Uptake was measured after 10 sec and with the amino acids present at 209
concentrations ranging from 0.9 μM to 5 μM. Expression of ten transporter genes resulted in 210
increased uptake of one or more amino acids (Table 1), while the remaining four genes 211
(yagE, yibG, yshA and gadC) showed no increased activity with any of the amino acids (data 212
not shown). The positive clones were studied in more detail by following uptake of the 213
identified substrate(s) with time (Fig. 2, left panels; Fig. 3) and by measuring the inhibition of 214
uptake by excess unlabelled amino acids (Fig. 2, right panels). 215
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Overexpression of lysP resulted in a strongly increased initial rate of uptake of lysine (Fig. 216
2A). The inhibition profile showed that LysP has narrow substrate specificity; none of the 217
other amino acids inhibited the uptake of lysine significantly. A 10-fold increase in the initial 218
rate of uptake of histidine was observed upon overexpression of lysQ (Fig. 2B). The 219
inhibition profile indicated that LysQ is mainly a histidine transporter but has low affinity for 220
the other basic amino acids arginine and lysine. The transporter was renamed HisP. The 221
adjacent genes ydgC and ydgB resulted in 20-fold and 5-fold increased initial rates of uptake 222
of serine, respectively (Fig. 2C, D). Uptake by cells expressing ydgC was followed by a rapid 223
efflux of label from the cells that was not observed in the case of ydgB or in the control cells. 224
The nature of this efflux is unclear. The inhibition studies revealed a different substrate 225
specificity of the two transporters. YdgC showed in addition to serine, high affinity for 226
threonine and cysteine, while serine uptake by YdgB was most strongly inhibited by alanine 227
and glycine, whereas threonine and cysteine had much less effect. Transporters YdgC and 228
YdgB were renamed SerP1 and SerP2, respectively. Initial rates of uptake of aspartate and 229
glutamate increased 30-fold and 7-fold, respectively, by cells expressing ylcA (Fig. 2E). The 230
inhibition profile of glutamate uptake revealed that the transporter is specific for the two 231
acidic amino acids. The transporter was renamed AcaP. Strongly increased rates of uptake of 232
phenylalanine and tyrosine were observed upon expressing ysjA. The inhibition profile of 233
tyrosine uptake showed that YsjA also has affinity for tryptophan and that the transporter is 234
highly specific for aromatic amino acids. YsjA was renamed FywP. The ArcD1 and ArcD2 235
genes found in the ADI cluster resulted in 3- to 4-fold increased initial rates of arginine 236
uptake (Fig. 2G, H). Also the uptake of ornithine, the end product of the ADI pathway and 237
counter substrate in arginine/ornithine exchange, was increased more than 10-fold (Table 1). 238
The inhibition profiles of both transporters indicated a main substrate specificity for arginine 239
with low affinity for lysine and histidine. The branched chain amino acids isoleucine, leucine 240
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and valine were taken up in cells expressing brnQ at a two- to four-fold higher initial rate 241
than in control cells (Fig. 3A). Although affinity for other amino acids was not determined, 242
BrnQ appears to be a branched chain amino acid transporter as suggested before (19) and 243
similar to the characterized homologue in Lactobacillus delbrueckii (31). Recently, the yrfD 244
gene present in strain IL1403 but not in MG1363, was proposed to encode the 245
precursor/product exchanger AguD that would mediate the uptake of extracellular agmatine 246
in exchange with intracellular putrescine, the product of the AgDI pathway (9). While control 247
cells did not show significant putrescine uptake activity, as observed previously (32), 248
expression of aguD enabled the cells to take up putrescine with an initial rate of 249
approximately 1.4 nmol/min/mg protein (Fig. 3B). Addition of a 10-fold excess of unlabelled 250
agmatine to cells that had accumulated putrescine led to a rapid efflux of the latter 251
demonstrating the exchange mode of transport. 252
253
Deletion of lysP kills high affinity lysine uptake without effecting growth on free amino acids 254
The lysP gene was deleted from L. lactis JP9000, a MG1363 derivative, using a two-step 255
integration and excision system (47). Lysine uptake activity in resting cells of strain JP9000 256
and JP9000∆lysP grown to mid-exponential phase in GM17 was measured at a lysine 257
concentration of 1.6 μM (Fig. 4A). The initial rate of uptake in the mutant had decreased 258
drastically by approximately 20-fold to 0.5 nmol/min/mg protein suggesting that LysP is the 259
major lysine transporter in L. lactis at low lysine concentrations, but that at least one other, 260
possibly low affinity, uptake system for lysine is present. 261
GM17 is a rich medium containing approximately 2.5 mM lysine and most other free amino 262
acids in the mM range as well as peptides derived from casein by partial hydrolysis (33). 263
Growth characteristics of the wild type and mutant strains in GM17 medium were the same 264
(not shown). Growth of the wild type strain JP9000 in SA medium, a chemically defined 265
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medium containing free amino acids (6) was slightly affected when the standard 1.4 mM of 266
lysine was omitted from the medium. The cells grew a little slower and to a 10% lower 267
optical density indicating that the lysine biosynthesis capacity is not a bottleneck in growth 268
(Fig. 4B). In line with these results, growth of the JP9000∆lysP strain was similar to the wild 269
type strain under the two conditions. A concentration as low as 56 μM lysine in the medium 270
was sufficient to overcome the small growth defect in the wild type strain, while the mutant 271
did not respond to this concentration (Fig. 4B). Apparently, uptake of lysine by LysP was 272
responsible for the growth enhancement at low lysine concentrations observed for the parent 273
strain. 274
275
HisP is essential for growth on free amino acids at low histidine concentrations 276
A hisP deletion mutant was constructed as described above for lysP. Histidine uptake by 277
JP9000∆hisP grown in GM17 medium was strongly reduced to a low, but still significant 278
level when measured at a concentration of 1.5 μM (Fig. 5A). Like observed for LysP above, 279
it appears that HisP is the main high affinity histidine transporter, but that a second transport 280
system, which is expressed at a low level and/or has low affinity for histidine, seems to be 281
present. 282
In rich GM17 medium, which contains approximately 86 μM histidine (33) as well as 283
peptides derived from casein, no significant difference in growth between the wild type and 284
mutant strain was observed (not shown). In the chemically defined SA medium that contains 285
250 μM histidine, JP9000∆hisP grew at a rate of approximately 50% of that of the wild type 286
while the biomass yield was approximately 70% of that of the wild type (Fig. 5B, C). 287
Increasing the concentration of histidine in the medium to 1 mM almost completely restored 288
the growth defect of the mutant which is in line with the presence of a second system able to 289
transport histidine, but with much lower affinity. With no histidine in the medium, both wild 290
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type and ∆hisP mutant cells were unable to grow, demonstrating the inability of L. lactis to 291
synthesize histidine. A histidine concentration as low as 10 μM allowed the wild type strain 292
to grow, but the biomass yield was limited to 40%. A concentration of 50 μM resulted in the 293
same growth as in standard SA medium (Fig. 5B). The mutant strain hardly showed any 294
growth at these two concentrations demonstrating that uptake of histidine by HisP is essential 295
for growth at low concentrations (Fig. 5C). 296
In the absence of free histidine, growth in SA medium of both strains could be fully restored 297
by the addition of 250 μM of the dipeptide histidine-leucine (not shown), demonstrating 298
separate systems for uptake of free histidine and the His-Leu dipeptide, which is probably 299
transported by the di- and tripeptide transporter DtpT (34). 300
301
302
AcaP is essential for growth on free amino acids in the absence of glutamine 303
Resting cells of the ∆acaP mutant JP9000∆acaP grown in SA medium, showed a strongly 304
reduced initial rate of uptake of aspartate at a concentration of 2.2 μM, while the initial rate of 305
uptake of glutamate at 1.9 μM was only marginally affected (Fig. 6A). Since AcaP was 306
shown to transport both aspartate and glutamate (Fig. 2E), the result indicates that a second 307
transporter is present that transports glutamate but not aspartate, and that this transporter is 308
responsible for the uptake of glutamate under the conditions of the experiment. Most likely, 309
the transporter is GlnPQ, an ABC type glutamate/glutamine transporter (20). 310
In GM17 media, rich in all amino acids, no difference in growth between the wild type and 311
acaP deletion mutant was observed. Similarly, in standard SA medium growth of the wild 312
type and mutant strain was not significantly different (compare Fig. 6B and C). SA medium 313
contains 2.1 mM glutamate but no aspartate. Clearly, AcaP plays no role in the uptake of 314
aspartate for biosynthetic purposes during growth. Omitting glutamate from the medium did 315
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not affect growth of wild type. Omitting glutamine, which is present at a concentration of 0.7 316
mM, resulted in only a marginally lower biomass yield. When both glutamate and glutamine 317
were omitted from the medium, no growth was observed (Fig. 6B). Apparently, L. lactis has 318
the capacity to efficiently take up and interconvert glutamate and glutamine. In line, growth 319
of the ΔacaP mutant was not affected when glutamate was omitted from the medium (not 320
shown). The cells are likely to take up glutamine from the medium by an appropriate 321
transporter and internally convert it to glutamate. In contrast, in medium without glutamine, 322
initially, the cells were not able to take up glutamate at a rate high enough to support cell 323
growth demonstrating the role of AcaP as glutamate uptake system during the growth of the 324
wild type cells under the same conditions (Fig. 6C). Apparently, the uptake of glutamate by 325
the mutant cells observed at μM concentrations (Fig. 6A) is not high enough to support the 326
need for glutamate and glutamine in biosynthesis under growth conditions. Remarkably, after 327
a long lag phase of approximately 5 hour the ΔacaP mutant started growing at approximately 328
half of the wild type rate (Fig. 6C). The lag phase indicates an adaptation process, possibly 329
involving the up regulation of a glutamate uptake system other than AcaP (glnPQ). The role 330
of the AcaP transporter during growth on free amino acids in the absence of glutamine is 331
further emphasized by the complete lack of growth of the mutant when, in addition, the 332
concentration of glutamate was reduced to 0.4 mM (Fig. 6C). Growth of the wild type under 333
these conditions was only marginally affected (not shown). 334
335
Deletion of fywP reduces the efficiency of growth at low concentrations of the aromatic 336
amino acids 337
Deletion of the fywP gene from strain JP9000 resulted in a 5-fold lower initial rate of uptake 338
of phenylalanine in cells grown in GM17 medium and when measured at 1 μM concentration 339
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(Fig. 7A). It follows that FywP is the main transporter for phenylalanine uptake under these 340
conditions. 341
No difference in growth was observed between wild type and mutant in rich GM17 medium 342
or chemically defined SA medium which contains 1.2 mM phenylalanine, 0.3 mM tyrosine 343
and 0.5 mM tryptophan. Although these amino acids are known to be not essential for L. 344
lactis MG1363, their presence usually stimulates growth (6,16) suggesting that the rate of 345
cellular biosynthesis is limited. Nevertheless growth of both strains was not affected when the 346
concentration of all three amino acids in SA medium was decreased 5-fold and even when the 347
concentrations were decreased 25-fold into the μM range, the wild type strain grew at the 348
same rate, but to a slightly lower biomass yield (Fig. 7B). Then, the ∆fywP mutant showed an 349
approximately 30% lower growth rate and 20% lower biomass yield, indicating that FywP is 350
important for optimal growth efficiency in an environment with low concentrations of 351
aromatic amino acids. Interestingly, in SA medium with none of the aromatic amino acids 352
present, also a lower growth rate for the fywP mutant was observed which may indicate that 353
FywP plays a role in the recovery of aromatic amino acids that have leaked out of the cells by 354
some other means. 355
356
HisP, AcaP and FywP play no significant role in excretion of accumulated histidine, 357
glutamate and phenylalanine 358
The ∆hisP, ∆acaP and ∆fywP mutants and the wild type were grown to mid-exponential 359
growth phase in SA medium and incubated in a 100 mM potassium phosphate pH 6.0 buffer 360
containing 25 mM glucose and 5 mM of the dipeptides His-Leu (strain NZ9000∆hisP and 361
NZ9000), Gly-Glu (JP9000∆acaP and JP9000) or Phe-Val (JP9000∆fywP and JP9000). The 362
consumption of the dipeptide and excretion of the corresponding free amino acids were 363
analysed by HPLC (Fig. 8). In all cases the dipeptide was taken up efficiently during the first 364
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5 min at rates of 0.17, 0.3 and 0.4 μmol/min/mg protein for His-Leu, Phe-Val and Gly-Glu, 365
respectively. Thereafter, the rates dropped considerably, especially for the Gly-Glu and Phe-366
Val peptides. The free amino acids produced by intracellular peptidase activity appeared 367
readily in the medium at a high rate during the first 5 min after which the rate slowed down in 368
line with the disappearance of the peptide from the medium. The His-Leu and Phe-Val 369
peptides were stoichiometrically converted to the free amino acids by the cells. With the 370
Gly-Glu peptide, the rate of glutamate excretion was 2 times lower than that of glycine 371
excretion during the first 5 min suggesting that part of the free intracellular glutamate was 372
further converted. No products other than glycine, glutamate and glycine-glutamate were 373
detected in the medium. More importantly, for none of the conversions, a difference was 374
observed between the wild type strains and the transporter mutants suggesting that HisP, 375
AcaP nor FywP played a role in the excretion of accumulated amino acids. 376
377
378
379
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Discussion 380
381
Since the emergence of genome sequencing and with the improvement of recombinant DNA 382
techniques, surprisingly little work has been carried out on the identification of amino acid 383
transport systems in lactic acid bacteria, more specifically in the model LAB L. lactis. Most 384
of the knowledge still originates from extensive transport studies in whole cells and 385
membrane vesicles derived from wild type L. lactis strains, performed in the late eighties and 386
early nineties of the previous century (8,14,16,17,35). Only two amino acid transport systems 387
have since been cloned for expression and functional characterization: the ABC-type 388
glutamate/glutamine transporter GlnPQ, encoded by glnP and glnQ (20) and BcaP, a 389
secondary, branched chain amino acid transporter, encoded by bcaP (19). Here, 13 genes of 390
L. lactis MG1363 and one of L. lactis IL1403 (aguD), all but one (brnQ) encoding members 391
of the APC family (21) and annotated either as unknown or as amino acid transporters based 392
on homology, were cloned and overexpressed in L. lactis NZ9000. Substrates were identified 393
(Fig. 2 and 3), and the specificity towards all proteinogenic amino acids was determined (Fig. 394
2, right panels). Four transporters, SerP1, SerP2, AcaP and FywP, for which no function was 395
assigned based on homology, are entirely newly identified amino acid transporters. SerP1 and 396
SerP2 transport serine, AcaP and FywP acidic amino acids and aromatic amino acids, 397
respectively. FywP has the same substrate specificity but, in sequence, is not closely related 398
to the aromatic amino acid transporter AroP. Hence, the different names. LysP and HisP 399
(formerly LysQ) were identified as lysine and histidine transporters, respectively, which was 400
predicted previously based on regulatory sequences in their promoter regions (36,37). The 401
predicted functions of AguD as an agmatine/putrescine exchanger (9) and ArcD1 and ArcD2 402
as arginine/ornithine exchangers (29) were also confirmed. BrnQ, which is not homologous 403
to the other APC family transporters, was shown to be a branched chain amino acid 404
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transporter, in agreement with transport studies with a brnQ knock-out strain described 405
previously (19). No activity could be determined for four genes as overexpression did not 406
result in increased uptake of any of the amino acids. The specificity of GadC that was 407
predicted to be a glutamate/γ-butyric acid exchanger (30) could not be confirmed in this 408
study. 409
Much of the data presented here is in agreement with earlier reports on transport of amino 410
acids studied in wild type cells or membrane vesicles of L. lactis. The presence of separate 411
secondary transport systems for lysine (14), histidine (15), serine and threonine (15), and 412
phenylalanine and tyrosine (16) is confirmed in here by the identification and characterization 413
of LysP, HisP, SerP1 and SerP2, and FywP. In contrast, a secondary transporter for aspartate 414
and glutamate uptake, i.e. AcaP, was not reported before. Instead, the ABC transporter 415
GlnPQ with high affinity for glutamate and glutamine and low affinity for aspartate has been 416
described (20,38). The physiological relevance for having both an ATP-driven glutamate 417
transporter together with a secondary, probably proton symport driven glutamate transporter 418
is unclear, but may be related to the importance of glutamate and glutamine in nitrogen 419
metabolism in the cell. 420
The physiological role of the LysP, HisP, AcaP and FywP transporters was demonstrated 421
using knock-out mutants. Uptake in resting cells demonstrated that the lysP, hisP, acaP and 422
fywP genes encode the major transporters for lysine, histidine, aspartate and tyrosine, 423
respectively, when the substrates were present at μM concentrations. For all these substrates, 424
unknown, low affinity transport systems appeared to be present as well. Glutamate uptake in 425
the acaP mutant, although being a substrate of AcaP, was similar to that in wild type, 426
indicating that a second transport system, most likely GlnPQ, is the major glutamate transport 427
system under these conditions. In chemically defined media containing free amino acids in a 428
range of 0.3 to 3.4 mM but no protein and peptides as additional sources of amino acids (SA 429
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medium (6)), growth of the mutants was similar to that of the wild type strain, which is in line 430
with the presence of additional low affinity uptake systems. The role of the transporters in 431
growth became apparent at low concentration but the effect was strongly dependent on the 432
capacity of L. lactis to synthesize the particular amino acid. While HisP was essential for 433
growth at low concentrations of histidine (Fig. 5B), LysP only restored a small growth defect 434
observed in the wild type when no lysine was present (Fig. 4B). The phenotype of the fywP 435
mutant was in between; a significantly lower growth rate was observed at low concentrations 436
of the aromatic amino acids and even without them (Fig. 7B). FywP transports aromatic 437
amino acids that can be synthesized by L. lactis itself. The effect at low concentrations is in 438
agreement with earlier observations that addition of phenylalanine, and in general, addition of 439
some non-essential free amino acids stimulates growth of L. lactis MG1363 in defined media 440
(6,16). Jensen et al. (39) demonstrated that in the intracellular amino acid pool of L. lactis 441
MG1363 growing in SA medium (containing free amino acids), only aspartate was derived to 442
a substantial proportion (38%) from de novo biosynthesis and that all other amino acids were 443
largely taken up from the medium. This probably reflects regulation to avoid the higher 444
energy costs of biosynthesis compared to the uptake of external free amino acids. The 445
observed difference between the wild type and fywP mutant in SA medium without aromatic 446
amino acids suggests that FywP is involved in retaining these amino acids in the cytoplasm. 447
Then, aromatic amino acids that would leak out of the cells by passive diffusion, would be 448
taken up again by FywP. Excretion or leakage of phenylalanine and tyrosine was indeed 449
observed by Whipp et al. (40) in an E. coli mutant strain that lacks aromatic amino acid 450
transporters. The situation in milk, with casein as the main amino acid source, is more 451
complicated. While casein is preferred over free amino acids as source of amino acids using 452
its efficient proteolytic system (3), it has been demonstrated at the same time that some 453
strains of L. lactis do not reach their maximal growth rate in milk and that this can be 454
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overcome by addition of certain amino acids (41). The implication is that in complex media 455
containing casein as well as free amino acids (for example released by other LAB in mixed 456
fermentations) amino acid transporters still play a role. In this light it is not surprising that 457
dairy strains of L. lactis that have lost their biosynthetic pathways for histidine and branched 458
chain amino acids during their adaptation to milk (1,2), have retained the transporters that can 459
import these as free amino acids, to not become fully dependent on casein as amino acid 460
source. 461
During fermentation in milk, some amino acids are excreted by L. lactis (42,43,44) indicating 462
that the amino acid composition of the peptides taken up after hydrolysis of casein by PrtP is 463
not matching the requirements for biosynthesis. Excretion of the excess amino acids has been 464
proposed to be coupled to the excretion of a proton (proton symport) which would contribute 465
to the generation of metabolic energy in the form of a proton motive force (pmf; energy 466
recycling model). A potential role for HisP, AcaP and FywP as the secondary transporters 467
involved in this process was investigated by feeding the cells dipeptides and measuring the 468
release of the free amino acids in wild type and mutants. The dipeptides histidine-leucine, 469
glycine-glutamate and phenylalanine-valine were efficiently taken up and the free amino 470
acids produced intracellularly by hydrolysis were readily excreted. No difference between the 471
knock-out mutants and wild type was observed suggesting that either a different transport 472
system or passive diffusion was responsible for the release from the cells. Passive diffusion 473
has previously been shown to be a mechanism of excretion of accumulated neutral amino 474
acids (12,45), however, it is unlikely that glutamate diffuses to a significant extent across the 475
membrane. Instead it was demonstrated that in Corynebacterium glutamicum glutamate 476
excretion is mediated by NCgl1221, a mechanosensitive channel which can also mediate 477
betaine transport (46,47,48). A deletion mutant accumulated up to 10-fold higher intracellular 478
glutamate concentrations. In the same organism, other transport systems were found 479
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mediating excretion of lysine and arginine (LysE) (49), threonine and serine (ThrE) (49) and 480
branched chain amino acids and methionine (BrnFE) (50,51). In L. lactis MG1363, only 481
distant homologues of NCgl1221 (llmg_0881 gene) and BrnFE (azlC-llmg_0882 gene pair) 482
can be found. Interestingly, llmg_0881 is located downstream of the gadCB operon (but 483
transcribed in the opposite direction), putatively encoding the glutamate/γ-aminobutyrate 484
pathway proteins (30). Apart from homologues of these systems, other types of carriers 485
mediating excretion of amino acids may be present. 486
487
488
489
Acknowledgements 490
491
We thank Agata Pudlik for assistance with the HPLC analysis of amino acids and dipeptides. 492
This work was supported by the European Community’s Seventh Framework Programme 493
grant agreement No. 211441-BIAMFOOD and by the Netherlands Organization for Scientific 494
Research (NWO-ALW). 495
496
497
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Table 1. Amino acid transporters of L. lactis ssp. lactis and cremoris. 498
Proteina Transporter Initial rate of uptakeb
(nmol/min/mg)
Substrate(s)c
substrate concentration
(μM)
wild type
cells
overexpressing
cells
LysQ HisP histidine 1.5 0.7 4.6 His, Arg, Lys
LysP LysP lysine 1.6 11.5 48 Lys
YdgC SerP1 serine 4.2 2.2 40 Ser, Thr, Cys
YdgB SerP2 serine 4.2 2.2 9.6 Ser, Ala, Gly
YibG - -d - - - -
YsjA FywP phenylalanine
tyrosine
1.0
1.0
0.2
0.2
7.5
9.6
Phe, Tyr, Trp
CtrA BcaP - - - - Leu, Ile, Vale
YagE - -d - - - -
ArcD1 ArcD1 arginine
ornithine
1.5
17.5
0.7
7.5
2.8
108
Arg, Orn, His
ArcD2 ArcD2 arginine
ornithine
1.5
17.5
0.2
7.5
0.9
72
Arg, Orn
GadC GadC -d - - - -
YshA - -d - - - -
YlcA AcaP aspartate
glutamate
2.2
1.9
0.2
0.4
6.6
3
Asp, Glu
YrfD AguD putrescine 4.7 0.2 1.4 Agm, Put
BrnQ BrnQ leucine
isoleucine
valine
1.4
1.5
1.8
0.5
0.7
0.2
1.0
1.2
1.3
Leu, Ile, Val
499 a Annotation of L. lactis ssp. lactis IL1403 genome except for BrnQ. 500 b Uptake measured during the first 10 sec. 501 c Data from uptake, inhibition and exchange studies (see also Fig. 2 and 3) 502
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d No significant difference between control cells and overexpressing cells were found for any 503
of the amino acids 504 e See reference (19) 505
506
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References 507
508
1. Delorme C, Godon JJ, Ehrlich SD, Renault P. 1993. Gene inactivation in Lactococcus 509
lactis: histidine biosynthesis. J. Bacteriol. 175:4391-4399. 510
2. Godon JJ, Delorme C, Bardowski J, Chopin MC, Ehrlich SD, Renault P. 1993. Gene 511
inactivation in Lactococcus lactis: branched-chain amino acid biosynthesis. J. Bacteriol. 512
175:4383-4390. 513
3. Kunji ER, Hagting A, De Vries CJ, Juillard V, Haandrikman AJ, Poolman B, 514
Konings WN. 1995. Transport of beta-casein-derived peptides by the oligopeptide transport 515
system is a crucial step in the proteolytic pathway of Lactococcus lactis. J. Biol. Chem. 516
270:1569-1574. 517
4. Poolman B, Kunji ER, Hagting A, Juillard V, Konings WN. 1995. The proteolytic 518
pathway of Lactococcus lactis. Soc. Appl. Bacteriol. Symp. Ser. 24:65S-75S. 519
5. Savijoki K, Ingmer H, Varmanen P. 2006. Proteolytic systems of lactic acid bacteria. 520
Appl. Microbiol. Biotechnol. 71:394-406. 521
6. Jensen PR, Hammer K. 1993. Minimal Requirements for Exponential Growth of 522
Lactococcus lactis. Appl. Environ. Microbiol. 59:4363-4366. 523
7. Konings WN. 2002. The cell membrane and the struggle for life of lactic acid bacteria. 524
Antonie Van Leeuwenhoek 82:3-27. 525
8. Driessen AJ, Molenaar D, Konings WN. 1989. Kinetic mechanism and specificity of the 526
arginine-ornithine antiporter of Lactococcus lactis. J. Biol. Chem. 264:10361-10370. 527
9. Ladero V, Rattray FP, Mayo B, Martin MC, Fernandez M, Alvarez MA. 2011. 528
Sequencing and transcriptional analysis of the biosynthesis gene cluster of putrescine-529
producing Lactococcus lactis. Appl. Environ. Microbiol. 77:6409-6418. 530
on May 3, 2018 by guest
http://jb.asm.org/
Dow
nloaded from
25
10. Molenaar D, Bosscher JS, ten Brink B, Driessen AJ, Konings WN. 1993. Generation 531
of a proton motive force by histidine decarboxylation and electrogenic histidine/histamine 532
antiport in Lactobacillus buchneri. J. Bacteriol. 175:2864-2870. 533
11. Driessen AJ, de Jong S, Konings WN. 1987. Transport of branched-chain amino acids 534
in membrane vesicles of Streptococcus cremoris. J. Bacteriol. 169:5193-5200. 535
12. Driessen AJ, Hellingwerf KJ, Konings WN. 1987. Mechanism of energy coupling to 536
entry and exit of neutral and branched chain amino acids in membrane vesicles of 537
Streptococcus cremoris. J. Biol. Chem. 262:12438-12443. 538
13. Driessen AJ, Kodde J, de Jong S, Konings WN. 1987. Neutral amino acid transport by 539
membrane vesicles of Streptococcus cremoris is subject to regulation by internal pH. J. 540
Bacteriol. 169:2748-2754. 541
14. Driessen AJ, van Leeuwen C, Konings WN. 1989. Transport of basic amino acids by 542
membrane vesicles of Lactococcus lactis. J. Bacteriol. 171:1453-1458. 543
15. Driessen AJ. 1987. Amino acid transport in lactic streptococci. PhD thesis. University of 544
Groningen, Groningen, The Netherlands. 545
16. Poolman B. Konings WN. 1988. Relation of growth of Streptococcus lactis and 546
Streptococcus cremoris to amino acid transport. J. Bacteriol. 170:700-707. 547
17. Poolman B, Smid EJ, Konings WN. 1987. Kinetic properties of a phosphate-bond-548
driven glutamate-glutamine transport system in Streptococcus lactis and Streptococcus 549
cremoris. J. Bacteriol. 169:2755-2761. 550
18. Molenaar D, Hagting A, Alkema H, Driessen AJ, Konings WN. 1993. Characteristics 551
and osmoregulatory roles of uptake systems for proline and glycine betaine in Lactococcus 552
lactis. J. Bacteriol. 175:5438-5444. 553
on May 3, 2018 by guest
http://jb.asm.org/
Dow
nloaded from
26
19. Den Hengst CD, Groeneveld M, Kuipers OP, Kok J. 2006. Identification and 554
functional characterization of the Lactococcus lactis CodY-regulated branched-chain amino 555
acid permease BcaP (CtrA). J. Bacteriol. 188:3280-3289. 556
20. Schuurman-Wolters GK, Poolman B. 2005. Substrate specificity and ionic regulation 557
of GlnPQ from Lactococcus lactis. An ATP-binding cassette transporter with four 558
extracytoplasmic substrate-binding domains. J. Biol. Chem. 280:23785-23790. 559
21. Saier MH Jr. 2000. Families of transmembrane transporters selective for amino acids 560
and their derivatives. Microbiology 146:1775-1795. 561
22. Kuipers OP, de Ruyter PG, Kleerebezem M, de Vos WM. 1998. Quorum sensing-562
controlled gene expression in lactic acid bacteria. J. Biotechnol. 64:15–21. 563
23. Pinto JP, Zeyniyev A, Karsens H, Trip H, Lolkema JS, Kuipers OP, Kok J. 2011. 564
pSEUDO, a genetic integration standard for Lactococcus lactis. Appl. Environ. Microbiol. 565
77:6687-6690. 566
24. De Ruyter PG, Kuipers OP, de Vos WM. 1996. Controlled gene expression systems for 567
Lactococcus lactis with the food-grade inducer nisin. Appl. Environ. Microbiol. 62:3662-568
3667. 569
25. Solem C, Defoor E, Jensen PR, Martinussen J. 2008. Plasmid pCS1966, a new 570
selection/counterselection tool for lactic acid bacterium strain construction based on the oroP 571
gene, encoding an orotate transporter from Lactococcus lactis. Appl. Environ. Microbiol. 572
74:4772-4775. 573
26. Pudlik AM. Lolkema JS. 2012. Rerouting Citrate Metabolism in Lactococcus lactis: 574
Citrate Driven Transamination. Appl. Environ. Microbiol. 78, 6665-6673 575
27. Lolkema JS, Slotboom DJ. 1998. Estimation of structural similarity of membrane 576
proteins by hydropathy profile alignment. Mol. Membr. Biol. 15:33-42. 577
on May 3, 2018 by guest
http://jb.asm.org/
Dow
nloaded from
27
28. Lolkema JS, Slotboom DJ. 2008. The major amino acid transporter superfamily has a 578
similar core structure as Na+-galactose and Na+-Leucine transporters. Mol. Membr. Biol. 579
25:567-570. 580
29. Budin-Verneuil A, Maguin E, AuffrayY, Ehrlich DS, Pichereau V. 2006. Genetic 581
structure and transcriptional analysis of the arginine deiminase (ADI) cluster in Lactococcus 582
lactis MG1363. Can. J. Microbiol. 52:617-622. 583
30. Sanders JW, Leenhouts K, Burghoorn J, Brands JR, Venema G, Kok J. 1998. A 584
chloride-inducible acid resistance mechanism in Lactococcus lactis and its regulation. Mol. 585
Microbiol. 27:299-310. 586
31. Stucky K, Hagting A, Klein JR, Matern H, Henrich B, Konings WN, Plapp R. 1995. 587
Cloning and characterization of brnQ, a gene encoding a low-affinity, branched-chain amino 588
acid carrier in Lactobacillus delbruckii subsp. lactis DSM7290. Mol. Gen. Genet. 249:682-589
690. 590
32. Romano A, Trip H, Lonvaud-Funel A, Lolkema JS, Lucas PM. 2012. Evidence of 591
two functionally distinct ornithine decarboxylation systems in lactic acid bacteria. Appl. 592
Environ. Microbiol. 78:1953-1961. 593
33. Guimont C. 2002. Change of free amino acids in M17 medium after growth of 594
Streptococcus thermophilus and identification of a glutamine transport ATP-binding protein. 595
Int. Dairy J. 12:729-736. 596
34. Fang G, Konings WN, Poolman B. 2000. Kinetics and substrate specificity of 597
membrane-reconstituted peptide transporter DtpT of Lactococcus lactis. J. Bacteriol. 598
182:2530-2535. 599
35. Driessen AJ. 1989. Secondary transport of amino acids by membrane vesicles derived 600
from lactic acid bacteria. Antonie Van Leeuwenhoek 56:139-160. 601
on May 3, 2018 by guest
http://jb.asm.org/
Dow
nloaded from
28
36. Rodionov DA, Vitreschak AG, Mironov AA, Gelfand MS. 2003. Regulation of lysine 602
biosynthesis and transport genes in bacteria: yet another RNA riboswitch? Nucleic Acids 603
Res. 31:6748-6757. 604
37. Vitreschak AG, Lyubetskaya EV, Shirshin MA, Gelfand MS, Lyubetsky VA. 2004. 605
Attenuation regulation of amino acid biosynthetic operons in proteobacteria: comparative 606
genomics analysis. FEMS Microbiol. Lett. 234:357-370. 607
38. Poolman B, Driessen AJ, Konings WN. 1987. Regulation of solute transport in 608
streptococci by external and internal pH values. Microbiol. Rev. 51:498-508. 609
39. Jensen NB, Christensen B, Nielsen J, Villadsen J. 2002. The simultaneous biosynthesis 610
and uptake of amino acids by Lactococcus lactis studied by (13)C-labeling experiments. 611
Biotechnol. Bioeng. 78:11-16. 612
40. Whipp MJ, Halsall DM, Pittard AJ. 1980. Isolation and characterization of an 613
Escherichia coli K-12 mutant defective in tyrosine- and phenylalanine-specific transport 614
systems. J. Bacteriol. 143:1-7. 615
41. Hugenholtz J, Dijkstra M, Veldkamp H. 1987. Amino acid limited growth of starter 616
cultures in milk. FEMS Microbiol. Ecol. 45:191-198. 617
42. Cretenet M, Laroute V, Ulve V, Jeanson S, Nouaille S, Even S, Piot M, Girbal L, Le 618
LY, Loubiere P, Lortal S, Cocaign-Bousquet M. 2011. Dynamic analysis of the 619
Lactococcus lactis transcriptome in cheeses made from milk concentrated by ultrafiltration 620
reveals multiple strategies of adaptation to stresses. Appl. Environ. Microbiol. 77:247-257. 621
43. Juillard V, Le BD, Kunji ER, Konings WN, Gripon JC, Richard J. 1995. 622
Oligopeptides are the main source of nitrogen for Lactococcus lactis during growth in milk. 623
Appl. Environ. Microbiol. 61:3024-3030. 624
on May 3, 2018 by guest
http://jb.asm.org/
Dow
nloaded from
29
44. Niven GW, Knight DJ, Mulholland F. 1998. Changes in the concentrations of free 625
amino acids in milk during growth of Lactococcus lactis indicate biphasic nitrogen 626
metabolism. J. Dairy Res. 65:101-107. 627
45. Anderson JJ, Oxender DL. 1978. Genetic separation of high- and low-affinity transport 628
systems for branched-chain amino acids in Escherichia coli K-12. J. Bacteriol. 136:168-174. 629
46. Hashimoto K, Nakamura K, Kuroda T, Yabe I, Nakamatsu T, Kawasaki H. 2010. 630
The protein encoded by NCgl1221 in Corynebacterium glutamicum functions as a 631
mechanosensitive channel. Biosci. Biotechnol. Biochem. 74:2546-2549. 632
47. Nakamura J, Hirano S, Ito H, Wachi M. 2007. Mutations of the Corynebacterium 633
glutamicum NCgl1221 gene, encoding a mechanosensitive channel homolog, induce L-634
glutamic acid production. Appl. Environ. Microbiol. 73:4491-4498. 635
48. Nottebrock D, Meyer U, Krämer R, Morbach S. 2003. Molecular and biochemical 636
characterization of mechanosensitive channels in Corynebacterium glutamicum. FEMS 637
Microbiol. Lett. 218:305-309. 638
49. Simic M, Sahm H, Eggeling L. 1996. A new type of transporter with a new type of 639
cellular function: L-lysine export from Corynebacterium glutamicum. Mol. Microbiol. 640
22:815-826. 641
50. Kennerknecht N, Sahm H, Yen MR, Patek M, Saier MH Jr, Eggeling L. 2002. Export 642
of L-isoleucine from Corynebacterium glutamicum: a two-gene-encoded member of a new 643
translocator family. J. Bacteriol. 184:3947-3956. 644
51. Trotschel C, Deutenberg D, Bathe B, Burkovski A, Krämer R. 2005. Characterization 645
of methionine export in Corynebacterium glutamicum. J. Bacteriol. 187:3786-3794. 646
647
648
649
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Figure legends 650
651
Figure 1. Phylogenetic tree of all APC family members and BrnQ (LIVCS family) of L. 652
lactis ssp. lactis (IL1403 and KF147) and ssp. cremoris (MG1363 and SK11). Nomenclature 653
of strain IL1403 was used, except for BrnQ. Newly assigned names are shown between 654
parantheses. Protein alignment and tree building was carried out using ClustalX. Amino acid 655
sequence identities between proteins were indicated. 656
657
Figure 2. Substrate specificity of amino acid transporters. Cells of L. lactis NZ9000 658
containing the empty vector pNZ8048 or pNZ8048 with lysP (A), lysQ (B), ydgC (C), ydgB 659
(D), ylcA (E), ysjA (F), arcD1 (G) or arcD2 (H) cloned behind the nisin inducible promoter, 660
were induced for 1 hour with 5 ng/ml nisin and tested for uptake of 14C-labeled amino acids 661
at a concentration of 1 – 5 μM (see Table 1). Left panels: time dependent uptake in cells 662
harbouring the empty vector (closed symbols) or the cells expressing a transporter (open 663
symbols). The substrates are indicated at the y-axis. In E, glutamate uptake is shown in 664
circles and aspartate uptake in triangles. In F, phenylalanine uptake is shown in circles and 665
tyrosine uptake in triangles. Right panels: substrate specificity as determined by inhibition of 666
substrate (indicated at the y-axis) by 1 mM of unlabelled amino acids, indicated at the x-axis 667
with one letter codes. Uptake was determined after 10 seconds of incubation with labelled 668
substrate. The control, i.e. without addition of unlabelled amino acid, was set to 100 % (last 669
bar, Co). Error bars represent standard deviations of the means of two experiments. 670
671
Figure 3. Amino acid uptake via BrnQ (A) and AguD (B). (A) Cells expressing brnQ (open 672
symbols) or harboring the empty vector pNZ8048 (closed symbols) were incubated with 14C-673
leucine (squares), 14C-isoleucine (triangles) or 14C-valine (circles) at concentrations shown in 674
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Table 1. (B) Cells expressing aguD or harbouring pNZ8048 were incubated with 4.7 μM 14C-675
putrescine. To demonstrate agmatine/putrescine exchange, 50 μM of unlabelled agmatine was 676
added to cells that were allowed to accumulate 14C-putrescine for 1 minute (triangles). 677
678
Figure 4. Phenotype of lysP deletion mutant. (A) Uptake of 14C-lysine (at a concentration of 679
1.6 μM) in wild type cells (strain JP9000, open circles) or lysP mutant cells (closed circles). 680
Cells were pregrown to mid-exponential phase in GM17 medium. (B) Growth of JP9000 681
(triangles) and JP9000∆lysP (circles) in SA medium containing 56 μM lysine (closed 682
symbols) or no lysine (open symbols). Shown is a representative experiment of at least two 683
independent experiments. 684
685
Figure 5. Phenotype of hisP deletion mutant. (A) Uptake of 14C-histidine (at a concentration 686
of 1.5 μM) in wild type cells (open circles) or hisP mutant cells (closed circles). Cells were 687
pregrown to mid-exponential phase in GM17 medium. (B and C) Growth of NZ9000 (B) and 688
NZ9000∆lysQ (C) in SA medium containing 1 mM (●), 250 μM (○), 50 μM (▼), 10 μM (∆) 689
or no histidine (■). Shown is a representative experiment of at least two independent 690
experiments. 691
692
Figure 6. Phenotype of acaP deletion mutant. (A) Uptake of 14C-glutamate (circles) or 693
aspartate (triangles) in wild type cells (open symbols) or acaP mutant cells (closed symbols). 694
Glutamate and aspartate were present at 1.9 and 2.2 μM, respectively. Cells were grown to 695
mid-exponential phase in SA medium. (B) Growth of JP9000 in standard SA medium (closed 696
circles) and SA medium from which either glutamate (closed triangles), glutamine (open 697
circles) or both (open triangles) were omitted. (C) Growth JP9000∆acaP in standard SA 698
medium (closed triangles), SA medium without glutamine(open circles) and SA medium 699
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without glutamine and containing 0.4 mM glutamate rather than the standard 2.1 mM (open 700
triangles). Shown is a representative experiment of at least two independent experiments. 701
702
Figure 7. Phenotype of fywP deletion mutant. (A) Uptake of 14C-phenylalanine (at a 703
concentration of 1 μM) in wild type cells (open circles) or ∆fywP mutant cells (closed 704
circles). Cells were grown to mid-exponential phase in GM17 medium. (B) Growth of 705
JP9000 (closed symbols) and JP9000∆fywP (open symbols) in SA medium containing 0.24 706
mM, 0.06 mM and 0.1 mM (circles) or 48 μM, 12 μM and 20 μM (triangles) of 707
phenylalanine, tyrosine and tryptophan, respectively or without any of these amino acids 708
(squares). Shown is a representative experiment of at least two independent experiments. 709
710
Figure 8. Dipeptide uptake and amino acid excretion by wild type and transporter mutant 711
strains. Cells grown to mid-exponential phase in SA medium were harvested and resuspended 712
to an OD600 of 2 in 100 mM KPi pH 6.0 buffer, containing 25 mM glucose and 5 mM 713
histidine-leucine (A), glycine-glutamate (B) or phenylalanine-leucine (C) and incubated at 714
30°C. Concentrations of free amino acids and dipeptide in the buffer were determined from 715
samples taken at indicated time points. Open symbols, wild type strain NZ9000 (A) or 716
JP9000 (B and C); closed symbols, NZ9000∆lysQ (A), JP9000∆ylcA (B) or JP9000∆ysjA (C). 717
Squares: dipeptide concentrations; circles: histidine (A), glycine (B) or phenylalanine 718
concentrations (C); triangles: leucine (A), glutamate (B) or valine concentrations (C). Amino 719
acid concentrations are indicated on the left y-axes; dipeptide concentrations on the right y-720
axes. 721
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