INFECTION AND IMMUNITY, Apr. 2004, p. 2386�2389 Vol. 72, No. 40019-9567/04/$08.00�0 DOI: 10.1128/IAI.72.4.2386–2389.2004Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Tec1p-Independent Activation of a Hypha-Associated Candida albicansVirulence Gene during Infection

Peter Staib,1 Ayfer Binder,1 Marianne Kretschmar,2 Thomas Nichterlein,2 Klaus Schroppel,3and Joachim Morschhauser1*

Institut fur Molekulare Infektionsbiologie, Universitat Wurzburg, D-97070 Wurzburg,1 Institut fur Medizinische Mikrobiologieund Hygiene, Fakultat fur Klinische Medizin Mannheim der Universitat Heidelberg, D-68167 Mannheim,2 and Institut

fur Klinische Mikrobiologie, Immunologie und Hygiene, Universitat Erlangen, D-91054 Erlangen,3 Germany

Received 27 October 2003/Returned for modification 10 December 2003/Accepted 22 December 2003

The Tec1p transcription factor is involved in the expression of hypha-specific genes in Candida albicans.Although the induction of the hypha-associated SAP5 gene by serum in vitro depends on Tec1p, deletion of allTec1p binding site consensus sequences from the SAP5 promoter did not affect its activation. In two differentanimal models of candidiasis, the SAP5 promoter was induced even in a �tec1 deletion mutant, demonstratingthat the requirement for Tec1p in gene expression in C. albicans depends on the environmental conditionswithin the host.

The ability to switch from a yeast to a filamentous, hyphalform, together with the expression of hypha-associated genes,is important for virulence of the opportunistic fungal pathogenCandida albicans (4). Morphogenesis and expression of hypha-associated genes are regulated in response to environmentalcues by several signal transduction pathways, including a mi-togen-activated protein kinase pathway that ends in the tran-scription factor Cph1p and a cyclic AMP-dependent pathwayinvolving the Efg1p transcription factor (3, 5, 12, 15, 24). De-pending on the growth conditions used to induce hyphal for-mation, mutants that are defective in these signaling pathwaysare unable to form filaments and do not express hypha-asso-ciated genes (2, 10, 13, 14, 23).

The TEA/ATTS transcription factor Tec1p also has beenshown to be necessary for proper hyphal formation in C. albi-cans in vitro (19). C. albicans mutants with a deletion of theTEC1 gene displayed attenuated virulence in a mouse model ofsystemic candidiasis, although they formed hyphae in the in-fected tissue (19). It has been proposed that Tec1p regulateshypha-associated genes in C. albicans by binding to the se-quence 5�-CATTCY-3�, which is the consensus binding se-quence for fungal TEA/ATTS transcription factors (1, 6, 19).This sequence is found in the promoter region of hypha-asso-ciated genes, and recent evidence suggests that different signaltransduction pathways involved in hyphal growth of C. albicansconverge on the Tec1p transcription factor, which then medi-ates the expression of hypha-associated genes (10).

The SAP5 gene encodes one of the secreted aspartic pro-teinases of C. albicans which have been shown to contribute tovirulence of the fungus (9, 17). Its expression is induced uponhyphal formation in vitro and during experimental infection invivo, and full SAP5 induction by host signals depends onCph1p and Efg1p (7, 20, 21). The SAP5 promoter containsthree direct repeats of a 25-bp sequence in which a Tec1p

consensus binding sequence is present, suggesting that Tec1pmight induce SAP5 expression by binding to this sequence.Therefore, using a recently developed in vivo expression tech-nology that is based on FLP-mediated deletion of a mycophe-nolic acid resistance marker (MPAR) from the genome as areporter of gene activation (20, 22), we investigated whetherSAP5 induction within the host is dependent on Tec1p andwhether the putative Tec1p binding sites mediate this activation.

To delete the three 25-mer repeats containing the putativeTec1p binding sites from PSAP5, a 0.8-kb fragment containingthe SAP5 promoter was PCR amplified with the primer pairSAP5P3 (5�-GTATAAATGCTCtaGAATTCTGTTTGGCG-3�) and SAP5P6 (5�-ACATTgtcgacTTGAGCTTAACTTTGGATTAGTTATAAAGGAGTGAAT*GACTATTTGAATG-3�). The lowercase letters represent nucleotide exchanges in-troduced to create an upstream XbaI site and a SalI site infront of the start codon (restriction sites are underlined, andthe reverse sequence of the start codon is shown in bold); theposition of the 75-bp deletion is indicated by an asterisk. ThePCR product was digested with XbaI and SalI, and the mu-tated SAP5 promoter (PSAP5�3) was substituted for the wild-type SAP5 promoter of plasmid pSFL53 (20) to generatepSFL56. Strain CFI1, which contains the FLP-deletable MPAR

marker, was transformed by electroporation (8) with the insertfrom this plasmid. The two SAP5 alleles of the parental straincan be distinguished by a BglII restriction site polymorphism(20), and two independent transformants which contained thePSAP5�3-ecaFLP fusion integrated into the SAP5-1 (strainS5FI5A) or the SAP5-2 allele (strain S5FI5B) were selected.Replacement of the original SAP5 promoter by the mutatedversion was confirmed by reamplification and sequencing.

To detect activation of the wild-type SAP5 promoter in theabsence of the Tec1p transcription factor, the FRT-MPAR-FRTcassette from plasmid pAFI3 containing the FLP-deletableMPAR marker between ACT1 flanking sequences (22) was firstintegrated into one of the ACT1 alleles of the �tec1 mutantstrain CaAS15 (19). The resulting strain, CFI6, was then trans-formed with the PSAP5-ecaFLP reporter gene fusion from

* Corresponding author. Mailing address: Institut fur MolekulareInfektionsbiologie, Universitat Wurzburg, Rontgenring 11, D-97070Wurzburg, Germany. Phone: 49-931-31 21 52. Fax: 49-931-31 25 78.E-mail: joachim.morschhaeuser@mail.uni-wuerzburg.de.

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pSFL53, and two independent transformants in which integra-tion had occurred into the SAP5-1 (strain C6S5F1D) or theSAP5-2 allele (strain C6S5F1E) were selected. All integrationevents were confirmed by Southern hybridization (data notshown).

We first assessed activation of the SAP5 promoter duringserum-induced hyphal growth in vitro. Five microliters of anovernight culture of each reporter strain was inoculated in 250�l of fresh serum from a healthy human donor and incubatedfor 4 h at 37°C. Appropriate dilutions were then spread onMPA indicator plates to determine the percentage of MPAS

cells in which induction of the SAP5 promoter had resulted inFLP-mediated deletion of the MPAR marker. As shown in Fig.1A, activation of the SAP5 promoter in a wild-type background(strains S5FI2A and S5FI2B [20]) during serum-induced hy-phal growth was readily detected with this reporter system,whereas virtually no SAP5 induction was seen in the �tec1mutant, confirming previous results from Northern hybridiza-tion experiments (19). Surprisingly, the deletion of the putativeTec1p binding sites did not affect the inducibility of the SAP5promoter by serum. Since there is no other Tec1p bindingconsensus sequence within 2 kb upstream of the SAP5 codingregion, this result suggests that Tec1p binding sites in C. albi-cans may differ from those in other organisms, as has recentlybeen reported for the Rim101p transcription factor (16), andTec1p might bind to sites other than the consensus sequence inthe SAP5 promoter to mediate activation of this gene. Alter-natively, the involvement of Tec1p in the transcriptional induc-tion of SAP5 may be indirect.

To analyze the dependence of SAP5 induction on the Tec1ptranscription factor during infection of an animal host, we useda mouse model of Candida peritonitis as described previously(20–22). We observed that in this model hyphal formationwithin the host was impaired in the �tec1 mutant strains. Incontrast to the wild-type strains, which formed hyphae andinvaded into the liver, the �tec1 mutants attached to the liversurface but did not invade and were surrounded by inflamma-tory cells (Fig. 2, left panels). Despite the failure of the �tec1mutants to invade the liver, activation of the SAP5 promoterwas clearly detected in the cells remaining at the organ surface,although the average percentage of MPAS cells was reduced(20% compared with 43% in the wild-type controls) (Fig. 1B).These results demonstrate that SAP5 activation also occurredin the absence of the Tec1p transcription factor within aninfected host. No significant differences were observed whenthe activations of the wild-type SAP5 promoter and the mu-tated version with a deletion of the putative Tec1p bindingsites were compared in a wild-type background (43 and 36%MPAS cells, respectively) (Fig. 1B), demonstrating that theTec1p binding consensus sequences are also not essential foractivation of the SAP5 promoter within an infected host. Thelower level of detectable activation of the SAP5 promoter inthe �tec1 mutants may be due to Tec1p binding to sites otherthan the consensus sequence in C. albicans or to indirect acti-vation, as discussed above. However, since the �tec1 mutantsresided in host niches other than the wild-type strains, it seemspossible that stronger host signals, and not Tec1p-mediatedtranscriptional activation, account for the increased SAP5 in-duction seen in the wild-type strains compared with that in the�tec1 mutants.

To investigate whether the Tec1p-independent SAP5 activa-tion within the host was a peculiarity of the intraperitonealinfection model, we also analyzed the behavior of our reporterstrains in mice after intravenous infection. Histological exam-ination showed that, as previously observed (19), the �tec1mutants formed hyphae in infected kidneys which were indis-

FIG. 1. Activation of the wild-type (SAP5) and mutated (SAP5�3)SAP5 promoters in a wild-type background and of the wild-type SAP5promoter in a �tec1 mutant (SAP5�tec1) by serum in vitro (A) andafter intraperitoneal (B) or intravenous (C) infection of mice. Fungalcells were recovered from the liver at 24 h after intraperitoneal infec-tion and from the kidneys at 4 days after intravenous infection. Eachbar shows the percentage of MPAS cells in one infected animal. Thelight grey bars show the results obtained with strains S5FI2A, S5FI5A,and C6S5F1D, and the dark grey bars show the results obtained withstrains S5FI2B, S5FI5B, and C6S5F1E.

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tinguishable from those formed by wild-type strains, demon-strating that the defect in hyphal formation of a �tec1 mutantdepends on the host niche (Fig. 2, right panels). Importantly,deletion of the putative Tec1p binding sites had no effect onthe inducibility of the SAP5 promoter and SAP5 activationwas also observed in the absence of TEC1 in this infectionmodel (Fig. 1C). These results demonstrate that the roles ofthe Tec1p transcription factor in the induction of hypha-associated genes can differ in vitro and in vivo and that SAP5can be activated within the host by Tec1p-independent sig-naling mechanisms. Staib et al. have shown previously thatSAP5 activation during infection depends on the transcrip-tional regulators Cph1p and Efg1p (21). In addition, no SAP5mRNA was detected in vitro in �efg1 or �tec1 mutants (18, 19).Therefore, the in vitro expression of the SAP5 gene mightrequire Efg1p-dependent activation by the Tec1p transcriptionfactor, whereas SAP5 induction by host signals involves Tec1p-independent mechanisms, possibly including direct activationof the SAP5 promoter by Cph1p and Efg1p (10, 11). Theresults of our study clearly demonstrate that the importance ofspecific signaling pathways for the induction of C. albicansvirulence-associated genes depends on the environmental con-ditions encountered within the host during an infection.

This study was supported by the Deutsche Forschungsgemeinschaft(DFG grants MO 846/1-3 and KR 2002/1-1). Joachim Morschhauserwas the recipient of a Heisenberg fellowship from the Deutsche For-schungsgemeinschaft.

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Editor: T. R. Kozel

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