autonomous linear dna clock richard j crossland. purpose internal count-down timer to any cellular...
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AUTONOMOUS LINEAR DNA CLOCK
Richard J Crossland
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Purpose
Internal count-down timer to any cellular event
Deployment of function at target site or at correct time
Internal control mechanism to prevent GM organisms evolving (does not require external signal)
Extra-chromosomal – can contain all GM genes
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dna replication
dna replication
repressor gene
kill switch
cell death
repression
telomere shortening
Mechanism – no telomerase
no repression
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Identified problems
Telomerase repair mechanism Replication machinery Repressor protein and cell death Getting linear DNA into cells other than
Streptomyces, Borellia. Horizontal gene transfer
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Telomerase repair mechanism
Two processes: 1. linear plasmid replication 2. patching gaps/ TIRS (terminal inverted repeats)
Disarm step 2
(Casjens, 1999)
280 nt
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Replication machinery -- given Bi-directional linear replication from
central internal origin Streptomyces, Borellia Also in: Yersinia enterocolitica, E.coli,
Klebsiella oxytoca, Salmonella Typhi. Either already on linear plasmid – viral
RNA polymerase, viral DNA polymerase Or see case study – it works!
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Repressor protein and cell death Tap into existing functionality, new sub-
project loads of possible mechanisms to choose from
nuclease mazEF stress-induced toxin-antitoxin suicide
module (E.coli) skf and sdp operons in Bacillus Subtilis -
nutrient limitation (Engleberg-Kulka et al. 2006) B.S – go into sporulation and don’t germinate
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Case study: Baker et al. 2007
S.Typi pBSSB2: 27kpb linear plasmid, 33 coding sequences Contains z66 flagellin antigen (flijBz66 gene)
pBSSB1 + kanamycin resistance cassette (1,432-bp) pBSSB2.
pBSSB2 E.coli SGB33
Plasmid isolated (alkaline lysis method), sequenced, and shown to be capable of autonomous, existence and stability in E.coli.
E.coli expressed antibiotic resistance. Although antigen could not be detected (interaction with flagella regulation
machinery) when retransformed E.coli SGB33 plasmid into S.Typhi, it was stably maintained and z66 antigen was dominantly expressed.
lambda red recombinase
electro-transformation
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cont ...
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Horizontal gene transfer
All GM genes on the linear plasmid Transferred to non-GM bacteria --> it
becomes GM and dies (death plasmid) Loss of GM linear DNA from GM bacteria
--> it is no longer GM, so no problem.
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Conclusion
1. Linear DNA exists, 2. It is stable, is expressed and replicated
when transformed into other bacteria ( E.coli) 3. It is large enough to contain all GM genes 4. What we need to achieve:
create a linear dna molecule with two genes (a repressor and a cell death protein)
knock-out the telomere patching mechanism Transform into B.Subtilis