Mei-Hui Lin and Shih-Tung Liu (2008) J. Bacteriol. 190: 3681-3689
Plasmids are selfish parasite? It must be partially true as long as plasmids confer no selective advantages on host bacteria. This article tells us one of such aspects of a small plasmid.
When host bacteria divide to produce daughter cells, plasmids are subjected to the risk of segregation from cells. To reduce the risk of segregational loss, most naturally occurring plasmids possess an active partitioning system (Funnell B. E. and Slavcev R. A., 2004; Graumann P.L. 2007) and/or post-segregational killing systems (VGP Blog Dec 28, 2008). If there was a plasmid that did not seem to have such systems, it could be just because we are not aware of the strategy of the tricky plasmid.
A plant pathogen Pantoea stewartii strain SW2 harbors more than 11 plasmids whose size are ranging from 4 kb to 320 kb. One of the plasmids pSW100 is a 4-kb ColE1-like plasmid that encodes two proteins related to DNA-transfer function. Despite that pSW100 does not seem to possess any genes related to the function to avoid segregational loss, pSW100 is stably maintained in the original host SW2 and E. coli strain HB101. Interestingly, pSW100 is unstable in another E. coli stain DH5α.
The question is why pSW100 is stable in HB101?.
The authors thus posit that pSW100 use a novel maintenance system, which is highly dependent on strain-specific factors. To find chromosomal factors responsible for the stable maintenance of pSW100, the authors created a mutant library of HB101 using transposon mutagenesis. From the mutant library, clones that became unable to support the stable maintenance of pSW100 were obtained. When transposon-insertion sites were sequenced, it turned out that most clones had an insertion of transposon in the sex pillus protein gene on the chromosome, which is considered to be remnant of F-plasmid. The authors also found that the 38-bp sequence of pSW100 was responsible for its stable maintenance in HB101, and they showed that the TraC protein provided from the F-plasmid remnant actually binds to the DNA segment containing the 38-bp sequence. This result leads us to speculate that pSW100 is stabilizing itself by attaching to the cytoplasmic component of the sex pillus of F-plasmid. The authors also showed that plasmid pSW1200 that coexists with pSW100 in strain SW2 carries traC homologue. This result suggests that pSW100 is parasitic on pSW100 in strain SW2.
This article raised novel and important ideas that (i) sex pillus components can stabilize plasmids by directly binding to plasmid DNA, and (ii) there could be other cases that remnants of plasmids on chromosomes positively affect the stability of other plasmids.
Personally, I am pretty surprised to know that E. coli HB101 has a remnant of F-plasmid on its chromosome because HB101 has been known for F-plasmid-free strain for decades!
Funnell B. E., and R. A. Slavcev. 2004. Partition Systems of Bacterial Plasmids in Plasmid Biology, ASM press, Washington D.C.
Graumann P. L. 2007. Cytoskeletal Elements in Bacteria. Annu. Rev. Microbiol. 61:589-618
Lin M.H. and S.T. Liu. 2008. Stabilization of pSW100 from Pantoea stewartii by the F conjugation system. J. Bacteriol. 190: 3681-3689
posted by H.Yano (University of Idaho)