The single-stranded DNA-binding protein gene of plasmid Colib-P9.

The Incil plasmid Colib-P9 was found to carry a single-stranded DNA-binding protein gene (ssb), and the cloned gene was able to suppress the UV and temperature-sensitivity of an ssb-l strain of Escherichia coli K-12. Determination of the nucleotide sequence of Colib ssb demonstrated that the gene shows considerable homology to the ssb gene of plasmid F. In contrast, Southern hybridization techniques indicated that the IncP plasmid RP4 lacks a gene with any extensive homology to F ssb. It was shown that the direction of transfer of Colib-P9 is such that the Colib ssb gene, which lies approximately 11 kb from the origin of transfer, is located within the region transferred early during conjugation. The Colib and F ssb genes are therefore similarly located on their respective plasmids. The Colib ssb gene was shown to be coordinately expressed with the transfer (tra) genes, suggesting that the Colib SSB protein may participate in the conjugative process. However, a mutant Colibdrd-1 derivative carrying a Tn903-derived insertion in ssb showed no defect in tests of conjugative efficiency and was apparently maintained stably both following mating and during vegetative growth. Thus no biological role for the Colib SSB protein was detected. However, unlike the parental plasmid, the Colib ssb mutants conferred a marked Psi- (plasmid- mediated SOS inhibition) phenotype on recA441 and recA730 strains. This may result from high level expression of a psi gene due to readthrough from the Tn903 insertion. It is now apparent that many conjugative plasmids previously thought to be unrelated may be derived from a common ancestral plasmid which possessed both ssb and psi genes. It is speculated that the function of the SSB proteins of conjugative plasmids such as Colib and F may subsequently have been duplicated by analogues derived from newly aquired conjugation systems.