MOSAIC STRUCTURE OF PLASMIDS FROM NATURAL-POPULATIONS OF ESCHERICHIA-COLI

The distribution of plasmids related to the fertility factor F was exa mined in the ECOR reference collection of Escherichia coli. Probes spe cific for four F-related genes were isolated and used to survey the co llection by DNA hybridization. To estimate the genetic diversity of ge nes in F-like plasmids, DNA sequences were obtained for four plasmid g enes. The phylogenetic relationships among the plasmids in the ECOR st rains is very different from that of the strains themselves. This find ing supports the view that plasmid transfer has been frequent within a nd between the major groups of ECOR. Furthermore, the sequences indica te that recombination between genes in plasmids takes place at a consi derably higher frequency than that observed for chromosomal genes. The plasmid genes, and by inference the plasmids themselves, are mosaic i n structure with different regions acquired from different sources. Co mparison of gene sequences from a variety of naturally occurring plasm ids suggested a plausible donor of some of the recombinant regions as well as implicating a chi site in the mechanism of genetic exchange. T he relatively high rate of recombination in F-plasmid genes suggests t hat conjugational gene transfer may play a greater role in bacterial p opulation structure than previously appreciated.