Heteroduplex analysis of P-plasmid evolution: the role of insertion and deletion of transposable elements.

DNA homology of thirteen R-plasmids of group P was examined by heteroduplex analysis and Southern blotting. Ten of these plasmids showed homology for extensive regions including all genes reported as necessary for replication and conjugational transfer. The differences between these plasmids could be explained by gain or loss of DNA sequences, many of which have been shown to be transposons. Of the other three plasmids, two showed unambiguous homology with the typical P-plasmids but this homology was imperfect, implying that these plasmids are products of lines which have evolved separately for long periods. One plasmid failed to produce heteroduplexes with the reference P plasmid.