SUMMARY: In most plasmids which have been studied to date the functions required for plasmid replication are clustered in a 2-3 kb region. However, in all known naturally occurring plasmids of the incompatibility group P the essential replication functions, the vegetative replication origin and which encodes proteins essential to activate are separated by blocks of DNA consisting of either known genes conferring resistance to antimicrobial agents and/or putative transposable elements. Nucleotide sequence comparisons reported here reveal that these blocks of DNA have inserted at different points into a backbone of DNA common to IncP plasmids. The results indicate that in the common ancestor of present IncP plasmids and must have been contiguous, whilst a ρ-independent transcriptional terminator, now lost in IncPα plasmids, may have prevented operon transcription from interfering with the activity of


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