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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 Escherichia coli incompatibility group P the essential replication functions, oriV, the vegetative replication origin and trfA, which encodes proteins essential to activate oriV, 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 oriV and trfA must have been contiguous, whilst a rho-independent transcriptional terminator, now lost in IncP alpha plasmids, may have prevented trfA operon transcription from interfering with the activity of oriV.

journal of general microbiology/journal of general microbiology

The ancestral IncP replication system consisted of contiguous oriV and trfA segments as deduced from a comparison of the nucleotide sequences of diverse IncP plasmids