SUMMARY: Plasmids determining I pili show a variety of intricate compatibility relationships. None the less, all I plasmids tested showed a ‘core’ of common DNA sequences of approximately 30 x 106 daltons. Plasmids of the O compatibility group do not appear to specify for the synthesis of I-like pili and yet they held about 20 % of their sequences in common with all tested I plasmids, suggesting a common phylogenetic origin. Members of the I complex (including group O) do not hold a significant proportion of their sequences in common with representative plasmids of other compatibility groups. We propose that the regions of similar polynucleotide sequences of the different I complex plasmids are due to phylogenetic homology between genes governing pili biosynthesis and/or transfer functions. The regions determining compatibility of the I complex plasmids in some cases may be either unrelated to or have diverged from a common ancestor.


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