The conjugation regions of IncF plasmids are closely related in that they share extensive DNA homology, and that they specify related pili. Variations between individual conjugation gene products of different IncF plasmids have, however, been noted. We have extended these observations by carrying out a systematic survey of twelve such plasmids, to examine the numbers and the groupings of the plasmid-specific alleles of several genes required for conjugation and its control.
Using vector plasmids carrying cloned origins of transfer (oriT), four different specificities were recognized, and these were correlated with the specificities of the genes with products that may act at this site (traM, traY and traZ). The traY gene is the first gene of the major transfer operon, and is therefore located close to the site at which the traJ protein acts to induce expression of the operon: correspondingly, correlation was observed between the oriT/traMYZ and traJ specificities in most of the plasmids. In turn, traJ is negatively regulated by the finO and finP products acting in concert: the finO product was relatively non-specific, but six finP alleles were identified, again with specificities correlated with those of traJ. Our explanation for this unexpectedly large number of finP alleles derives from the concept that the finP product is an RNA molecule rather than a protein. Although the conjugative pili encoded by IncF plasmids are closely related, they confer different efficiencies of plating of the various F-specific bacteriophages. We distinguished four groups on this basis, presumably resulting from differences in the primary amino-acid sequences of the pilin proteins. These groups could be related to the surface exclusion system specificities, consistent with the hypothesis that surface exclusion acts at least in part by preventing interaction between the pilus and the recipient cell surface.
From these data, information about the evolutionary relationships between the twelve IncF plasmids can be deduced.