Interactions between the F conjugal transfer system and CloDF13::Tna plasmids.

It was confirmed that all the F transfer genes required for the formation of stable mating pairs, including traN and traG, are essential for transfer of the two small multicopy plasmids ColE1 and cloDF13, whereas the traM, traI and traZ genes that are required for F conjugal DNA metabolism, are not. Differences between ColE1 and CloDF13 were that the F traD gene was needed for transfer of ColE1 but not of CloDF13, and that R100-1 efficiently transferred CloDF13 but not ColE1. A copy number mutant of CloDF13 inhibited F transfer and reduced plaque formation by the F-specific RNA phage f2, but not by Q beta or by the single-strand DNA phage f1. This phenotype suggests that the inhibition system (FinC) acts on traD, mutations in which give a similar phenotype. Hybridisation experiments with lambda tra phage DNA showed that transcription of traD was not reduced, and FinC probably inhibits function of the traD product rather than its synthesis. FinC-insensitive Flac mutants were isolated and characterised. One was shown to have an uppromoter mutation resulting in increased transcription of traJ and hence of the traY leads to Z operon including traD: the raised level of the traD product presumably then counteracted FinC inhibition.