Two classes of Flac mutants insensitive to transfer inhibition by an F-like R factor

Summary Mutants of Flac episomes whose transfer is no longer inhibited by the fi⁺ R factor R100 are shown to be of at least two types. One is recessive in transient heterozygotes containing Fhis and R100 in addition to the Flac mutant. The other is dominant. The occurrence of recessive mutants suggests that inhibition of F transfer by R100 requires an F-specified gene product in addition to that produced by the R factor. Synthesis of the F product or its interaction with the R100 product to give the true inhibitor seems to be a slow process. Since the inhibitor and mutations of the transfer gene traJ both affect a plasmid-specific transfer product, F-pilus formation, and surface exclusion, we propose that the inhibitor prevents the synthesis or function of the traJ product.Supported by a George Murray Scholarship from the University of Adelaide, Australia.