finP and fisO mutations in FinP anti-sense RNA suggest a model for FinOP action in the repression of bacterial conjugation by the Flac plasmid JCFL0.

Expression of the transfer operon in the F plasmids is negatively regulated by FinOP which has two components, the finP and finO gene products. Mutations in either gene result in increased expression of the positive regulator of transcription, traJ, leading to derepressed levels of conjugal transfer. Five mutations in the finP gene have been previously characterised by Finnegan and Willetts (1971). Three were complementable in trans and were named finP mutations and two were complementable at low levels (fisO) presumably because they affected the site of action of the finO gene product. In this study, DNA sequence analysis revealed three different mutations shared by the five mutants which were located in the stems of the predicted stem-and-loop structures in the finP anti-sense RNA. The properties of three mutants created by site-specific mutagenesis suggested that the stability of the stem structure was important in FinP action and that a small region in one of the stems appears to be the target of the finO gene product. Analysis of wild-type and fisO FinP RNA showed that FinO increased the amount of an 80 nuceotide FinP RNA, probably by stabilizing this transcript or preventing its degradation. The fisO mutation decreased the amount of 80 nucleotide RNA substantially. FinP transcripts from either the finP promoter of the lac promoter appeared to be stabilized by FinO.