Repression and derepression of conjugation of plasmid R1 by wild-type and mutated finP antisense RNA.

The finP gene of plasmid R1 is located between the genes traM and traJ, partially overlapping the first few nucleotides of the latter. It codes for a repressor of the conjugation system. The product of this gene is a small RNA of 72 nucleotides and, because it is transcribed from the opposite DNA strand, it is complementary to the 5' non-translated sequences, the ribosome-binding site, and the first two codons of traJ mRNA. The finP transcript is present in much higher concentrations in R1 than in R1-19 containing cells, the latter being a derepressed mutant of the former. A synthetic finP gene expressed from a synthetic lambda PL promoter markedly reduced the conjugation frequency of pDB12, a multicopy derivative of R1-19. Mutagenesis of finP showed that only finP loop II mutants have lost the ability to repress conjugation of R1-19 in trans. They are also the only ones which derepress conjugal DNA transfer of R1, probably by competing for the finO product, a molecule needed as corepressor for maximal activity. Mutations interrupting potential open reading frames of finP do not abolish finP repressor activity. Hence finP acts as an antisense RNA blocking the expression of the traJ gene by interacting with traJ mRNA through loop II.