The F plasmid of Escherichia coli allows horizontal DNA transfer between an F+ donor cell and an F− recipient. Expression of the transfer genes is tightly controlled by a number of factors, including the following plasmid-encoded regulatory proteins: TraJ, the primary activator of the 33-kb tra operon, and the autoregulators TraM and TraY. Here, we demonstrate that the host RNA binding protein, Hfq, represses TraJ and TraM synthesis by destabilizing their respective mRNAs. Mating assays and immunoblot analyses for TraM and TraJ showed that transfer efficiency and protein levels increased in host cells containing a disruption in hfq compared to wild-type cells in stationary phase. The stability of transcripts containing a putative Hfq binding site located in the intergenic untranslated region between traM and traJ was increased in hfq mutant donor cells, suggesting that Hfq destabilizes these transcripts. Electrophoretic mobility shift assays demonstrated that Hfq specifically binds this region but not the antisense RNA, FinP, encoded on the opposite strand. Together, these findings indicate that Hfq regulates traM and traJ transcript stability by a mechanism separate from FinOP-mediated repression.
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