Differential levels of fertility inhibition among F-like plasmids are related to the cellular concentration of finO mRNA.

The FinOP system of F-like plasmids consists of an antisense RNA (FinP) and a 22 kDa protein (FinO) which act in concert to prevent the translation of TraJ, the positive regulator of the transfer operon. Earlier studies suggested that two different variants of finO were responsible for differential levels of fertility inhibition among F-like plasmids. We have shown that these variations are due to the presence of an additional open reading frame (orf286) upstream of the finO gene of conjugative plasmids that are highly repressed for transfer. When orf286 and finO are linked in cis, the level of FinO expression is increased because of a rise in the cellular concentration of finO mRNA. orf286 frameshift and deletion mutants also gave the same concentration of finO transcript, suggesting that the effect is due to mRNA stabilization. We suggest that the levels of fertility inhibition exhibited by F-like plasmids are a function of their cellular FinO concentration.