Regulation of the F-factor pif operon: pifO, a site required in cis for autoregulation, titrates the pifC product in trans.

F factor pifC, pifA, and pifB gene expression is subject to negative regulation by the product of the pifC locus (J.F. Miller and M. H. Malamy, J. Bacteriol. 156:338-347, 1983). In this paper, we describe the properties of a new regulatory site in the pif region, pifO, which is required in cis for autoregulation of pif gene expression. Spontaneous pifO mutations were isolated that allow expression of a pifC-lacZ protein fusion in the presence of pifC product in trans. Recombination of these pifO mutations onto F'lac results in increased pifA and pifB activity. Thus, a single regulatory element, pifO, regulates pifC, pifA, and pifB expression in cis. The presence of multiple copies of a fragment from the pif region carrying wild-type pifO sequences (F coordinates 42.9 to 42.43 kilobases) in trans to F'lac results in an increase in pifA and pifB activity as measured by inhibition of T7 plating. When the pifO mutations are recombined onto a plasmid carrying pifO, the resulting recombinants are greatly decreased in the ability to increase F'lac pif expression. These results suggest that increased F'lac pifA and pifB expression caused by pifO sequences in trans is a consequence of titration of pifC product and derepression of the pif operon.