The F plasmid centromere, sopC, is required for full repression of the sopAB operon

The SopB protein of the F plasmid has a dual role in the partition of F pla smid copies to daughter cells prior to division. It binds to the sopC centr omere site to form the partition complex needed for stabilizing the plasmid , and it interacts with SopA to repress transcription of the sopAB operon, thus preventing the destabilization that results from excess SopB. We have isolated sop mutants by screening for unstable inheritance of mutagenized m ini-F DNA. Four of the mutants resulted from different missense mutations i n sopB. All four were deficient, to varying degrees, in autoregulation of S op protein synthesis. The mutant proteins showed diminished capacity for re ducing the linking number of mini-F and for destabilizing a plasmid carryin g sopC, indicating that reduced ability to form a normal complex with sopC might underlie the autoregulation defect. Repression of the transcription o f a sop promoter-lacZ fusion by SopA and SopB was strongly enhanced in the presence of sopC, in cis or in trans, and the enhancement was reduced or nu llified when wild-type sopB was replaced by the mutant sopB alleles. A sing le 43 bp unit of sopC was almost as effective as sopC itself in enhancing r epression. The results show that sopC is necessary for full repression of t he sop promoter. They thus indicate a previously unsuspected role for this centromere site, and suggest that autoregulation and partition might normal ly be coordinated processes. (C) 1999 Academic Press.