Autogenous regulation of synthesis of the replication protein in plasmid pSC101.

A 1.3-kb segment of plasmid pSC101 includes the replication origin (ori) and the gene (rep) encoding the 37 kilodalton (K) protein required for autonomous replication of the plasmid. The present work describes the regulation of the rep gene expression. The gamma promoters PR and PL fail to promote rep gene expression when located upstream of a sequence with dyad symmetry overlapping the rep promoter, whereas elimination of this sequence allows expression and results in over-production of the rep protein. When expression of trpA'-lacZ is controlled under the rep promoter, beta-galactosidase is produced without the lac inducer. However, this enzyme synthesis is effectively reduced when the complete rep sequence is provided in trans. A partial disruption of the sequence with dyad symmetry relieves the repression. These results suggest that expression of the rep gene is negatively regulated by its own product and that the sequence with dyad symmetry plays the role of a receptor site for the rep protein.