The inactive pT181 initiator heterodimer, RepC/C, binds but fails to induce melting of the plasmid replication origin.

Staphylococcus aureus plasmid pT181 replicates via a rolling circle mechanism. The synthesis of the pT181 initiator protein (RepC) is regulated by antisense RNAs, and RepC is inactivated after usage by the attachment of an oligonucleotide to one of its subunits. The inactivated heterodimeric RepC/C* has been shown be unable to initiate replication in vitro (Rasooly, A., and Novick, R. P. (1993) Science 262, 1048-1050). The inactive RepC/C* has been found to be very stable and constitute about 90-95% of the total RepC antigen inside the cell. We studied the specific interaction of the RepC/C and RepC/C* complex with the pT181 double strand origin. The results indicated that RepC/C and RepC/C* footprint supercoiled DNA differently although their footprints on linear DNA are similar; we also find that RepC/C is able to enhance cruciform extrusion while RepC/C* cannot. RepC/C* binds and bends the double strand origin much more weakly than does RepC/C. These results suggest that the attached oligonucleotide induces a conformational change in the RepC/C* molecule that is responsible for its lack of activity.