Bacterial two-hybrid studies of randomly cloned Escherichia coli DNA identified a physical interaction between GyrA, subunit A of gyrase, and MarR, a repressor of the marRAB operon. GyrA-His immobilized on Ni-nitrilotriacetic acid (NiNTA) resin bound MarR, while MarR alone did not bind. GyrA interfered with MarR binding to marO, as detected by electrophoretic mobility assays. In a strain bearing the marRAB operon and a marO-lacZ reporter, overexpression of GyrA increased LacZ activity, indicating decreased repression of marO-lacZ by MarR. These results were confirmed by an increased survival of cells treated with quinolones and other antibiotics when GyrA was overexpressed. This work, like a previous study examining TktA (12), shows that unrelated proteins can regulate MarR activity. The findings reveal an unexpected regulatory function of GyrA in antibiotic resistance.