INHIBITION OF A NATURALLY-OCCURRING ROLLING-CIRCLE REPLICON IN DERIVATIVES OF THE THETA-REPLICATING PLASMID PLP501

The mechanisms ensuring regulation of DNA replication in genomes conta ining multiple replicons are poorly understood. In this report, we add ressed this question by analysing in Bacillus subtilis the replication of a derivative of the promiscuous plasmid pIP501 that carries a roll ing-circle and a theta replicon. Genetic analyses revealed that the ro lling-circle replicon is strongly inhibited in the derivative and that inhibition requires three elements involved in theta replication: the replication origin, the initiator RepR protein and strong transcripti on of the repR gene. Inhibition is, however, independent of DNA synthe sis at the theta origin. We conclude that rolling-circle inhibition is caused by an inhibitory signal encoded by the theta replicon and prop ose that the signal is composed, at least, of the RepR protein bound t o its cognate origin.