Inhibition of a naturally occurring rolling-circle replicon in derivatives of the theta-replicating plasmid pIP501

The mechanisms ensuring regulation of DNA replication In genomes containing multiple replicons are poorly understood. In this report, we addressed this question by analysing in Bacillus subtilis the replication of a derivative of the promiscuous plasmid pIP501 that carries a rolling-circle and a theta replicon. Genetic analyses revealed that the rolling-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 transcription of the repR gene. Inhibition is, however, independent of DNA synthesis at the theta origin. We conclude that rolling circle inhibition is caused by an inhibitory signal encoded by the theta replicon and propose that the signal is composed, at least, of the RepR protein bound to its cognate origin.