ROLLING-CIRCLE REPLICATION OF BACTERIAL PLASMIDS

Many bacterial plasmids replicate by a rolling-circle (RC) mechanism. Their replication properties have many similarities to as well as sign ificant differences from those of single-stranded DNA (ssDNA) coliphag es which also replicate by an RC mechanism. Studies on a large number of RC plasmids have revealed that they fall into several families base d on homology in their initiator proteins and leading-strand origins. The leading-strand origins contain distinct sequences that are require d for binding and nicking by the Rep proteins. Leading-strand origins also contain domains that are required for the initiation and terminat ion of replication. RC plasmids generate ssDNA intermediates during re plication, since their leading-strand synthesis does not usually initi ate until the lending strand has been almost fully synthesized. The le nding-and lagging-strand origins are distinct, and the displaced leadi ng-strand DNA is converted to the double-stranded form by using solely the host proteins. The Rep proteins encoded by RC plasmids contain sp ecific domains that are involved in their origin binding and nicking a ctivities. The replication and copy number-of RC plasmids, in general, are regulated at the level of synthesis of their Rep proteins, which are usually rate limiting for replication. Some RC Rep proteins are kn own to be inactivated after supporting one round of replication. A num ber of in vitro replication systems have been developed for RC plasmid s and have provided insight into the mechanism of plasmid RC replicati on.