Rolling circle-replicating plasmids from gram-positive and gram-negative bacteria: a wall falls.

Rolling circle-replicating plasmids constitute a group of small, promiscuous multicopy replicons spread among eubacteria. Until recently, rolling circle replication seemed to be limited to small plasmids from Gram-positive hosts and to single-stranded bacteriophages from Gram-negative bacteria. However, characterization of two small plasmids from Gram-negative hosts has shown that this replication mechanism is general among eubacteria. This review focuses on a family of highly related promiscuous plasmids that replicate by the rolling circle mechanism, and that have been isolated from various Gram-positive bacteria and from the Gram-negative bacterium Helicobacter. They all share homologies at the leading-strand origins and at the initiator of replication proteins. The plasmids of this family have directly repeated sequences at their plus origin of replication, which is located 5' from the start point of the mRNA for the initiation of replication protein. Replication is controlled by an antisense RNA and by a transcriptional repressor protein. The features and regulatory circuits of replication of this plasmid family seem to be unique among rolling circle-replicating plasmids. Members of this family replicate autonomously in Gram-positive and -negative hosts.