ROLLING CIRCLE-REPLICATING PLASMIDS FROM GRAM-POSITIVE AND GRAM-NEGATIVE BACTERIA - A WALL FALLS

Rolling circle-replicating plasmids constitute a group of small, promi scuous multicopy replicons spread among eubacteria. Until recently, ro lling circle replication seemed to be limited to small plasmids from G ram-positive hosts and to single-stranded bacteriophages from Gram-neg ative bacteria. However, characterization of two small plasmids from G ram-negative hosts has shown that this replication mechanism is genera l among eubacteria. This review focuses on a family of highly related promiscuous plasmids that replicate by the rolling circle mechanism, a nd that have been isolated from various Gram-positive bacteria and fro m the Gram-negative bacterium Helicobacter. They all share homologies at the leading-strand origins and at the initiator of replication prot eins. The plasmids of this family have directly repeated sequences at their plus origin of replication, which is located 5' from the start p oint of the MRNA for the initiation of replication protein. Replicatio n 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 plas mids. Members of this family replicate autonomously in Gram-positive a nd -negative hosts.