A 4TH CLASS OF THETA-REPLICATING PLASMIDS - THE PAM-BETA-1 FAMILY FROM GRAM-POSITIVE BACTERIA

Plasmid pAMbeta1 from Enterococcus faecalis uses a unidirectional theta mode of replication. We show here that this replication (i) is dependent on a plasmid-encoded replication protein (Rep) but not on a DNA structure typical for origins of most Rep-dependent plasmids and (ii) is initiated by DNA polymerase I (PolI). pAMbeta1 minimal replicon shares no homology with highly conserved ColE1-type replicons, which use Poll for initiation but do not encode a Rep, or with ColE2 and ColE3 replicons, which require PolI for replication and encode a Rep. We propose that pAMbeta1 and a number of other naturally occurring and closely related plasmids form a distinct plasmid class.