Genetic and Physical Studies on the Replication of ColE1-type Plasmids

The mechanisms of replication of extrachromosomal elements uch as plasmid ColE1 are determined by both host-specific and replicon-specific components. Replication of ColE1 differs from that of many other prokaryotic replicons in several ways: it is unidirec-tional (Inselburg 1974; Lovett et al. 1974), requires the polymerizing activity of DNA polymerase I (Kings-bury and Helinski 1973), proceeds in the absence of protein synthesis (Clewell 1972), and maintains a high number of genomes per cell (Clewell and Helinski 1972). There is evidence that no ColEl-encoded polypeptide is required for its replication (Donoghue and Sharp 1978; Kahn and Helinski 1978). We are investigating the replication of plasmids derived from pMB1, a plasmid structurally and functionally very similar to ColE1 (Betlach et al. 1976). We have identified the plasmid-specified genetic information of pMB1 which is required for its own replication, and we have constructed in vitro site-specific alterations of that genetic information. Examination of the effects of these alterations on replication, together with observa-tions in vivo of a unique replication aberration, leads us to propose a model for ColEl-type replication.