UvrD-dependent replication of rolling-circle plasmids in Escherichia coli

The Escherichia coli UvrD helicase (or helicase II) is known for its involvement in DNA repair. We report that UvrD is required for DNA replication of several different rolling-circle plasmids in E. coli, whereas its homologue, the Rep helicase, is not. Lack of UvrD helicase does not impair the first step of plasmid replication, nicking of the double-stranded origin by the plasmid initiator protein. However, replication proceeds no further without UvrD. Indeed, the nicked plasmid molecules accumulate to a high level in uvrD mutants. We conclude that UvrD is the replicative helicase of various rolling-circle plasmids. This is the first description of a direct implication of UvrD in DNA replication in vivo.