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

The Escherichia coil UvrD helicase (or helicase II) is known for its involv ement in DNA repair. We report that UvrD is required for DNA replication of several different rolling-circle plasmids in E. coil, whereas its homologu e, the Rep helicase, is not. Lack of UvrD helicase does not impair the firs t step of plasmid replication, nicking of the double-stranded origin by the plasmid initiator protein. However, replication proceeds no further withou t 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 implicat ion of UvrD in DNA replication in vivo.