Lagging-Strand DNA Replication Origins Are Required for Conjugal Transfer of the Promiscuous Plasmid pMV158

Abstract

The promiscuous streptococcal plasmid pMV158 is mobilizable by auxiliary plasmids and replicates by the rolling-circle mechanism in a variety of bacterial hosts. The plasmid has two lagging-strand origins, ssoA and ssoU , involved in the conversion of single-stranded DNA intermediates into double-stranded plasmid DNA during vegetative replication. Transfer of the plasmid also would involve conversion of single-stranded DNA molecules into double-stranded plasmid forms in the recipient cells by conjugative replication. To test whether lagging-strand origins played a role in horizontal transfer, pMV158 derivatives defective in one or in both sso 's were constructed and tested for their ability to colonize new hosts by means of intra- and interspecies mobilization. Whereas either sso supported transfer between strains of Streptococcus pneumoniae , only plasmids that had an intact ssoU could be efficiently mobilized from S. pneumoniae to Enterococcus faecalis . Thus, it appears that ssoU is a critical factor for pMV158 promiscuity and that the presence of a functional sso plays an essential role in plasmid transfer.