Isolation of a derivative of Escherichia coli-Enterococcus faecalis shuttle vector pAM401 temperature sensitive for maintenance in E. faecalis and its use in evaluating the mechanism of pAD1 par-dependent plasmid stabilization.

A derivative of the Escherichia coli-Enterococcus faecalis shuttle vector pAM401 was isolated by mutagenesis in an E. coli mutator strain. This plasmid, designated pAM401ts, was more than an order of magnitude less stable at 38 degreesC than at 30 degreesC in the E. faecalis host strain JH2-2. The E. faecalis plasmid pAD1-encoded par stability locus was cloned onto pAM401ts, and its effects on plasmid stability and host cell viability were assessed. It was found that par stabilized pAM401ts at 38 degreesC but also caused a substantial drop in cell viability three to four generations after a temperature shift from 30 to 38 degreesC. After a maximum viability drop of 94%, culture growth recovered as plasmid-free cells began to accumulate. Provision of excess RNAII, the putative par antidote, in trans attenuated cell killing. These characteristics support a postsegregational killing mechanism for par-mediated plasmid stabilization.