Characterization of pES213, a small mobilizable plasmid

Most Vibrio Wscheri strains isolated from the Euprymna scolopes light organ carry plasmids, often including both a large (>40 kb) plasmid, and one or more small (<12 kb) plasmids. The large plasmids share homology with pES100, which is the lone plasmid in V. Wscheri type strain ES114. pES100 appears to encode a conjugative system similar to that on plasmid R721. The small plasmids lack extensive similarity to pES100, but they almost always occur in cells that also harbor a large plasmid resembling pES100. We found that many or all of these small plasmids share homology with pES213, a plasmid in strain ES213. We determined the 5501-bp pES213 sequence and generated selectable antibiotic resistance encoding pES213 derivatives, which enabled us to examine replication, retention, and transfer in V. Wscheri. An 863-bp fragment of pES213 with features characteristic of -type replicons conferred replication without requiring any pES213 open reading frame (ORF). We estimated that pES213 derivatives were maintained at 9.4 copies per genome, which corresponds well with a model of random plasmid segregation to daughter cells and the »10¡4 per gen-eration frequency of plasmid loss. pES213 derivatives mobilized between V. Wscheri strains at frequencies up to »10¡4 in culture and in the host, apparently by employing the pES100 conjugative apparatus. pES213 carries two homologs of the putative pES100 origin of transfer (oriT), and V. Wscheri strains lacking the pES100 conjugative relaxase, including a relaxase mutant, failed to serve as donors for transmission of pES213 derivatives. In other systems, genes directing con