A Single Gene on the Staphylococcal Multiresistance Plasmid pSK1 Encodes a Novel Partitioning System

Abstract

ABSTRACT The orf245 gene is located immediately upstream of, and divergently transcribed from, the replication initiation gene, rep , of the Staphylococcus aureus multiresistance plasmid pSK1, and related genes have been found in association with a range of evolutionarily distinct replication genes on plasmids from various gram-positive genera. orf245 has been shown previously to extend the segregational stability of a pSK1 minireplicon. Here we describe an investigation into the basis of orf245 -mediated stabilization. orf245 was not found to influence transcription of pSK1 rep , indicating that it is not directly involved in plasmid replication. This was confirmed by demonstrating that orf245 is able to enhance the segregational stability of heterologous theta- and rolling-circle-replicating replicons, suggesting that it encodes a plasmid maintenance function. Evidence inconsistent with postsegregational killing and multimer resolution mechanisms was obtained; however, the intergenic region upstream of orf245 was found to mediate orf245 -dependent incompatibility, as would be expected if it encodes a cis -acting centromere-like site. Taken together, these findings implicate active partitioning as the probable basis of the activity of orf245 , which is therefore redesignated par . Since it is unrelated to any gene known to play a role in plasmid segregation, it seems likely that pSK1 par potentially represents the prototype of a novel class of active partitioning systems that are distinguished by their capacity to enhance plasmid segregational stability via a single protein-encoding gene.