A single gene on the staphylococcal multiresistance plasmid pSK1 encodes a novel partitioning system.

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.