Mini-P1 plasmid partitioning: excess ParB protein destabilizes plasmids containing the centromere parS.

The partition system of the unit-copy plasmid P1 consists of two proteins, the parA and parB gene products, and a cis-acting site, parS. Production of high levels of the P1 ParB protein, from an external promoter on a high-copy-number vector, inhibits the propagation of lambda-mini-P1 prophages and destabilizes other P1-derived plasmids. The interference by ParB protein depends on the parS site, or centromere, of the P1 partition region; plasmids lacking parS are unaffected. The defect is more severe than the defect due to mutations that simply eliminate par function. In the presence of excess ParB protein, plasmids carrying parS are more unstable than would be predicted from a random distribution at cell division. The destabilization is a segregation defect, as the copy number of parS-bearing plasmids is not decreased under these conditions. Thus, it appears that ParB protein binds to parS; if too much protein is present, it sequesters such plasmids so they cannot be properly, or even randomly, partitioned. This suggests that under normal conditions, ParB protein recognizes and binds to parS and may be the protein responsible for pairing plasmids during the process of partitioning at cell division.