The P1 plasmid-partition system synthesizes two essential proteins from an autoregulated operon.

The P1 partition region contains two large open reading frames that encode the proteins ParA and ParB. It was previously shown that ParA is essential for partition activity. Using a novel assay, we show that ParB protein is also an absolute requirement for partition and that it is active in trans to the partitioning plasmid. Development of complementation tests for parA and parB allowed us to assign a number of partition-defective point mutants of a P1 miniplasmid to the parA and parB cistrons. Using gene fusion techniques, it was shown that parA and parB constitute an operon controlled from a promoter proximal to the start of parA. Transcription from this promoter is autoregulated by a feedback loop that is sensitive to the ParA and ParB proteins in concert. The parB gene also appears to be expressed at a low level from a second promoter at the intercistronic boundary. This results in a low level of expression and tight autoregulation for the ParA protein and slightly less stringent control for ParB synthesis.