Structural and genetic analyses of a par locus that regulates plasmid partition in Bacillus subtilis

The Bacillus plasmid pLS11 partitions faithfully during cell division. Using a partition-deficient plasmid vector, we randomly cloned DNA fragments of plasmid pLS11 and identified the locus that regulates plasmid partition (par) by cis complementation in Bacillus subtilis. The cloned par gene conferred upon the vector plasmid a high degree of segregational stability. The par locus was mapped to a 167-base-pair segment on pLS11, and its nucleotide sequence was determined. The cloned par fragment regulated the partition of several different Bacillus replicons, and it only functioned in cis; it did not contain the replication function nor elevate the plasmid copy number in B. subtilis. The expression of par was orientation specific with respect to the replication origin on the same plasmid. We propose that the pLS11-derived par functions as a single-stranded site that interacts with other components involved in plasmid partition during cell division.