Partitioning of plasmid R1. Ten direct repeats flanking the parA promoter constitute a centromere-like partition site parC, that expresses incompatibility.

The parA partitioning system of plasmid R1 consists of three different components: the cis-acting centromere-like parC site, and the two trans-acting proteins ParM and ParR. These three components are contained within a region of 1.6 kb. The parC site is located upstream of the two genes, parM and parR, which are expressed as an operon from the parA promoter. The parC site contains an array of ten 11 base-pair direct repeats, organized in two sets of five repeats flanking the parA core promoter sequences. Deletions and point mutations were introduced in the parA locus, resulting in partially stable and unstable plasmids. An analysis of these parA- plasmids showed that ParM and ParR are transacting. The 160 bp minimal parC region contained sufficient in cis information for efficient trans-complementation. Both proteins were required for maximal stabilization of a parC+ mini-R1 plasmid, although ParR alone, donated either in cis or in trans, yielded partial stabilization. Plasmids that overexpressed ParR caused destabilization of a co-resident parA+ plasmid, whereas overexpression of ParM had no such effect. The parC site exerted incompatibility (incA) at high but not at low copy number. Likewise, the entire parA system exerted incompatibility in a copy number-dependent fashion, and stronger than the incompatibility expressed by parC alone.