Effects of the P1 Plasmid Centromere on Expression of P1 Partition Genes

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Abstract
The partition operon of P1 plasmid encodes two proteins, ParA and ParB, required for the faithful segregation of plasmid copies to daughter cells. The operon is followed by a centromere analog,parS, at which ParB binds. ParA, a weak ATPase, represses theparpromoter most effectively in its ADP-bound form. ParB can recruit ParA toparS, stimulate its ATPase, and significantly stimulate the repression. We report here thatparSalso participates in the regulation of expression of thepargenes. A single chromosomalparSwas shown to augment repression of several copies of theparpromoter by severalfold. The repression increase was sensitive to the levels of ParA and ParB and to their ratio. The increase may be attributable to a conformational change in ParA mediated by theparS-ParB complex, possibly acting catalytically. We also observed an inciseffect ofparSwhich enhanced expression ofparB, presumably due to a selective modulation of the mRNA level. Although ParB had been earlier found to spread into and silence genes flankingparS, silencing of theparoperon by ParB spreading was not significant. Based upon analogies between partitioning and septum placement, we speculate that the regulatory switch controlled by theparS-ParB complex might be essential for partitioning itself.

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