Silencing of Genes Flanking the P1 Plasmid Centromere

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Abstract
Partition modules stabilize bacterial plasmids and chromosomes by actively promoting their segregation into daughter cells. The partition module of plasmid P1 is typical and consists of a centromere site, parS , and genes that encode proteins ParA and ParB. We show that ParB can silence genes flanking parS (to which ParB binds), apparently by polymerizing along the DNA from a nucleation site at parS . Wild-type ParB contacts an extensive region of P1 DNA; silencing-defective ParB proteins, which were found to be partition-defective, are less able to spread. Hence, the silenced structure appears to function in partitioning.