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.