Identification and characterization of the dif site from Bacillus subtilis

Bacteria with circular chromosomes have evolved systems that ensure multime ric chromosomes, formed by homologous recombination between sister chromoso mes during DNA replication, are resolved to monomers prior to cell division . The chromosome dimer resolution profess in Escherichia coli is mediated b y two tyrosine family site-specific recombinases, XerC and XerD, and requir es septal localization of the division protein FtsK. The Xer recombinases a ct near the terminus of chromosome replication at a site known as dif (Ecdi f). In Bacillus subtilis the RipX and CodV site-specific recombinases have been implicated in an analogous reaction. We present here genetic and bioch emical evidence that a 28-bp sequence of DNA (Bsdif), lying 6 degrees count er-clockwise;ise from the B. subtilis terminus of replication (172 degrees) , is the site at which RipX and CodV catalyze site-specific recombination r eactions required for normal chromosome partitioning. Bsdif in vivo recombi nation did not require the B. subtilis FtsK homologues, SpoIIIE and YtpT, W e also show that the presence or absence of the B. subtilis SPP-bacteriopha ge, and in particular its yopP gene product, appears to strongly modulate t he extent of the partitioning defects seen in codV strains and, to a lesser extent, those seen in ripX and dif strains.