The Saccharomyces Pif1p DNA helicase and the highly related Rrm3p have opposite effects on replication fork progression in ribosomal DNA

Replication of Saccharomyces ribosomal DNA (rDNA) proceeds bidirectionally from origins in a subset of the similar to 150 tandem repeats, but the left ward-moving fork stops when it encounters the replication fork barrier (RFB ). The Pif1p helicase and the highly related Rrm3p were rDNA associated in vivo. Both proteins affected rDNA replication but had opposing effects on f ork progression. Pif1p helped maintain the RFB. Rrm8p appears to be the rep licative helicase for rDNA as it acted catalytically to promote fork progre ssion throughout the rDNA. Loss of Rrm3p increased rDNA breakage and accumu lation of rDNA circles, whereas breakage and circles were less common in pi f1 cells. These data support a model in which replication fork pausing caus es breakage and recombination in the rDNA.