A mechanism for Rad53 to couple leading- and lagging-strand DNA synthesis under replication stress in budding yeast.

In response to DNA replication stress, DNA replication checkpoint kinase Mec1 phosphorylates Mrc1, which in turn activates Rad53 to prevent the generation of deleterious single-stranded DNA, a process that remains poorly understood. We previously reported that lagging-strand DNA synthesis proceeds farther than leading strand in mutant cells defective in replication checkpoint under replication stress, resulting in the exposure of long stretches of the leading-strand templates. Here, we show that asymmetric DNA synthesis is also observed in and cells defective in replication checkpoint but, surprisingly, not in cells in which both DNA replication and checkpoint functions of Mrc1 are missing. Furthermore, depletion of either Mrc1 or its partner, Tof1, suppresses the asymmetric DNA synthesis in mutant cells. Thus, the DNA replication checkpoint pathway couples leading- and lagging-strand DNA synthesis by attenuating the replication function of Mrc1-Tof1 under replication stress.