Abstract

To ensure duplication of the whole genome, DNA replication initiates from thousands of replication origins. The replication forks move through the chromatin until they encounter forks from the neighbouring origins. During the termination of replication forks the replisomes disassemble by an unknown mechanism and topisomerase II resolves the daughter DNA molecules. If not resolved properly, the terminating forks are at high risk of stalling and fork reversal, leading to DNA damage and genomic instability. Using the X.laevis egg extract system, we have shown that blocking polyubiquitylation results in the prolonged association of the active helicase with replicating chromatin and replication termination defect. It has lead us to propose that it is the disassembly of the active helicase at the termination of replication forks that is defective upon polyubiquitylation inhibiton. The Mcm7 protein is the only component of the active helicase that we find polyubiquitylated during S-phase and this ubiquitylation is blocked when forks cannot terminate. Once ubiquitylated, the disassembly of the helicase is dependent on p97/VCP/Cdc48 segregase. Altogether, our data provides first insight into the mechanism of replisome disassembly during eukaryotic DNA replication termination and sheds light onto the way the terminating replication forks are resolved efficiently to maintain a stable genome.