Mechanical tuning of replication stress tolerance and genomic stability through the checkpoint mediator Mrc1

Mrc1 is a replication fork component that mediates communication between the replication checkpoint and fork progression. Here, we find that Mrc1 forms a dynamic mechanical linkage between CMG helicase and DNA Polymerase ε that balances fork stability and flexibility. In its coupled state, Mrc1 bridges CMG helicase and the Pol2 C-terminal domain of Polymerase ε to maintain helicase–polymerase coordination, promoting tolerance to replication stress but constraining fork remodeling. Upon checkpoint activation, Mec1/Rad53-dependent phosphorylation of Mrc1 shifts this equilibrium toward a loosened state, reducing coupling and enabling fork flexibility and lower mutation rate at the cost of reduced stress resistance.