Homologous recombination and Mus81 promote replication completion in response to replication fork blockage

Impediments toDNAreplication threaten genome stability. The homologous recombination (HR) pathway has been involved in the restart of blocked replication forks. Here, we used a method to increase yeast cell permeability in order to study at the molecular level the fate of replication forks blocked byDNAtopoisomerase I poisoning by camptothecin (CPT). Our results indicate that Rad52 and Rad51HRfactors are required to completeDNAreplication in response toCPT. Recombination events occurring during S phase do not generally lead to the restart ofDNAsynthesis but rather protect blocked forks until they merge with convergent forks. This fusion generates structures requiring their resolution by the Mus81 endonuclease in G(2)/M. At the global genome level, the multiplicity of replication origins in eukaryotic genomes and the fork protection mechanism provided byHRappear therefore to be essential to completeDNAreplication in response to fork blockage.