The yeast Shu complex couples error-free post-replication repair to homologous recombination

DNA post-replication repair (PRR) functions to bypass replication-blocking lesions and prevent damage-induced cell death. PRR employs two different mechanisms to bypass damaged DNA. While translesion synthesis has been well characterized, little is known about the molecular events involved in error-free bypass, although it has been assumed that homologous recombination (HR) is required for such a mode of lesion bypass. We undertook a genome-wide synthetic genetic array screen for novel genes involved in error-free PRR and observed evidence of genetic interactions between error-free PRR and HR. Furthermore, this screen identified and assigned four genes, CSM2, PSY3, SHU1 and SHU2, whose products form a stable Shu complex, to the error-free PRR pathway. Previous studies have indicated that the Shu complex is required for efficient HR and that inactivation of any of these genes is able to suppress the severe phenotypes of top3 and sgs1. We confirmed and further extended some of the reported observations and demonstrated that error-free PRR mutations are also epistatic to sgs1. Based on the above analyses, we propose a model in which error-free PRR utilizes the Shu complex to recruit HR to facilitate template switching, followed by double-Holliday junction resolution by Sgs1-Top3. This mechanism appears to be conserved throughout eukaryotes.