Control of spontaneous and damage-induced mutagenesis by SUMO and ubiquitin conjugation

Protein modification by ubiquitin is emerging as a signal for various biological processes in eukaryotes, including regulated proteolysis, but also for non-degradative functions such as protein localization, DNA repair and regulation of chromatin structure1,2,3,4. A small ubiquitin-related modifier (SUMO) uses a similar conjugation system that sometimes counteracts the effects of ubiquitination5. Ubiquitin and SUMO compete for modification of proliferating cell nuclear antigen (PCNA), an essential processivity factor for DNA replication and repair6. Whereas multi-ubiquitination is mediated by components of the RAD6 pathway and promotes error-free repair, SUMO modification is associated with replication6,7,8,9. Here we show that RAD6-mediated mono-ubiquitination of PCNA activates translesion DNA synthesis by the damage-tolerant polymerases η and ζ in yeast. Moreover, polymerase ζ is differentially affected by mono-ubiquitin and SUMO modification of PCNA. Whereas ubiquitination is required for damage-induced mutagenesis, both SUMO and mono-ubiquitin contribute to spontaneous mutagenesis in the absence of DNA damage. Our findings assign a function to SUMO during S phase and demonstrate how ubiquitin and SUMO, by regulating the accuracy of replication and repair, contribute to overall genomic stability.