error-free component of the RAD6/RAD18 DNA damage tolerance pathway of budding yeast employs sister-strand recombination

Evidence for an error-free DNA damage tolerance process in eukaryotes (also called postreplication repair) has existed for more than two decades, but its underlying mechanism, although known to be different from that in prokaryotes, has remained elusive. We have investigated this mechanism in Saccharomyces cerevisiae, in which it is the major component of the RAD6/RAD18 pathway, by transforming an isogenic set of rad1delta excision-defective strains with plasmids that carry a single thymine-thymine pyrimidine (6-4) pyrimidinone photoadduct in each strand at staggered positions 28 base pairs apart. C-C mismatches placed opposite each of the T-T photoproducts permit unambiguous detection of the events that can lead to the completion of replication: sister-strand recombination or translesion replication on one or the other strand. Despite the severe block to replication that these lesions impose, we find that more than half of the plasmids were fully replicated in a rad1delta strain and that >90% of them achieved this end by recombination between partially replicated sister strands within the interlesion region. Approximately 60-70% of these events depended on the error-free component of the RAD6/RAD18 pathway, with the remaining events depended on RAD52; these two processes account for almost all of the recombination, which depended neither on DNA polymerase zeta nor on mismatch repair. We conclude that the error-free component of the RAD6/RAD18 pathway completes replication by a mechanism employing recombination between partially replicated sister strands, possibly by means of transient template strand switching or copy choice.