The eukaryotic replisome tolerates leading-strand base damage by replicase switching

The high-fidelity replicative DNA polymerases, Pol epsilon and Pol delta, are generally thought to be poorly equipped to replicate damaged DNA. Direct and complete replication of a damaged template therefore typically requires the activity of low-fidelity translesion synthesis (TLS) polymerases. Here we show that a yeast replisome, reconstituted with purified proteins, is inherently tolerant of the common oxidative lesion thymine glycol (Tg). Surprisingly, leading-strand Tg was bypassed efficiently in the presence and absence of the TLS machinery. Our data reveal that following helicase-polymerase uncoupling a switch from Pol epsilon, the canonical leading-strand replicase, to the lagging-strand replicase Pol delta, facilitates rapid, efficient and error-free lesion bypass at physiological nucleotide levels. This replicase switch mechanism also promotes bypass of the unrelated oxidative lesion, 8-oxoguanine. We propose that replicase switching may promote continued leading-strand synthesis whenever the replisome encounters leading-strand damage that is bypassed more efficiently by Pol delta than by Pol epsilon.

Automated summary

The study shows that a yeast replisome is inherently tolerant to the oxidative lesion Tg, efficiently bypassing leading-strand Tg even in the absence of TLS machinery. A switch from Pol epsilon to Pol delta after helicase-polymerase uncoupling promotes rapid, efficient, and error-free lesion bypass at physiological nucleotide levels. Replicase switching may facilitate continued leading-strand synthesis when Pol delta is more effective at bypassing damage than Pol epsilon.