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

The high-fidelity replicative DNA polymerases, Pol ε and Pol δ, 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 ε, the canonical leading-strand replicase, to the lagging-strand replicase Pol δ, 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 δ than by Pol ε.