TLS mediates the age-dependent increase in genomic instability. (A) Chronological survival and (B) mutation frequency (Can^r) of the wild type (BY4741) and mutants deficient of Polζ, rev3Δ, and rev7Δ. *, P < 0.05; **, P < 0.01; mutants versus wild type (WT) two-tailed t test at the indicated time points (n = 4). (C) Chronological survival and (D) mutation frequency (Can^r) of the wild type (DBY746), rev3Δ, and rad30Δ mutants. *, P < 0.05; **, P < 0.01; ***, P < 0.001; mutants versus WT, two-tailed t test at the indicated time points (n = 6–10). Error bars indicate ±SEM. (E) Abolished TLS in Sch9-deficient cells. Nuclear extracts from 3-d-old stationary phase wild-type (DBY746) and sch9Δ mutant cells were incubated with undamaged or abasic site-containing DNA templates for 30 min at 30°C. Primer extension products were analyzed on 19% polyacrylamide gels. There was no lesion bypass observed in sch9Δ mutants. Free primers are indicated by the arrow. TLS products are indicated by solid (undamaged) or broken (damaged) lines. On the right, the percentage of incorporation opposite the abasic site is given.