Fig. 3. Bypass of a (6–4)TT photoproduct in S.cerevisiae wild-type and rad30 mutant strains. Error-free and mutagenic TLS events are determined as described in Materials and methods. Mutagenic TLS (T→C transitions) is strongly reduced either when Pol η is absent (rad30Δ) or catalytically defective (rad30AA). Error-free TLS is unaffected in the Pol η deletion strain, but is significantly reduced in the catalytically defective Pol η strain as if the mutant protein is able to compete with the DNA polymerase that mediates error-free bypass. Over 1000 colonies were analyzed by hybridization in each strain.