UV-induced T-->C transition at a TT photoproduct site is dependent on Saccharomyces cerevisiae polymerase eta in vivo.

UV-induced reversion of the arg4-17 ochre allele in Saccharomyces cerevisiae is largely dependent on translesion polymerase eta (Rad30p), known to bypass cyclobutane-type TT dimers in an error-free fashion. arg4-17 locus reversion was predominantly due to T-->C transition of T127, the 3' T of a TT photoproduct site. This event was at least 20-fold reduced in a rad30 deletion mutant, irrespective of the status of nucleotide excision repair. These data correlate with known properties of 6-4 TT photoproducts and in vitro characteristics of polymerase eta and suggest that polymerase eta plays an important in vivo role in inserting G opposite the 3' T of 6-4 TT photoproducts at this site. Alternatively, an unprecedented error-prone processing of cyclobutane-type photoproducts at this site by polymerase eta must be assumed as the critical mechanism. Whereas photoreactivation results indeed hint at the latter possibility, a possible regulatory influence of reducing the overall UV damage load on the bypass probability of non-cyclobutane-type pyrimidine dimer photoproducts should not be dismissed.