The two-step model of UV mutagenesis reassessed: deamination of cytosine in cyclobutane dimers as the likely source of the mutations associated with photoreactivation.

A large increase in the incidence of bacteriophage mutants is found after photoreactivation of UV-irradiated phage S13. The increase was seen only when the irradiated phage were stored before they were photoreactivated; the maximum mutation frequency was achieved after storage for 2 h at 4 degrees C or 30 min at 37 degrees C. The mutations can be attributed entirely to deamination of cytosine in cyclobutane dimers. Naked S13 DNA was stored for 2 h at 37 degrees C after being irradiated with wavelengths greater than or equal to 290 nm in the presence of 0.2% acetophenone, which sensitizes the formation of thymine-thymine but not cytosine-containing dimers; the specific mutation frequency was 7.2-fold lower compared to the frequency produced by irradiation in the absence of the photosensitizer, confirming that cytosine dimers are a major source of mutations. These results undermine the basis for the two-step model of UV mutagenesis in which a distinctly separate misincorporation step is supposed to precede the lesion bypass step; instead the results support a different two-step model, in which a deamination step precedes the bypass. The S13 capsid appears to completely inhibit the putative deamination reaction at about 75% of the dimer sites.