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.
Tessman
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Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
Kennedy
M A
MA
D003090:Coliphages / Q000235:genetics* / Q000528:radiation effects; D003596:Cytosine / Q000528:radiation effects*; D004277:DNA, Single-Stranded / Q000235:genetics / Q000528:radiation effects*; D004926:Escherichia coli / Q000235:genetics / Q000528:radiation effects; D008957:Models, Genetic; D016296:Mutagenesis; D011740:Pyrimidine Dimers; D013696:Temperature; D014466:Ultraviolet Rays
D004277:DNA, Single-Stranded; D011740:Pyrimidine Dimers; D003596:Cytosine