The two-step model of UV mutagenesis reassessed: deamination of cytosine in cyclobutane dimers as the likely source of the mutations associated with photoreactivation |
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Authors: | Irwin Tessman and Matthew A Kennedy |
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Institution: | (1) Department of Biological Sciences, Purdue University, 47907 West Lafayette, IN, USA |
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Abstract: | Summary 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° C or 30 min at 37° C. The mutations can be attributed entirely to deamination of
cytosine in cyclobutane dimers. Naked S13 DNA was stored for 2 h at 37° C after being irradiated with wavelengths ≥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. |
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Keywords: | UV mutagenesis Pyrimidine dimers Photoreactivation Acetophenone SOS repair |
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