Mutagenesis, Vol. 16, No. 1, 1-6,
January 2001
© 2001 UK Environmental Mutagen Society/Oxford University Press
Effects of photoreactivation of cyclobutane pyrimidine dimers and pyrimidine (6–4) pyrimidone photoproducts on ultraviolet mutagenesis in SOS-induced repair-deficient Escherichia coli
1 Biological Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, 2 Department of Radiation Biophysics, Nagasaki University School of Medicine, Nagasaki 852-8523 and 3 Division of Radiation Biology, Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa 920-0934, Japan
Using purified photolyases for pyrimidine (6–4) pyrimidone photoproducts [(6–4)PP] and cyclobutane pyrimidine dimers (CPD), the effects of photoreactivation on mutagenesis were examined in the supF gene on a plasmid transfected into repair-deficient SOS-induced Escherichia coli host cells. More than 95% of CPD and (6–4)PP were removed from plasmid DNA by treatment with CPD photolyase and (6–4)photolyase, respectively. In each photolyase treatment, base substitutions at dipyrimidine sequences were predominantly observed. Of the singlebase substitutions observed after CPD photoreactivation, 83% were A:T
G:C transitions at 5'-TT-3' sites. After (6–4)photolyase treatment, 81% were G:CA:T transitions at 5'-CC-3' and 5'-TC-3' sequences. Thus, the major mutagenic photoproducts of single-base substitutions were CPD at 5'-CC-3' or 5'-TC-3' sites and (6–4)PP at 5'-TT-3' sites. Tandem double mutations occurred mainly at 5'-CC-3' sites and were CPD-photoreactivated, suggesting that CPD at 5'-CC-3' was responsible for tandem double mutations. After photoreactivation of both CPD and (6–4)PP, single-base substitutions were primarily G:CA:T transitions at 5'-CC-3' or 5'-TC-3' sites and A:TG:C transitions at 5'-TT-3' sites, and secondarily G:CT:A transversions at 5'-CC-3' sites, G:CC:G transversions at 5'-CC-3' sites and A:TT:A transversions at 5'-TT-3' sites, which were essentially the same as those observed after photoreactivation of CPD alone, (6–4)PP alone and without photoreactivation. Thus, these transversions were not derived from unknown UV adducts but from incompletely repaired CPD and (6–4)PP.
4 To whom correspondence should be addressed. Email: tnkm{at}mail.cc.tohoku.ac.jp
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