Accelerated Deamination of Cytosine Residues in UV-Induced Cyclobutane Pyrimidine Dimers Leads to CC→TT Transitions

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Abstract
The rate of UV-induced deamination of cytosine to uracil at a specific site in double-stranded (ds) DNA was monitored using a genetic reversion assay. M13mp2C141 ds DNA was exposed to 160 J/m2 UV (254 nm), incubated at 37 °C, pH 7.4, for various time intervals to allow for deamination, and treated with Escherichiacoli photolyase in the presence of 365 nm light to reverse cyclobutane-type pyrimidine dimers. Upon transfection into uracil-glycosylase deficient (ung-) E. coli cells, the mutation (i.e., reversion) frequencies in the CCCC target sequence increased greatly with post-UV time of incubation at 37 °C, nearly doubling every day that the DNA had been held at 37 °C. After 8 days, the reversion frequencies had increased by two orders of magnitude upon transfection into ung- cells, relative to isogenic ung+ cells, indicating that most of the mutations arising in UV/photolyase-treated ds DNA were C→T mutations mediated by a uracil intermediate. Sequencing of the revertants revealed that all mutations were single C→T or tandem double CC→TT mutations. An increasing percentage of tandem double CC→TT mutations was found with longer post-UV incubation times, yet none occurred if the post-UV delay time step was omitted before photoreversal. After a 4-day delay between UV and photoreversal at 37 °C, greater than 84% of the total revertants had tandem double CC→TT mutations. Thus, the generation of a tandem double mutation is a time-dependent process that arises in DNA after the initial UV exposure. The rate of appearance (with a pseudo-first-order rate constant ca. 10-6 s-1) of tandem double mutations during incubation of UV-irradiated DNA is inconsistent with two random, independently occurring mutational events and suggests a concerted deamination of both residues in a tandem cytosine pyrimidine (C<>C) dimer. Considering that deamination in a C<>C dimer occurred here with a half-life of ca. 5 days, in contrast to the measured half-life of ca. 20 000 years for spontaneous (non-UV-treated) cytosine deamination for the same target, these studies show that the formation of pyrimidine dimers in DNA increases the rate of deamination by six orders of magnitude, leading to the accelerated formation of single C→T and tandem double CC→TT mutations.