The specificity of UV-induced mutations at an endogenous locus in mammalian cells.

Abstract

We have used a rapid in vivo recombinational method to clone and completely sequence 34 UV-induced mutants at the adenine phosphoribosyltransferase (APRT) locus of Chinese hamster ovary cells. Among the mutants recovered, 26 were single base substitutions including 17 G .cntdot. C .fwdarw. A .cntdot. T transitions and a single A .cntdot. T .fwdarw. G .cntdot. C transition. Three of the 4 possible transversions accounted for the remaining 8 mutations. The G .cntdot. C .fwdarw. T .cntdot. A transversion was not recovered. Six tandem double or closely neighboring was not recovered. Six tandem double or closely neighboring double-base substitutions, one double mutation consisting of a G .cntdot. C .fwdarw. T .cntdot. A transversion and an adjacent frameshift, as well as one single frameshaft mutation were also recovered. UV-induced mutation appears to be targeted to dipyrimidine sites with only two exceptions. These include two double mutations where only one of the base substitutions occurred at a dipyrimidine site. The observed specificity of UV-light-induced mutations at the APRT locus is consistent with the argument that G .cntdot. C .fwdarw. A .cntdot. T transitions result primarily from the (6-4) pyrimidine pyrimidone lesion. A striking resemblance in the distribution of UV-induced mutants and a collection of 30 spontaneous mutants identified recently in our laboratory was noted. Both share a common strong site of multiple occurrence and a considerable degree of overlap with respect to site specificity. We speculate therefore that DNA context plays a significant role in mutation fixation in mammalian cells.