UV-induced hprt mutations in a UV-sensitive hamster cell line from complementation group 3 are biased towards the transcribed strand.

The molecular nature of 254 nm ultraviolet light (UV)-induced mutations at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus in UV24 Chinese hamster ovary (CHO) cells, which are defective in nucleotide excision repair, was determined. Sequence analysis of 19 hprt mutants showed that single base substitutions (9 mutants) and tandem base changes (7 mutants) dominated the UV mutation spectrum in this cell line. Sixty-five percent of the base substitutions were GC greater than AT transitions, whereas the rest consisted of transitions and transversions at AT base pairs. Analysis of the distribution of dipyrimidine sites over the two DNA strands, where the photoproducts causing these mutations presumably were formed, showed that 12 out of 14 mutations were located in the transcribed strand of the hprt gene. A similar strand distribution of mutagenic photoproducts as in UV24 has previously been found in two other UV-sensitive Chinese hamster cell lines (V-H1 and UV5), indicating that under defective nucleotide excision repair conditions the induction of mutations is strongly biased towards lesions in the transcribed strand of the hprt gene. A plausible explanation for this phenomenon is that during DNA replication large differences exist in the error rate with which DNA polymerase(s) bypass lesions in the templates for the leading and lagging strand, respectively.