Impact of DNA nucleotide excision repair on methyl methanesulfonate-induced mutations in Drosophila melanogaster.

To study the impact of DNA nucleotide excision repair (NER) on the spectrum of mutations induced by alkylating agents, postmeiotic male germ cell stages of Drosophila melanogaster were exposed to methyl methanesulfonate (MMS) and the males then mated with nucleotide excision repair deficient (exr-; mus(2)201) females. MMS (s value = 0.86) has a strong preference for alkylating the nitrogen positions in DNA, whereas < 1% of all DNA lesions are on oxygen. For genetic and molecular analysis of the types of mutations induced by MMS the vermilion locus was used as target gene. Mutation induction by MMS was increased 10-fold under the exr- conditions compared to a normal functioning repair system. The genetic analysis showed that < 15% of the mutants represented inter-locus mutations, which were classified as multi-locus deletions. Of the intra-locus mutations (18 F1 and 8 F2 mutants) 78% were transversions with a clear dominance of AT-->TA (11 in the F1, 3 in the F2) and few GC-->TA (2 in the F1, 3 in the F2) type of transversions. In comparison to the MMS spectrum produced under repair proficient (exr+) condition (Nivard, M.J.M., Pastink, A. and Vogel, E.W., 1992), the exr- spectrum shows a significant decrease in the percentage of deletions and a relative increase in transversions. These data are consistent with previously published papers suggesting that under normal repair conditions the nitrogen DNA adducts are efficiently repaired in Drosophila and that the hypermutability of MMS in the exr- strain is caused by an increased formation of apurinic sites either formed from 3-methyladenine or 7-methylguanine. This suggests that also in Drosophila 'the A-rule' is valid, indicating that during DNA replication an adenine (A) is preferentially incorporated opposite to non-instructive apurinic sites.