LEADING VERSUS LAGGING-STRAND MUTAGENESIS INDUCED BY 7,8-DIHYDRO-8-OXO-2'-DEOXYGUANOSINE IN ESCHERICHIA-COLI

We have previously shown that a single N-2-acetylaminofluorene (AAF) a dduct bound to the C-8 position of a guanine residue located within pl asmids containing the unidirectional ColE1 origin of replication induc es a 20-fold higher mutation frequency when the adduct is located in t he lagging strand as compared to the leading strand. In this study, si ngle 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) lesions have been i ntroduced in the leading and lagging strand orientation within the sam e sequence context as for the AAF adducts. The induced frequency of gu anine to thymine transversions has been measured, using a specific PCR -based quantitative assay, in strains deficient in the repair of the o xidative lesion. The potential involvement of the UvrABC excision repa ir system in the removal of 8-oxodG has also been investigated and rul ed out. Concerning the mutation frequency asymmetry, in contrast to AA F adducts, 8-oxodG adducts induce the same mutation frequency, irrespe ctive of their location in the leading or lagging strands. This striki ng difference between 8-oxodG and dGuo-C8-AAF adducts is discussed in terms of their differential capacity to block DNA replication. (C) 199 7 Academic Press Limited