DNA-SEQUENCE DETERMINANTS OF CARCINOGEN-INDUCED FRAMESHIFT MUTAGENESIS

In order to examine the mechanisms of induced frameshift mutagenesis, we constructed double-stranded DNA plasmids which contain single N-2-a cetylaminofluorene (AAF) adducts at specified positions within a run o f contiguous guanine residues. The length of the homopolymeric run and the nature of the bases flanking the contiguous sequence were systema tically varied. Monomodified plasmids were introduced into SOS-induced Escherichia coti, and -1 frameshift mutations were scored by means of a phenotypic assay. A strong positional effect of the DNA adduct with in the contiguous sequence was observed irregardless of the nature of the flanking bases: the AAF-induced mutation frequency wag 20-200-fold higher at the 3'-end of the contiguous sequence than at the 5'-end. I n addition, for a given number of guanine residues flanking the G(AAF) adduct on its 5'-side, differences (up to 5-fold) in the induced muta tion frequency were observed as a function of the base 3' to the adduc t (CGGG(AAF)AT approximate to CGGG(AAF)GT > (CGGG(AAF)CT > CGGG(AAF)TT ). These results are discussed, within the frame of an incorporation s lippage model, in terms of differences in stability and occurrence of the slipped mutagenic intermediates.