The major stable reaction product of activated aflatoxin B1 (AFB1) with DNA is the N7-guanine adduct. By using a simple extension of the Maxam-Gilbert sequencing technique on defined DNA sequences modified by activated AFB1, we have shown [Muench, K. F., Misra, R. M., & Humayun, M. Z. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 6–10] that the induction of alkali labile sites is strongly and predictably influenced by the nucleotide sequence context in double-stranded DNA. In this paper, we present data that show the following: (a) sequence-specific effects are abolished in single-stranded DNA while at the same time the overall reactivity of guanine (G) residues is strongly suppressed when compared to double-stranded DNA; (b) in single-stranded DNA capable of forming intrastrand hairpin stem-loop structures, AFB1 reacts strongly with base-paired G residues in a sequence-specified manner but not as efficiently with non-base-paired G residues; (c) certain chemicals related in structure to AFB1, and the intercalating dye ethidium bromide, inhibit the reaction of AFB1 with DNA. These data are consistent with the possibility that a sequence-specific precovalent association between double-stranded DNA and AFB1 is a factor in the observed specificity in covalent reactions. We also present data on another kind of covalent AFB1 binding, namely, the formation of photoadducts. Previous work showed that AFB1, in analogy with related coumaryl chemicals, the psoralens, can be photoactivated, resulting in stable adduct formation with DNA. We show here that a significant reaction is with G residues, presumably at the N7 position.
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