Mechanisms of frameshift mutagenesis by aflatoxin B₁-2,3-dichloride

In order to characterize frameshift mutagenesis by aflatoxin B₁-2,3-dichloride (AFB1C12), we have introduced a + 1 (BK8) or a - 1 (HS8) frameshift within the lacZ α gene segment contained in the phage M13mp8 to obtain lacZ α^- derivatives. BK8 or HS8 replicative form DNA was modified with AFB1C12 in vitro, transfected into appropriate Escherichia coli hosts and lacZ α⁺ revertants scored and defined by DNA sequencing. The -1 frameshift (BK8) results suggest the following. (1) The E. coli recA gene is not absolutely required for AFB1C12-induced frameshift mutagenesis; however, in recA⁺ cells, ultraviolet light (SOS) induction enhances AFB1C12 mutagenesis, but such ultraviolet induction is not required. The plasmid pGW270 (mucAB⁺) significantly enhances the AFB1C12-induced frameshift mutagenesis. The uvrABC⁺ excision system plays a major role in the repair of AFBlC12-induced damage. (2) Sequence analysis reveals that AFB1C12 induces two classes of -1 frameshift mutations: the simple class in which the frameshift is due to the loss of one base-pair, and the complex class in which the loss of a base-pair is coupled to a vicinal base substitution. Both types of mutations occur predominantly at G · C runs, which are hotspots for AFB1C12 damage. The complex mutations appear to be concerted events targeted by a single AFB1C12 adduct. The frequency of these complex mutations is significantly enhanced by mucAB activity. In this system, recA activity is required for generation of significant levels of complex mutations. An analysis of the +1 frameshifts (HS8) reveals that AFB1C12 induces +1 frameshifts with an efficiency comparable to that for -1 frameshifts. Most + 1 frameshifts occur by the addition of a base, and a third of the additions are complex mutations because they are accompanied by at least one base substitution. All simple additions occur at G · C runs; however, in a striking contrast to spontaneous insertions, a majority of the induced events introduce an A · T pair at these sites. Our data suggest a model for the generation of base substitution as well as simple and complex frameshift mutations induced by AFB1C12. To the extent determined, the frameshift specificity of aflatoxin B₁ activated by metabolic enzymes is similar to that of AFB1C12.