Quantitation of aflatoxin B1 adduction within the ribosomal RNA gene sequences of rat liver DNA.

The in vivo formation of covalent aflatoxin B1 (AFB1)-DNA adducts within the rRNA gene sequences of nuclear DNA has been studied in AFB1-treated rats. Liver nuclear DNA, enriched in ribosomal DNA (rDNA) by one round of cesium salt density gradient centrifugation, was treated under buffered alkaline conditions to convert unstable AFB1-N7-guanine adducts to stable AFB1-formamidopyrimidine derivatives. The alkali-treated DNA was hybridized to 18S and 28S rRNA in 70% formamide buffer to form rRNA X rDNA hybrids. These hybrids were separated from the bulk of nuclear DNA by two rounds of centrifugation in CsCl, and the level of AFB1 adduction to rDNA versus total nuclear DNA was compared as a function of dose 2 hr after AFB1 administration. Over an 8-fold dose range (0.25-2.0 mg of AFB1 per kg of body weight), rDNA contained 4- to 5-fold more AFB1 residues than nuclear DNA, indicating that rDNA is preferentially accessible to carcinogen modification in vivo. While aflatoxin B1 forms adducts with DNA principally at guanine residues, the guanine enrichment of rDNA was insufficient to explain the magnitude of observed preferential AFB1 modification of rDNA. These results support the hypothesis that rDNA regions are preferentially accessible to carcinogen modification because of the diffuse conformation maintained within transcribed genes. This experimental approach permits the quantitative description of carcinogen modification within a defined gene sequence; further refinement of this approach may be useful in defining the precise relationships between covalent chemical-DNA interactions and the alterations in gene expression that result.