Mutations induced by aromatic amine DNA adducts in pBR322.

Abstract

A 276 bp region from the tetracycline resistance gene of the plasmid pBR322 was modified with 2-acetylaminofluorene (AAF), 2-aminofluorene (AF), 4-aminobiphenyl (ABP), N'-acetylbenzidine or 1-aminopyrene (AP) in order to determine the effect of adduct structure upon mutation induction. Each modification reaction gave one major adduct and these adducts had chromatographic properties, as determined by 32P-postlabeling, identical to those in which substitution had occurred at C8 of deoxyguanosine through the amine or amide nitrogen. The types and distribution of mutations were then characterized following introduction of the modified plasmids into SOS-induced Escherichia coli using Hanahan et al.'s procedure (Methods Enzymol., 204, 63-113, 1991). With AAF-modified plasmid, 60% of the mutations were deletions or additions, and these were detected primarily at NarI sites or in repetitive G sequences. Modification with AF gave -G deletions, primarily in runs of Gs, and base substitution mutations, which were mainly G to T transversions. Substitution with ABP or N'-acetylbenzidine resulted in G to T and G to C transversions, the latter being a mutation not detected with AF; in addition, -G deletions were detected at only very low frequency. AP modification gave both -G frameshift and base substitution mutations, of which G to T transversions predominated. A comparison of the mutation frequencies per adduct indicated that the mutagenic efficiencies of the adducts decreased in the order AP > AF > AAF approximately ABP approximately N'-acetylbenzidine. AAF- and ABP-modified pBR322 were also introduced with a CaCl2 method. The mutation frequency per adduct increased with this transformation procedure, and this appeared to be a reflection of a greater percentage of frameshift mutations. These data indicate that a series of structurally related aromatic amines will induce both base substitution and frameshift mutations when incorporated into pBR322, but that frameshift mutations occur almost exclusively with the planar derivatives. Furthermore, the ability to induce frameshift mutations increases the mutagenic efficiency of an adduct.