A role for DNA polymerase in the specificity of nucleotide incorporation opposite N-acetyl-2-aminofluorene adducts.
Escherichia coli DNA polymerase I (Klenow fragment), DNA polymerase alpha from both calf thymus and human lymphoma cells and DNA polymerase beta from calf thymus and Novikoff hepatoma cells can incorporate nucleotides opposite N-guanin-8-yl-acetyl-2-aminofluorene in DNA. The polymerases incorporate dCTP opposite some AAF-dG lesions when Mg2+ is the divalent cation. Substitution of Mn2+ for Mg2+ broadens the specificity of insertion: E. coli DNA polymerase I (Klenow fragment) also inserts A, and at specific sites G or T; DNA polymerase alpha inserts any of the four dNTPs with A and C incorporated preferentially to G and T. Polymerase beta is specific, inserting mainly C even in the presence of Mn2+. The Km for addition of dATP opposite a lesion by E. coli polymerase I (Klenow fragment) in the presence of Mn2+ is about 0.5 mM. dNMPs increase the insertion of nucleotides opposite AAF-dG in the presence of Mg2+ and increase both the rate and number of sites at which incorporation occurs in the presence of Mn2+. dNTP alpha S and recA protein increase only the insertion of C. We suppose that the incorporation of dCTP reflects normal base-pairing with the AAF-deoxyguanine in the anti conformation, whereas insertion of the other nucleotides (including some of the C) reflects insertion opposite the AAF adduct in its preferred syn conformation. The fact that the DNA polymerase plays a role in determining the specificity of insertion opposite a lesion terminating DNA synthesis suggests that the spectrum of base substitution mutagenesis seen in vivo may reflect the properties of the protein components, including the polymerase, involved in bypass synthesis.