Perturbations of enzymic uracil excision due to guanine modifications in DNA.

Phage PBS2 DNA, which contains uracil in place of thymine, was used as substrate for purified Bacillus subtilis uracil:DNA glycosylase. Incubation of this DNA with the ultimate carcinogen N-acetoxy-N-2-acetylaminofluorene resulted in the production of N-(deoxyguanosin-8-yl)acetylaminofluorene. A decreased Vmax resulted from the reaction of the glycosylase with this arylamidated substrate. Addition of a 2-fold excess of control PBS2 DNA following initiation of the reaction with the modified substrate showed delayed dissociation of the enzyme from the arylamidated DNA. This shows that the presence of a carcinogen-modified DNA base can reduce the capacity for uracil excision. Therefore, interference with enzymic release of uracil from DNA may be an indirect mechanism of mutagenesis by carcinogen:DNA adducts.