Perturbations of Enzymic Uracil Excision due to Guanine Modifications in DMA1

Phage PBS2 DMA, which contains uracil in place of thymine, was used as substrate for purified Bacillus subtilis uracil:DNA glycosylase. Incubation of this DMA with the ultimate carcinogen A/-acetoxy-A/-2-acetylaminofluorene resulted in the production of A/-(deoxyguanosin-8-yl)acetylaminofluorene. A decreased v^ 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 sub strate showed delayed dissociation of the enzyme from the arylamidated DNA. This shows that the presence of a carcino gen-modified DNA base can reduce the capacity for uracil exci sion. Therefore, interference with enzymic release of uracil from DNA may be an indirect mechanism of mutagenesis by carcinogen:DNA adducts.