Misincorporation in DNA synthesis after modification of template or polymerase by MNNG, MMS and UV radiation

Abstract

The synthetic DNA polymers, poly(dG-dC), poly(dC), poly(dA-dT), poly(dA) and poly(dT), were treated with N-methyl- N′-nitro- N-nitrosoguanidine (MNNG), methyl methanesulfonate (MMS) and UV irradiation. The modified polymers were use as templates to examine the incorporation of non-complementary nucleotides by E. coli DNA polymerase I. Methylation of poly(dG-dC) by MNNG predominantly induced the misincorporation of dTMP, whereas methylation by MMS induced that of dAMP. Treatment of poly(dT) with MNNG caused the misincorporation of dGMP to a considerable extent, but MMS did not enhance the error on poly(dT). The misincorporation of dAMP on poly(dC) and that of dGMP on poly(dA) were also increased by these chemicals. UV irradiation of poly(dT) and poly(dC) induced the error of dGMP and dAMP, respectively. These data on MNNG and MMS in vitro were in fair agreement with the directions of mutation in vivo. But the predominant induction of transitions by UV in vitro did not agree with the UV-induced transversions in E. coli. This inconsistency suggested the participation of other factors than direct mispairing in UV-induced transversion. Modification of DNA polymerase I by MNNG changed the ratio of polymerase to 3′→5∼ exonuclease activity altering the fidelity of this enzyme, whereas MMS and UV-irradiation did not alter the fidelity of the enzyme.