Uracil glycol deoxynucleoside triphosphate is a better substrate for DNA polymerase I Klenow fragment than thymine glycol deoxynucleoside triphosphate

A major-stable oxidation product of DNA cytosine is 5,6-dihydroxy-5,6-dihydrouracil (Ug). Ug can be formed directly in DNA or in the cellular nucleotide pools by deamination of the unstable primary product, cytosine glycol. Here, we synthesized dUgTP and showed that dUgTP was incorporated In place of dTTP and was a much better substrate for the model enzyme DNA, polymerase I Klenow fragment lacking proofreading activity, Kf (exo(-)), than deoxythymidine glycol triphosphate (dTgTP). The relative efficiency for dUgTP insertion opposite A was 10 times higher than for dTgTP; however, the extension of a primer with 3' dUg was about 100 times more efficient than the extension of a primer with 3' dTg, At the insertion step, the differences in V-max appeared to be responsible since the apparent K(m)s for dUgTP and dTgTP were about the same. In contrast, both the apparent K-m and V-max for elongation of dUg were markedly different from those of dTg. Molecular modeling was performed with both Tg and Ug and provides a rational structural explanation for these observations.