Effect of the Y955C Mutation on Mitochondrial DNA Polymerase Nucleotide Incorporation Efficiency and Fidelity

The human mitochondrial DNA polymerase (pol gamma) is responsible for the replication of the mitochondrial genome. Mutation Y955C in the active site of pol gamma results in early onset progressive external ophthalmoplegia, premature ovarian failure, and Parkinson's disease. In single-turnover kinetic studies, we show that the Y955C mutation results in a decrease in the maximal rate of polymerization and an increase in the K-m for correct incorporation. The mutation decreased the specificity constant for correct incorporation of dGTP, TTP, and ATP to values of 1.5, 0.35, and 0.044 mu M-1 s(-1), respectively, representing reductions of 30-, 110-, and 1300-fold, respectively, relative to the value for the wild-type enzyme. The fidelity of incorporation was reduced 6-130-fold, largely because of the significant decrease in the specificity constant for correct dATP:T incorporation. For example, K-cat/K-m for forming a TTP:T mismatch was decreased 10-fold from 0.0002 to 0.00002 mu M-1 s(-1) by the Y955C mutant, but the 1300-fold slower incorporation of the correct dATP:T relative to that of the wild type led to a 130-fold lower fidelity. While correct incorporation of 8-oxo-dGTP was largely unchanged, the level of incorporation of 8-oxo-dG with dA in the template strand was reduced 500-fold. These results support a role for Y955 in stabilizing A:T base pairs at the active site of pol gamma and suggest that the severe clinical symptoms of patients with this mutation may be due, in part, to the reduced efficiency of incorporation of dATP opposite T, and that the autosomal dominant phenotype may arise from the resulting higher mutation frequency.