Exonuclease proofreading by human mitochondrial DNA polymerase

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• DNA polymerases are crucial for maintaining genomic integrity.
• Mitochondrial DNA polymerase plays a key role in replicating mitochondrial DNA.
• Error correction mechanisms, like proofreading, are essential for preventing mutations.

Purpose of the Study:

• To investigate the error-correcting ability of human mitochondrial DNA polymerase.
• To quantify the kinetics of DNA excision and polymerization during proofreading.
• To elucidate the mechanism of mismatch recognition and removal.

Main Methods:

• Single turnover kinetic assays were employed.
• Rates of single-stranded and duplex DNA excision were measured.
• The effect of mismatches and free nucleotides on excision rates was analyzed.

Main Results:

• Excision of correctly base-paired DNA was slow (0.05 s⁻¹), indicating minimal proofreading cost.
• Excision rates for mismatched DNA increased with the number of mismatches, reaching up to 9 s⁻¹.
• Free nucleotides enhanced excision rates 7-fold (to 21 s⁻¹) by facilitating DNA transfer to the exonuclease site.
• Mitochondrial DNA polymerase fidelity increased to approximately 200-fold in the presence of nucleotides.

Conclusions:

• Human mitochondrial DNA polymerase possesses an efficient proofreading mechanism.
• The cost of proofreading is low, with minimal impact on polymerization rate.
• Free nucleotides significantly enhance the error correction efficiency of mitochondrial DNA polymerase.
• The intramolecular transfer mechanism facilitates rapid and effective error correction.