How asymmetric DNA replication achieves symmetrical fidelity

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Accurate DNA replication requires polymerase selectivity, exonucleolytic proofreading, and DNA mismatch repair (MMR).
• DNA polymerases (Pols) δ and ε possess intrinsic 3'-5' exonuclease activity for cis-excision of mismatches.
• The interplay between these mechanisms in achieving genome-wide replication fidelity remains incompletely understood.

Purpose of the Study:

• To investigate the in vivo role of extrinsic proofreading by Pol δ in DNA replication fidelity.
• To quantify the contributions of nucleotide selectivity, intrinsic proofreading, extrinsic proofreading, and MMR to overall fidelity.
• To determine if extrinsic proofreading by Pol δ impacts fidelity symmetrically across both DNA strands.

Main Methods:

• Utilized Saccharomyces cerevisiae as a model organism.
• Employed high-accuracy sequencing and genetic analyses to assess DNA replication fidelity.
• Dissected the contributions of individual fidelity mechanisms, including nucleotide selectivity, intrinsic proofreading, extrinsic proofreading, and MMR.

Main Results:

• Provided strong evidence that Pol δ performs extrinsic proofreading of mismatches, independent of its polymerization activity and MMR.
• Demonstrated that Pol δ's extrinsic proofreading efficiently corrects errors made by both Pol δ and Pol ε across the genome.
• Showed that extrinsic proofreading by Pol δ significantly improves and balances DNA replication fidelity between the two strands.

Conclusions:

• Pol δ's extrinsic proofreading is a critical and efficient mechanism for maintaining DNA replication fidelity.
• A comprehensive model is presented where nucleotide selectivity, intrinsic proofreading, extrinsic proofreading, and MMR collaborate for high and symmetrical DNA strand fidelity.
• This study elucidates a previously underappreciated layer of DNA repair contributing to genome stability.