DNA polymerase ε leading strand signature mutations result from defects in its proofreading activity

Area of Science:

• Molecular Biology
• Genetics
• DNA Replication

Background:

• DNA polymerase ε (Pol ε) has been implicated in leading strand replication due to specific mutation signatures.
• Previous studies suggested Pol ε's role based on its misincorporation bias and observed A > T mutations in yeast.

Purpose of the Study:

• To investigate whether A > T signature mutations are caused by defects in Pol ε's proofreading activity.
• To differentiate between Pol ε's role as a leading strand replicase versus its proofreading function in mutation generation.

Main Methods:

• Analysis of A > T mutation rates in yeast strains with specific Pol ε proofreading-defective mutations (pol2-4 and pol2-M644G).
• Assessment of mutation rates in the absence of PCNA ubiquitination or Pol ζ.
• Comparison of mutation rates between strains with different Pol ε proofreading efficiencies.

Main Results:

• A > T signature mutations were highly elevated in both pol2-4 and pol2-M644G strains, irrespective of Pol ε's specific misincorporation bias.
• The elevated rate of A > T mutations was significantly reduced when PCNA ubiquitination or Pol ζ was absent.
• The findings challenge the hypothesis of Pol ε's primary role in leading strand replication.

Conclusions:

• Leading strand A > T signature mutations result from impaired Pol ε proofreading activity, not its replicative function.
• This evidence supports a major role for DNA polymerase δ (Pol δ) in the replication of both DNA strands.
• The study clarifies the distinct roles of different DNA polymerases in maintaining genome integrity.