Proof-reading 3'-->5' exonucleases isolated from rat liver nuclei

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Mammalian nuclear DNA polymerases alpha and beta lack intrinsic 3'-->5' exonucleolytic editing activity.
• This crucial editing function is presumed to be carried out by non-covalently associated exonucleases.

Purpose of the Study:

• To isolate and characterize novel 3'-->5' exonucleases from mammalian nuclei.
• To investigate the role of these exonucleases in enhancing DNA replication fidelity in conjunction with DNA polymerase alpha.

Main Methods:

• Isolation and purification of two distinct 3'-->5' exonucleases (exo-40 and exo-5) from rat liver nuclei.
• Assaying exonuclease activity on mismatched nucleotides using a poly[d(A-T)] template.
• Evaluating the effect of adding purified exonucleases to DNA polymerase alpha on in-vitro DNA synthesis fidelity using bacteriophage phi X174 DNA.

Main Results:

• Exo-40 and exo-5 preferentially excised mismatched nucleotides, with exo-40 being 10-fold more efficient than exo-5.
• Co-addition of either exonuclease with DNA polymerase alpha increased in-vitro DNA replication fidelity by 5-10 fold.
• In vitro findings suggest that exonucleolytic proofreading can enhance DNA synthesis accuracy by 2-3 orders of magnitude at cellular activity levels.

Conclusions:

• Rat liver nuclei contain at least two distinct 3'-->5' exonucleases that can function as proofreading enzymes.
• These exonucleases significantly enhance the fidelity of DNA synthesis mediated by DNA polymerase alpha.
• Exonucleolytic proofreading is a critical mechanism for maintaining genomic integrity in mammalian cells.