Hypermutability and error catastrophe due to defects in ribonucleotide reductase

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Ribonucleotide reductase (RNR) regulates deoxynucleoside-5 riphosphates (dNTPs), essential for DNA synthesis.
• Cellular dNTP levels are critical for maintaining DNA replication fidelity and controlling mutation rates.
• Mutator mutants of Escherichia coli RNR with altered dNTP pools and increased mutation rates have been previously identified.

Purpose of the Study:

• Investigate the impact of specific RNR mutations on dNTP pools and mutation rates.
• Determine the mechanism underlying the hypermutability observed in certain RNR mutants.
• Explore the relationship between dNTP pool deviations and DNA mismatch repair system function.

Main Methods:

• Isolation and characterization of Escherichia coli RNR mutants.
• Measurement of dNTP pools in wild-type and mutant strains.
• Assessment of mutation rates using a mutational forward assay.
• Analysis of DNA mismatch repair system saturation.

Main Results:

• RNR mutants with alterations at the allosteric specificity site exhibited modest dNTP pool deviations.
• These mutants showed exceptionally high mutator phenotypes (>1,000-fold increases in mutation rates).
• Evidence suggests saturation of the DNA mismatch repair system, leading to hypermutability and error catastrophe.

Conclusions:

• Modest deviations in cellular dNTP pools can dramatically increase mutation rates.
• Specific dNTP pool imbalances can promote replication errors that overwhelm DNA repair mechanisms.
• The study highlights the critical role of precise dNTP pool regulation in preventing genomic instability.