Fidelity of DNA ligase I is sensitive to physiological Mg²⁺ level

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Free intracellular Mg2+ concentration varies significantly across human tissues.
• DNA ligase I (LIG1) is essential for DNA replication, recombination, and repair.
• LIG1 deficiency causes immunodeficiency-96 (IMD96).

Purpose of the Study:

• To investigate the effect of free Mg2+ on the fidelity of human DNA ligase I (LIG1).
• To explore the implications of Mg2+ concentration on LIG1 function in tissues with low Mg2+ levels, such as the brain and immune system.
• To examine the fidelity of a specific LIG1 variant (K845N) associated with neuroprotection in Huntington's disease.

Main Methods:

• Steady-state kinetics were used to compare the fidelity of LIG1.
• LIG1 fidelity was assessed using a nicked DNA substrate with a damaged nucleobase at the 3´-OH position.
• The release of the AMP-DNA intermediate from a pro-mutagenic DNA substrate was analyzed.

Main Results:

• Decreasing free Mg2+ concentration from 1.0 mM to 0.2 mM increased LIG1 fidelity 21-fold.
• LIG1 fidelity enhancement was observed at physiological Mg2+ concentrations found in the brain and immune system (0.2–0.4 mM).
• The K845N LIG1 variant also showed enhanced fidelity at lower Mg2+ concentrations, primarily due to improved AMP-DNA intermediate release.

Conclusions:

• Free Mg2+ availability is a critical determinant of DNA ligation fidelity.
• Tissue-specific Mg2+ concentrations may influence the fidelity of DNA transactions, impacting disease risk.
• Understanding Mg2+ regulation of LIG1 is crucial for addressing neurodegenerative and immune disorders.