DNA polymerase λ Loop1 variant yields unexpected gain-of-function capabilities in nonhomologous end-joining

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• DNA polymerases lambda (Polλ) and mu (Polmicro) are X-Family polymerases involved in DNA double-strand break (DSB) repair via nonhomologous end-joining (NHEJ).
• These polymerases synthesize DNA from one DSB end using a template from another, facilitating NHEJ and minimizing sequence loss.
• Polλ functions with paired primer ends, while Polmicro is required for unpaired primer ends.

Purpose of the Study:

• To investigate the functional divergence and substrate specificity of DNA polymerases lambda and mu in NHEJ.
• To characterize a novel Polλ variant (PolλKGET) with altered substrate specificity.
• To elucidate the role of the Loop1 region in the unique activities of Family X polymerases.

Main Methods:

• Site-directed mutagenesis to generate the PolλKGET variant.
• Biochemical assays to assess DNA polymerase activity and fidelity.
• Analysis of the role of the Loop1 region in PolλKGET function during NHEJ.

Main Results:

• The PolλKGET variant retained canonical Polλ activity on paired ends but with reduced fidelity.
• PolλKGET unexpectedly acquired the ability to synthesize from unpaired primer termini, a function previously unique to Polmicro.
• The Loop1 region of PolλKGET was found to be essential for this novel unpaired end synthesis activity, despite not directly contacting the DNA substrate.

Conclusions:

• The Loop1 region plays distinct and critical roles in the differing substrate specificities of Family X DNA polymerases.
• Modifications in Polλ can lead to acquisition of novel enzymatic activities, expanding our understanding of DNA repair mechanisms.
• These findings provide insights into the structural and functional basis of DNA polymerase diversity in genome stability.