Visualization of local DNA unwinding by Mre11/Rad50/Nbs1 using single-molecule FRET

Area of Science:

• Molecular biology
• DNA repair mechanisms
• Biophysics

Background:

• The Mre11/Rad50/Nbs1 (MRN) complex is a key initiator and coordinator of DNA double-strand break repair and signaling.
• MRN interaction with DNA ends is vital for recruiting DNA-processing enzymes, tethering DNA ends, and activating ATM kinase.

Purpose of the Study:

• To visualize and characterize the binding of the MRN complex to duplex DNA ends.
• To elucidate the role of MRN-mediated DNA end processing in DNA repair.

Main Methods:

• Single-molecule Förster Resonance Energy Transfer (smFRET) to visualize MRN-DNA interactions.
• Biochemical assays to assess DNA end resection in vitro.
• Analysis of DNA break repair in human cells using a Rad50 mutant.

Main Results:

• MRN complex unwinds 15-20 base pairs at duplex DNA ends, holding the structure open for extended periods.
• This unwinding is an ATP-dependent process.
• A Rad50 mutant deficient in ATP-dependent DNA opening showed impaired DNA end resection and resection-dependent repair.

Conclusions:

• MRN-mediated unwinding of DNA ends generates essential single-stranded DNA.
• This MRN-generated single-stranded DNA is critical for the efficient repair of DNA double-strand breaks in human cells.