Human RPA activates BLM's bidirectional DNA unwinding from a nick

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• The Bloom syndrome, RecQ helicase-like (BLM) enzyme is crucial for genome stability, involved in DNA replication and repair.
• BLM processes various DNA substrates but its mechanism for handling nicked DNA was unclear.

Purpose of the Study:

• To investigate the mechanisms by which BLM interacts with and processes nicked DNA.
• To understand the role of human replication protein A (hRPA) in modulating BLM's DNA unwinding activity.

Main Methods:

• Utilized a combined single-molecule approach to observe BLM and hRPA interactions with DNA substrates.
• Applied external forces to DNA substrates to mimic cellular conditions and analyze unwinding dynamics.

Main Results:

• High concentrations of BLM can unwind double-stranded DNA (dsDNA) from a nick under applied force.
• Human replication protein A (hRPA) enables BLM to processively unwind nicked dsDNA under reduced force.
• hRPA also facilitates BLM translocation on single-stranded DNA (ssDNA), leading to bidirectional unwinding.

Conclusions:

• BLM exhibits novel DNA unwinding activities on nicked DNA, dependent on hRPA and specific targeting.
• These findings suggest a mechanism for BLM's functional switching in DNA repair pathways.
• The interaction highlights the importance of accessory proteins in regulating helicase activity.