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DNA unwinding helicase enzymes are a type of motor protein. Motor proteins can translocate along filaments or polymers using energy generated from ATP hydrolysis. Helicases are involved in all the important cellular processes where DNA unwinding is required, such as DNA replication, repair, recombination, and transcription. They are present in all living organisms, but vary in their structure, function, and mechanism of action. For example, in prokaryotes, DnaB helicase binds and translocates...
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For successful DNA replication, the unwinding of double-stranded DNA must be accompanied by stabilization and protection of the separated single strands of the DNA. This crucial task is performed by single-strand DNA-binding (SSB) proteins. They bind to the DNA in a sequence-independent manner, which means that the nitrogenous bases of the DNA need not be present in a specific order for binding of SSB proteins to it. The binding of SSB proteins straightens single-stranded DNA (ssDNA) and makes...
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DNA replication is initiated at sites containing predefined DNA sequences known as origins of replication. DNA is unwound at these sites by the minichromosome maintenance (MCM) helicase and other factors such as Cdc45 and the associated GINS complex.The unwound single strands are protected by replication protein A (RPA) until DNA polymerase starts synthesizing DNA at the 5’ end of the strand in the same direction as the replication fork. To prevent the replication fork from falling apart,...
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The basic reaction of homologous recombination (HR) involves two chromatids that contain DNA sequences sharing a significant stretch of identity. One of these sequences uses a strand from another as a template to synthesize DNA in an enzyme-catalyzed reaction. The final product is a novel amalgamation of the two substrates. To ensure an accurate recombination of sequences, HR is restricted to the S and G2 phases of the cell cycle. At these stages, the DNA has been replicated already and the...
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Single-Molecule Fluorescence Visualization of DNA Polymerase Dynamics at G-Quadruplexes
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BLM unfolds G-quadruplexes in different structural environments through different mechanisms.

Wen-Qiang Wu1, Xi-Miao Hou2, Ming Li3

  • 1College of Life Sciences, Northwest A&F University, Yangling, Shaanxi 712100, China.

Nucleic Acids Research
|April 22, 2015
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Summary

Bloom's syndrome helicase (BLM) unwinds G-quadruplexes (G4s) using ATP. BLM employs distinct mechanisms to unfold G4s based on their surrounding DNA structures, revealing context-dependent activity.

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Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • Bloom's syndrome results from mutations in the BLM gene, leading to genetic instability and cancer.
  • BLM helicase is crucial for DNA repair and is known to interact with G-quadruplexes (G4s).
  • G4s are non-canonical DNA structures implicated in genomic instability.

Purpose of the Study:

  • To elucidate the molecular mechanisms by which BLM helicase unfolds G-quadruplexes.
  • To investigate the role of ATP in BLM-mediated G4 unfolding.
  • To determine if BLM utilizes different mechanisms for G4 unfolding in varied molecular contexts.

Main Methods:

  • Single-molecule Förster Resonance Energy Transfer (smFRET) assays were employed.
  • Studies focused on BLM helicase activity in the presence of ATP.
  • G4 structures with different flanking DNA sequences (3'-ssDNA tail, ssDNA-dsDNA junction) were analyzed.

Main Results:

  • ATP is essential for BLM helicase to unfold G-quadruplex structures.
  • BLM exhibits distinct G4 unfolding mechanisms dependent on the G4's molecular environment.
  • One mechanism involves unidirectional translocation for G4s with a 3'-ssDNA tail.
  • Another mechanism involves repetitive reeling of ssDNA for G4s connected to dsDNA via ssDNA, with BLM anchored at the junction.

Conclusions:

  • BLM helicase utilizes ATP-dependent, context-specific mechanisms for G-quadruplex unfolding.
  • This adaptability suggests a versatile role for BLM in managing diverse G4 structures in the genome.
  • Understanding these mechanisms provides insight into maintaining genomic stability and preventing cancer predisposition in Bloom's syndrome.