BLM helicase measures DNA unwound before switching strands and hRPA promotes unwinding reinitiation

EMBO J. 2009 Feb 18;28(4):405-16. doi: 10.1038/emboj.2008.298. Epub 2009 Jan 22.


Bloom syndrome (BS) is a rare genetic disorder characterized by genomic instability and a high predisposition to cancer. The gene defective in BS, BLM, encodes a member of the RecQ family of 3'-5' DNA helicases, and is proposed to function in recombinational repair during DNA replication. Here, we have utilized single-molecule fluorescence resonance energy transfer microscopy to examine the behaviour of BLM on forked DNA substrates. Strikingly, BLM unwound individual DNA molecules in a repetitive manner, unwinding a short length of duplex DNA followed by rapid reannealing and reinitiation of unwinding in several successions. Our results show that a monomeric BLM can 'measure' how many base pairs it has unwound, and once it has unwound a critical length, it reverses the unwinding reaction through strand switching and translocating on the opposing strand. Repetitive unwinding persisted even in the presence of hRPA, and interaction between wild-type BLM and hRPA was necessary for unwinding reinitiation on hRPA-coated DNA. The reported activities may facilitate BLM processing of stalled replication forks and illegitimately formed recombination intermediates.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Bloom Syndrome / genetics
  • Bloom Syndrome / metabolism
  • DNA / chemistry
  • DNA / metabolism*
  • DNA Helicases / chemistry
  • DNA Helicases / genetics
  • DNA Repair
  • DNA Replication
  • Fluorescence Resonance Energy Transfer
  • Humans
  • Models, Genetic
  • Nucleic Acid Conformation
  • Oligonucleotides / chemistry
  • Protein Binding
  • RecQ Helicases / metabolism*


  • Oligonucleotides
  • DNA
  • Bloom syndrome protein
  • DNA Helicases
  • RecQ Helicases