Spring-loaded mechanism of DNA unwinding by hepatitis C virus NS3 helicase

Science. 2007 Jul 27;317(5837):513-6. doi: 10.1126/science.1144130.


NS3, an essential helicase for replication of hepatitis C virus, is a model enzyme for investigating helicase function. Using single-molecule fluorescence analysis, we showed that NS3 unwinds DNA in discrete steps of about three base pairs (bp). Dwell time analysis indicated that about three hidden steps are required before a 3-bp step is taken. Taking into account the available structural data, we propose a spring-loaded mechanism in which several steps of one nucleotide per adenosine triphosphate molecule accumulate tension on the protein-DNA complex, which is relieved periodically via a burst of 3-bp unwinding. NS3 appears to shelter the displaced strand during unwinding, and, upon encountering a barrier or after unwinding >18 bp, it snaps or slips backward rapidly and repeats unwinding many times in succession. Such repetitive unwinding behavior over a short stretch of duplex may help to keep secondary structures resolved during viral genome replication.

Publication types

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

MeSH terms

  • Adenosine Triphosphate / metabolism
  • Base Pairing
  • DNA / chemistry
  • DNA / metabolism*
  • DNA Helicases / metabolism*
  • Fluorescence Resonance Energy Transfer
  • Hepacivirus / enzymology*
  • Models, Biological
  • Models, Molecular
  • Nucleic Acid Conformation
  • Temperature
  • Viral Nonstructural Proteins / chemistry
  • Viral Nonstructural Proteins / metabolism*


  • NS3 protein, hepatitis C virus
  • Viral Nonstructural Proteins
  • Adenosine Triphosphate
  • DNA
  • DNA Helicases