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Science 27 July 2007:
Vol. 317. no. 5837, pp. 513 - 516
DOI: 10.1126/science.1144130
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Reports

Spring-Loaded Mechanism of DNA Unwinding by Hepatitis C Virus NS3 Helicase

Sua Myong,1,2* Michael M. Bruno,3,4 Anna M. Pyle,3,4 Taekjip Ha1,2,4

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.

1 Physics Department, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA.
2 Institute for Genomic Biology, University of Illinois, 1206 West Gregory Drive, Urbana, IL 61801, USA.
3 Molecular Biophysics and Biochemistry, Yale University, 266 Whitney Avenue, Room 334A, Bass Building, New Haven, CT 06511, USA.
4 Howard Hughes Medical Institute.

* To whom correspondence should be addressed. E-mail: smyong{at}uiuc.edu

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