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ReportsSpring-Loaded Mechanism of DNA Unwinding by Hepatitis C Virus NS3 HelicaseNS3, 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. * To whom correspondence should be addressed. E-mail: smyong{at}uiuc.edu
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Science. ISSN 0036-8075 (print), 1095-9203 (online)