1. Sections of
2. Microbiology and of
3. Molecular and Cellular Biology and
4. Department of Applied Science, University of California at Davis, Davis, California 95616, USA
5. Electronics Engineering Technologies Division, and Biology and Biotechnology Research Program, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

Correspondence to: Stephen C. Kowalczykowski^1,2 Correspondence and requests for materials should be addressed to S.C.K.
(e-mail: Email: sckowalczykowski@ucdavis.edu).

Abstract

RecBCD enzyme is a processive DNA helicase¹ and nuclease² that participates in the repair of chromosomal DNA through homologous recombination^{3, 4}. We have visualized directly the movement of individual RecBCD enzymes on single molecules of double-stranded DNA (dsDNA). Detection involves the optical trapping of solitary, fluorescently tagged dsDNA molecules that are attached to polystyrene beads, and their visualization by fluorescence microscopy^{5, 6}. Both helicase translocation and DNA unwinding are monitored by the displacement of fluorescent dye from the DNA by the enzyme⁷. Here we show that unwinding is both continuous and processive, occurring at a maximum rate of 972 172 base pairs per second (0.30 m s^-1), with as many as 42,300 base pairs of dsDNA unwound by a single RecBCD enzyme molecule. The mean behaviour of the individual RecBCD enzyme molecules corresponds to that observed in bulk solution.