Abstract
Major pathways of recombinational DNA repair in Escherichia coli require the RecBCD protein—a heterotrimeric, ATP-driven, DNA translocating motor enzyme. RecBCD combines a highly processive and exceptionally fast helicase (DNA-unwinding) activity with a strand-specific nuclease (DNA-cleaving) activity (refs 1, 2 and references therein). Recognition of the DNA sequence ‘χ’ (5′-GCTGGTGG-3′) switches the polarity of DNA cleavage and stimulates recombination at nearby sequences in vivo. Here we attach microscopic polystyrene beads to biotin-tagged RecD protein subunits and use tethered-particle light microscopy to observe translocation of single RecBCD molecules (with a precision of up to ∼30 nm at 2 Hz) and to examine the mechanism by which χ modifies enzyme activity. Observed translocation is unidirectional, with each molecule moving at a constant velocity corresponding to the population-average DNA unwinding rate. These observations place strong constraints on possible movement mechanisms. Bead release at χ is negligible, showing that the activity modification at χ does not require ejection of the RecD subunit from the enzyme as previously proposed; modification may occur through an unusual, pure conformational switch mechanism.