Figure 7.

(A) Schematic drawing of a RecBCD helicase translocate along an immobilized DNA molecule. (B) Unwinding time courses of RecBCD helicase moving along a 4.1 kb long duplex χ-containing DNA (▴) and χ-zero DNA (○) were determined by FTPM. The χ sequence is located at 1404 bp from the surface. The arrow indicates the first χ position where RecBCD would encounter the χ sequence. The solid line represents the linear fittings of unwinding rates before (95 bp/s) and after (17 bp/s) the χ position for χ-containing DNA. The unwinding rate for a χ-zero DNA does not change (93 bp/s). This experiment was carried out at 35 μM ATP and 1 mM magnesium, below the K_M of ATP for RecBCD. (C) Of 55 RecBCD unwinding processes observed, 34 lowered the translocation rate at the χ sequence. The plot includes data from both 1967 bp and 4141 bp DNA substrates containing the χ sequence. All of the rate changes at χ position lead to a slower rate, as they all fall into the bottom-right section.