Figure 5.

Power spectrum analysis of the variance in bead height during (circles) and in the absence of (squares) protein activity. Each set of points is the average of 26 segments of a data trace acquired at 5 mM ATP and F=19 pN. The spectrum of noise during protein activity shows a clear 1/f² increase over the spectrum of the measurement noise alone. Note that the latter spectrum is calculated from quiescent segments between the bursts of activity that give rise to the former spectrum, and is thus a nearly simultaneous measure of the background noise. This increase is due to stochastic variations of the motion of the protein, and is proportional to the protein's step size δ. The line is a fit to the predicted increase (Charvin et al, 2002): y=δ〈v〉/2π²f²+b, where 〈v〉 is the mean velocity and b accounts for the (white) measurement noise. From many such fits, we find that, if the FtsK_50C enzymatic cycle has a single rate-determining step, it advances 12±2 bp per cycle.