FIGURE 1. Set-up and initial results.

a, Diagrams showing the experimental set-up at the start of a measurement (left), constant force feedback mode (middle) and no feedback mode (right) measurements. A single 29 packaging complex is tethered between two microspheres. Optical tweezers are used to trap one microsphere and measure the forces acting on it, while the other microsphere is held by a micropipette. To insure measurement on only one complex, the density of complexes on the microsphere is adjusted so that only about one out of five–ten microspheres yielded hook-ups. Such attachments break in one discrete step as the force is increased, indicating that only one DNA molecule carries the load. b, DNA tether length against time for four different complexes with a constant force of 5 pN using a 34.4-kb 29- DNA construct. Inset, increased detail of regions, indicated by arrows, showing pauses (curves have been shifted). The solid lines are a 100-point average of the raw data. c, Packaging rate against the amount of DNA packaged, relative to the original 19.3-kb 29 genome. Grey line, trace for a single complex (derived from black trace in b). Rates were obtained by linear fitting in a 1.5-s sliding window. The red line is an average of eight such measurements. Large pauses (velocity drops >30 bp s^-1 below local average) were removed, and the curves were horizontally shifted to account for differences in microsphere attachment points. The red line was smoothed using a 200-nm sliding window. The standard deviation for the ensemble of measurements varies from 20 bp s^-1 at the beginning, down to 10 bp s^-1 at the end.