Fig. 1.

Schematic of the experiment. T4 prohead–gp17 complexes were attached to antibody-coated microspheres and captured in one optical trap (lower left). Biotinylated DNA molecules were tethered to streptavidin-coated microspheres and captured in a second optical trap (upper left). The bottom trap was moved with respect to the top trap while measuring the force. To initiate packaging, the microspheres were brought into near contact for 1 s (middle) and then quickly separated to detect DNA binding and translocation (right).