Figure 7. Phage DNA packaging via a torsional compression mechanism.

(a) A fully assembled packasome complex is sketched, with the empty prohead (red) carrying the gp20 portal ring (blue) shown at one vertex, and the DNA duplex drawn in black. The multimeric state of the gp17 terminase (orange) remains to be precisely determined: only two subunits are shown for clarity. The terminase subunits are sketched with a minor lobe, representing a flexible region that undergoes a conformational change (black dashed arrow) coupled with ATP binding and hydrolysis (white solid arrow). No detailed gp17 structure is implied; nor do we specify the exact steps and temporal order by which the binding and hydrolysis of ATP is coupled to movement of gp17 during the reaction cycle. (b) Directed linear motion of the flexible arm of the lower gp17 subunit engages the DNA substrate and translocates this towards the prohead. This movement coupled with interaction of the DNA with the portal region causes induced changes to the helical pitch and temporarily stores energy. (c) The stored energy is released by translocation into the prohead (green arrow), enabling a restoration of the B-form helical repeat. For clarity, only one power stroke from one gp17 subunit is shown: the model also allows for multiple power strokes occurring at this stage before translocation occurs. (d) A nick in the DNA strand disrupts the accumulation and translocation potential of energy temporarily stored in the duplex, preventing translocation. Instead, loss of torsional energy in the compressed nicked duplex might even facilitate backward release of the substrate, as suggested by the red arrow in (e).