Old, new, and widely true: The bacteriophage T4 DNA packaging mechanism

- Small terminase subunit (TerS) synapses two pac homologs by a twin ring mechanism.
- Both ends of TerL packaged DNA are held at portal, showing loop of DNA is packaged.
- Transient spring-like compression of B form to A form DNA accompanies translocation.
- The C-terminal domain of TerL is docked to the portal, moves toward it when stalled.
- Portal bound resolvase releases stalled Y-DNA A compression to allow translocation.

DNA packaging into empty viral procapsids by ATP-driven motor proteins applies widely among viruses. Recent fluorescence studies of phage T4 reveal: 1) the small terminase subunit (TerS) synapses pac homologs by a twin ring mechanism to gauge DNA maturation and allow packaging by the large terminase subunit (TerL); 2) translocation of linear DNA is efficient by TerL acting alone; expansion of the procapsid is controlled by the portal-terminase assembly; 3) both ends of the packaged DNA are held at the portal, showing a loop of DNA is packaged; 4) transient spring-like compression of B form to A form-like DNA accompanies translocation; 5) the C-terminal domain of TerL is docked to the portal and moves toward it when stalled; 6) a portal bound resolvase can release stalled Y-DNA compression and allow translocation in vitro; and 7) ATP powered translocation on A form dsDNA is supported by recent hexameric helicase studies.