Fig. 1.

A model for DNA gating in SPP1 and tailed bacteriophages. (A) The nascent prohead portal binds to the packaging proteins and a DNA packaging machine is assembled. (B) After packaging 1 viral genome (headful), a conformational change (pink) causes termination of packaging and dissociation of the motor. The last packaged DNA is restrained by the portal channel. (C) The neck proteins (adaptor and stopper) assemble, forming a closed DNA gate. Another conformational change widens the portal channel. The DNA descends and lands on the stopper, primed for delivery. The tail assembles on the neck. (D) Interaction with the host receptor triggers conformational changes that transmit a signal to the stopper, unzipping the DNA gate. The DNA is released into the bacterium through the tail tube.