Bacteriophage T4 has long served as an extraordinary model for tailed phages. During virion assembly, the viral DNA genome is tightly packed into the head, to which tail attaches via a portal-neck connector. Keeping this highly pressurized head leak-proof during these transactions is a challenge, yet the mechanisms remain poorly understood. Here we show that T4 seals its DNA-filled capsid using a double "genome-gate" mechanism. By reconstituting portal (gp20)-neck (gp13/gp14) assembly intermediates in vitro and determining their structures, we find that the gp14 hexamer forms a primary gate that closes the portal-neck opening. This gate is reinforced by a second gate formed by the host protein Hfq, which is hijacked by the virus as an accessory structural component. Hfq also stabilizes neck assembly and prevents its mis-assembly with portal. These studies define a viral genome retention mechanism in which a pre-assembled gp13/gp14/Hfq neck complex seals the pressurized, genome-filled capsid during virion maturation.
Han et al. (Sat,) studied this question.