In situ structures of the portal-neck-tail complex of bacteriophage T4 inform a viral genome positioning mechanism.

The post-genome packaging mechanisms that govern the assembly of an infectious virion are poorly understood in bacteriophages and other viruses. Here, our near-atomic resolution cryo-EM structural analyses uncovered an assembly- and conformation-driven genome positioning mechanism in the tailed bacteriophage T4. We show that following headful packaging, which generates a pressurized head, a global conformational change occurs in the portal structure, probably triggering packaging termination and ejection of the packaging motor. Our high-resolution structures of the neck of the virion further show that the neck undergoes conformational changes upon docking of a pre-assembled tail onto the sealed neck, which then opens a genome-gate. Driven by the pressure of the packaged DNA, the genome travels through open neck channels, binds and compresses the resident tape-measure protein, and halts at the bottom of the second topmost disk of the tail tube. Pressure-suspended within the virion's innermost tunnel and secured by a baseplate plug, the genome remains poised to flow through the tunnel into a host cell upon receiving the host receptor recognition signal.