Insight into DNA and protein transport in double-stranded DNA viruses: The structure of bacteriophage N4

N4 is a lytic double-stranded DNA (dsDNA) bacteriophage that infects the Escherichia coli K12 strains. This article reports the structure of this bacteriophage as determined to 14-Å resolution by cryoelectron microscopy (Fig. 1), The identity and placement of the different parts of the virion in the three-dimensional structure was confirmed by a combination of bioinformatics and analysis of the difference density maps of the wild type (WT) and mutants lacking some virion gene products. The virion has an icosahedral head housing the vRNAP, the viral RNA polymerase, and the DNA. The head is supported by a neck composed of 12 subunits forming a pentameric base. The tail assembly is formed by a hollow cylindrical tube covered by a noncontractile sheath. The structure of the virion provides insight into the process of injection of material into the host cell. Briefly, the virion adsorbs onto the host's surface by the tail sheath and the materials within it, starting with the vRNAP, are injected into the host cell. The incompatibility of the vRNAP size with the small diameter of the tail suggests that vRNAP is released in a denatured or partially unfolded form that must be quickly renatured in the host cell's cytoplasm.