Assisted assembly of bacteriophage T7 core components for genome translocation across the bacterial envelope

Abstract

In most bacteriophages, the genome transport across bacterial envelopes is carried out by the tail machinery. In Podoviridae viruses, where the tail is not long enough to traverse the bacterial wall, it has been postulated that viral core proteins are translocated and assembled into a tube within the periplasm. T7 bacteriophage, a member from the Podoviridae family, infects E. coli gram-negative bacteria. Despite extensive studies, the precise mechanism by which this virus translocates its genome remains unknown. Using cryo-electron microscopy, we have resolved the structure two different assemblies of the T7 bacteriophage DNA translocation complex, built by core proteins gp15 and gp16. Gp15 alone forms a partially folded hexamer, which is further assembled by interaction with gp16, resulting in a tubular structure with dimensions compatible with traversing the bacterial envelope and a channel that allows DNA passage. The structure of the gp15-gp16 complex also shows the location in gp16 of a canonical transglycosylase motif essential in the bacterial peptidoglycan layer degradation. Altogether these results allow us to propose a model for the assembly of the core translocation complex in the periplasm, which helps in the understanding at the molecular level of the mechanism involved in the T7 viral DNA release in the bacterial cytoplasm. SIGNIFICANCE STATEMENTT7 bacteriophage infects E. coli bacteria. During this process, the DNA transverses the bacterial cell wall, but the precise mechanism used by the virus remains unknown. Previous studies suggested that proteins found inside the viral capsid (core proteins) disassemble and reassemble in the bacterial periplasm to form a DNA translocation channel. In this article we solved by cryo-electron microscopy two different assemblies of the core proteins that reveal the steps followed by them to finally form a tube large enough to traverse the periplasm, as well as the location of the transglycosylase enzyme involved in peptidoglycan degradation. These findings confirm previously postulated hypothesis and make experimentally visible the mechanism of DNA transport trough the bacterial wall.