Structural Studies of Bacteriophage Φ6 and Its Transformations during Its Life Cycle

Abstract

From the first isolation of the cystovirus bacteriophage Φ6 from Pseudomonas syringae 50 years ago, we have progressed to a better understanding of the structure and transformations of many parts of the virion. The three-layered virion, encapsulating the tripartite double-stranded RNA (dsRNA) genome, breaches the cell envelope upon infection, generates its own transcripts, and coopts the bacterial machinery to produce its proteins. The generation of a new virion starts with a procapsid with a contracted shape, followed by the packaging of single-stranded RNA segments with concurrent expansion of the capsid, and finally replication to reconstitute the dsRNA genome. The outer two layers are then added, and the fully formed virion released by cell lysis. Most of the procapsid structure, composed of the proteins P1, P2, P4, and P7 is now known, as well as its transformations to the mature, packaged nucleocapsid. The outer two layers are less well-studied. One additional study investigated the binding of the host protein YajQ to the infecting nucleocapsid, where it enhances the transcription of the large RNA segment that codes for the capsid proteins. Finally, I relate the structural aspects of bacteriophage Φ6 to those of other dsRNA viruses, noting the similarities and differences.

Bacteriophages and their unique components provide limitless resources for exploitation

“…Pseudomonas syringae phage ϕ6 was the first enveloped dsRNA Cystovirus identified more than 50 years ago (Gottlieb and Alimova, 2023 ) and has served as the model phage to understand phage RNA biosynthesis, packaging and host lipid bilayer recruitment. Reminiscent of eukaryotic viruses, the phages use protein-triggered membrane fusion for entry into their bacterial hosts (Bamford et al, 1987 ) and the phages exit surrounded by host phospholipid bilayers from the inner membrane (Heymann, 2023 ). These envelopes are generated through the insertion of one specific phage protein (that must be expressed together with its chaperone) and vesicles of similar buoyant density as ϕ6 phospholipid precursors can be generated independently through the sole expression of these proteins (Johnson and Mindich, 1994 ).…”