Structure of NO16, a marine non-tailed vibriophage with an unusual symmetry-mismatched vertex arrangement.

Non-tailed phages remain underexplored in marine environments, as tailed phages have long dominated sequence and culture collections. Yet recent surveys suggest that non-tailed phages may be more abundant and have distinct impacts on microbial mortality and gene transfer. Here, we solve the structure of Vibrio anguillarum bacteriophage NO16, one of the simplest members of the Varidnaviria realm. Mass spectrometry detected at least nine different proteins in the virion, which has a pseudoT = 21 capsid similar to that of related double jelly roll (DJR) phages, but differs in the organization of minor capsid proteins, particularly those mediating membrane-capsid contacts. The DJR major capsid protein GP19 is stabilized by strong electrostatic interactions between monomers, and possibly by a cation at its base, as seen in corticovirus PM2. Localized reconstruction revealed a symmetry mismatch at the vertex, where two trimeric GP13 spikes attach to the pentameric GP14 penton base. GP13 carbohydrate-binding sites and predicted glycosylase activity point to a role in host entry. Using structural and functional predictions for its entire proteome, we propose a complete atlas of the NO16 infectious cycle.