Cryo-EM structure of bacteriophage Bas63 reveals structural conservation and diversity in the Felixounavirus genus

The BASEL phage collection was developed to provide access to diverse bacteriophages, distinct from model phages. Escherichia phage JohannRWettstein (Bas63), a myophage in the collection, is a member of the subfamily Ounavirinae and the Felixounavirus genus. Using cryo-electron microscopy, we investigated Bas63's structure to explore its evolutionary relationships and functional adaptations. Our structures reveal a series of gene products: (i) a capsid decorated with β-tulip proteins at three-fold symmetry axes and a Hoc-like protein at hexamer centers, (ii) a conserved connector with an additional 12-fold ring of collar proteins that extend unique whisker proteins that are structurally related to podophage GP4 tail fibers, and (iii) a baseplate with long tail fibers resembling a contracted form of T4's long tail fibers. Sequence conservation analysis of Bas63 structural proteins across ICTV-recognized Felixounavirus' supports its role as a structural model for Felixounavirus evolution. This study advances the mechanistic understanding of phage architecture and reinforces the structural mosaicism of bacteriophages.