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

The BASEL phage collection was developed to provide access to diverse bacteriophages, distinct from model phages. <i>Escherichia</i> phage JohannRWettstein (Bas63), a myophage in the collection, is a member of the subfamily Ounavirinae and the <i>Felixounavirus</i> 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 <i>Felixounavirus'</i> supports its role as a structural model for <i>Felixounavirus</i> evolution. This study advances the mechanistic understanding of phage architecture and reinforces the structural mosaicism of bacteriophages.