Structures of a T1-like siphophage reveal capsid stabilization mechanisms and high structural similarities with a...

Bacteriophage T1, a member of Siphoviruses, infects Escherichia coli with high efficiency, making it a promising candidate for phage therapy. Here, we report the near-atomic structures of FCWL1, a T1-like phage that belongs to the T1 phage family. We focus on the head, the head-to-tail interface, and its surrounding components. The hexameric capsomer displays unique gaps between neighboring A domains of the major capsid proteins. These gaps are partially filled by the N-loop of the decoration protein, which adopts a unique conformation. These structural features suggest that the phage might employ a novel strategy for stabilizing its head. Furthermore, despite being a siphophage, the head and head-to-tail connector of the phage show high structural similarity to those of a myophage. These findings enhance our understanding of the structure, capsid stabilization mechanism, and evolution of phages in the T1 family.