A Classic Fold With a Twist: Structural Architecture of Dhillonvirus Phage Bas18

Although over 15,000 unique phage genomes have been sequenced, many isolates remain uncharacterised and accessible only within individual laboratories. The BASEL (BActeriophage SElection for your Laboratory) collection comprises 106 newly isolated and characterised virulent bacteriophages that infect the laboratory strain Escherichia coli K-12 and provide an open resource for phage biology. Here, we used cryo-electron microscopy (cryo-EM) to determine the structure of Bas18, a Dhillonvirus siphophage from the BASEL collection. Bas18 assembles an icosahedral capsid with T = 7(d) triangulation. The asymmetric unit contains seven copies of the major capsid protein (MCP; gp09) and one dimeric decoration protein (gp64) bound at the centre of each hexamer. The MCP adopts the canonical HK97 fold but features a distinct insertion between the A and P domains, which we designate as G² and E² loop. The neck assembly consists of the portal, adaptor, stopper and terminator. The helical tail is built from hexameric rings of the tail tube protein, and the tail tip consists of the distal tail protein, hub and central fibre. Despite low sequence similarity, the overall architecture of the neck, tail, and tail tip closely resembles that of bacteriophage T1. In contrast, structural protein sequences are highly conserved (85-99% sequence similarity) across Dhillonviruses indicating a conserved virion architecture within this genus. These results expand the structural knowledge of the BASEL collection and provide a detailed architectural framework for Dhillonvirus phages, contributing to a broader understanding of siphophage structural diversity.