The flagellotropic bacteriophage χ (Chi) infects bacteria via the flagellar filament. Despite years of study, its structural architecture remains partly characterized. Through cryo-EM, we unveil χ′s nearly complete structure, encompassing capsid, neck, tail, and tail tip. While the capsid and tail resemble phage YSD1, the neck and tail tip reveal new proteins and their arrangement. The neck shows a unique conformation of the tail tube protein, forming a socket-like structure for attachment to the neck. The tail tip comprises four proteins, including distal tail protein (DTP), two baseplate hub proteins (BH1P and BH2P), and tail tip assembly protein (TAP) exhibiting minimal organization compared to other siphophages. Deviating from the consensus in other siphophages, DTP in χ forms a trimeric assembly, reducing tail symmetry from 6-fold to 3-fold at the tip. These findings illuminate the previously unexplored structural organization of χ’s neck and tail tip.

嗜鞭毛噬菌体 χ (Chi) 通过鞭毛丝感染细菌。尽管经过多年的研究，其结构体系仍然具有部分特征。通过冷冻电镜，我们揭示了 χ 几乎完整的结构，包括衣壳、颈部、尾巴和尾尖。虽然衣壳和尾部类似于噬菌体 YSD1，但颈部和尾部尖端揭示了新的蛋白质及其排列。颈部显示出尾管蛋白的独特构象，形成用于附着在颈部的插座状结构。尾尖包含四种蛋白质，包括远端尾蛋白 (DTP)、两种底板枢纽蛋白（BH1P 和 BH2P）以及尾尖组装蛋白 (TAP)，与其他噬菌体相比，尾尖组装蛋白表现出最小的组织。与其他噬菌体中的共识不同，χ 中的 DTP 形成三聚体组装，将尾部对称性从尖端的 6 倍减少到 3 倍。这些发现阐明了先前未探索过的 χ 颈部和尾尖的结构组织。