Structural studies of the phage G tail demonstrate an atypical tail contraction

Phage G is recognized as having a remarkably large genome and capsid size among isolated, propagated phages. Negative stain electron microscopy of the host–phage G interaction reveals tail sheaths that are contracted towards the distal tip and decoupled from the head–neck region. This is different from the typical myophage tail contraction, where the sheath contracts upward, while being linked to the head–neck region. Our cryo-EM structures of the non-contracted and contracted tail sheath show that: (1) The protein fold of the sheath protein is very similar to its counterpart in smaller, contractile phages such as T4 and phi812; (2) Phage G’s sheath structure in the non-contracted and contracted states are similar to phage T4’s sheath structure. Similarity to other myophages is confirmed by a comparison-based study of the tail sheath’s helical symmetry, the sheath protein’s evolutionary timetree, and the organization of genes involved in tail morphogenesis. Atypical phase G tail contraction could be due to a missing anchor point at the upper end of the tail sheath that allows the decoupling of the sheath from the head–neck region. Explaining the atypical tail contraction requires further investigation of the phage G sheath anchor points.