The bacterial flagellar hook is a short, curved tubular structure made of FIgE. The hook connects the basal body as a rotary motor and the filament as a helical propeller and functions as a universal joint to smoothly transmit torque produced by the motor to the filament. Salmonella FIgE consists of DO, Dc, D1 and D2 domains. Axial interactions between a triangular loop of domain Dl (D1-loop) and domain D2 are postulated to be responsible for hook supercoiling. In contrast, Bacillus FlgE lacks the Dl-loop and domain D2. Here, to clarify the roles of the D1-loop and domain D2 in the mechanical function, we carried out deletion analysis of Salmonella FlgE. A deletion of the Dl-loop conferred a loss-of-function phenotype whereas that of domain D2 did not. The Dl-loop deletion inhibited hook polymerization. Suppressor mutations of the Dl-loop deletion was located within FIgD, which acts as the hook cap to promote hook assembly. This suggests a possible interaction between the Dl-loop of FIgE and F1gD. Suppressor mutant cells produced straight hooks, but retained the ability to form a flagellar bundle behind a cell body, suggesting that the loop deletion does not affect the bending flexibility of the Salmonella hook. (C) 2017 Elsevier Inc. All rights reserved.