The N-terminal FleQ domain of the Vibrio cholerae flagellar master regulator FlrA plays pivotal structural roles in stabilizing its active state.

In Vibrio cholerae, the master regulator FlrA controls transcription of downstream flagellar genes in a σ 54 -dependent manner. However, the molecular basis of regulation by VcFlrA, that contains a phosphorylation-deficient N-terminal FleQ domain, was elusive yet. Our studies on VcFlrA, four of its constructs, and a mutant showed that the AAA + domain of VcFlrA, with or without the linker 'L', remains in ATPase-deficient monomeric states. In contrast, the FleQ domain plays a pivotal role in promoting higher-order functional oligomers, providing the required conformation to 'L' for ATP/c-di-GMP binding. The crystal structure of VcFlrA-FleQ at 2.0Å suggests that distinct structural features of VcFlrA-FleQ presumably assist in inter-domain packing. VcFlrA in high concentration forms ATPase-efficient oligomers when the intracellular c-di-GMP level is low. Conversely, excess c-di-GMP locks VcFlrA in a non-functional lower oligomeric state, causing repression of flagellar biosynthesis.