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

In Vibrio cholerae, the master regulator FlrA controls transcription of downstream flagellar genes in a σ⁵⁴ -dependent manner. However, the molecular basis of regulation by VcFlrA, which contains a phosphorylation-deficient N-terminal FleQ domain, has remained elusive. 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. By contrast, the FleQ domain plays a pivotal role in promoting higher-order functional oligomers, providing the required conformation to 'L' for ATP/cyclic di-GMP (c-di-GMP) binding. The crystal structure of VcFlrA-FleQ at 2.0 Å suggests that distinct structural features of VcFlrA-FleQ presumably assist in inter-domain packing. VcFlrA at a 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.