The N‐terminal FleQ domain of the Vibrio cholerae flagellar master regulator FlrA plays pivotal structural roles in stabilizing its act

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 Vc FlrA, which contains a phosphorylation‐deficient N‐terminal FleQ domain, has remained elusive. Our studies on Vc FlrA, four of its constructs, and a mutant showed that the AAA + domain of Vc FlrA, 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 Vc FlrA‐FleQ at 2.0 Å suggests that distinct structural features of Vc FlrA‐FleQ presumably assist in inter‐domain packing. Vc FlrA at a high concentration forms ATPase‐efficient oligomers when the intracellular c‐di‐GMP level is low. Conversely, excess c‐di‐GMP locks Vc FlrA in a non‐functional lower oligomeric state, causing repression of flagellar biosynthesis.