PAS domain of flagellar histidine kinase FlrB exhibits novel architecture, and binds Heme as sensory signal in unconventional fashion

SUMMARY Phosphorylation of the σ 54 -dependent transcription activator FlrC by the sensor histidine kinase FlrB is essential in flagellar synthesis of Vibrio cholerae . Despite that, the structure, sensory signal, and mechanistic basis of function of FlrB were elusive. Here we report the crystal structure of the sensory PAS domain of FlrB in functional dimeric state that exhibits a novel architecture. Series of biochemical/biophysical experiments unequivocally established heme as sensory ligand that packs hydrophobically in the ligand binding cleft of FlrB-PAS. Intriguingly, ATP binding to the C-terminal ATP binding (CA) domain assists PAS domain to bind heme, vis-à-vis, heme binding to the PAS facilitates ATP binding to CA; suggesting a synergistic mode of heme and ATP binding to FlrB. We propose that such synergistic binding triggers conformational signaling in FlrB, leading to the downstream flagellar gene transcription. Enhanced swimming motility of V. cholerae with increased heme uptake further supports this proposition.