flhC

FlhC is a protein subunit of the FlhDC master regulator complex that controls flagellar biosynthesis in many Gram-negative bacteria. It is encoded by the flhC gene and, together with FlhD, forms the essential regulatory unit that initiates the flagellar gene expression cascade.

Functionally, the FlhDC complex acts as the primary transcriptional activator of class II flagellar operons, triggering the production of components required for the basal body and hook structures and setting in motion the assembly of the flagellum. The complex binds promoter regions upstream of target flagellar genes and facilitates RNA polymerase recruitment, thereby coordinating the early stages of motility machinery assembly.

Regulation of FlhDC activity integrates environmental cues and cellular status. In organisms such as Escherichia coli, flhDC expression and activity respond to factors like nutrient availability, growth phase, and temperature. Post-translational regulation, proteolysis, and inhibitors such as YdiV can modulate FlhDC activity in some strains, linking motility to metabolic state and environmental conditions. The cAMP-CRP system and other regulatory inputs can influence FlhDC levels or activity to optimize motility.

Structure and interactions: FlhC contributes the DNA-binding component of the FlhDC complex, often described as a heteromeric regulator with FlhD, forming a FlhD2C2 assembly in many bacteria. This complex embodies the master regulatory function that governs the onset of flagellar gene expression.

Impact: FlhC and FlhDC are essential for proper flagellum formation and bacterial motility, with mutations typically reducing or abolishing motility and affecting related cellular processes. Their regulation links environmental sensing to the production of the flagellar apparatus.