Functions in complex with FlhC as a master transcriptional regulator that regulates transcription of several flagellar and non-flagellar operons by binding to their promoter region. Activates expression of class 2 flagellar genes, including fliA, which is a flagellum-specific sigma factor that turns on the class 3 genes. Also regulates genes whose products function in a variety of physiological pathways. Homodimer; disulfide-linked. Forms a heterohexamer composed of two FlhC and four FlhD subunits. Each FlhC binds a FlhD dimer, forming a heterotrimer, and a hexamer assembles by dimerization of two heterotrimers. The C-terminal region contains a putative helix-turn-helix (HTH) motif, suggesting that this region may bind DNA. Belongs to the FlhD family. Extended N-terminus.