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. Expression is regulated by a large number of systems, including induction by quorum sensing via the two-component regulatory system QseB/QseC, induction by cAMP-CRP, repression by high osmolarity via OmpR and repression by H-NS. The C-terminal region contains a putative helix-turn-helix (HTH) motif, suggesting that this region may bind DNA. Has been reported to be involved in cell division regulation, but it was later shown that this is not the case. Belongs to the FlhD family. Extended N-terminus. Extended N-terminus.