FlhBC

FlhBC is a term occasionally used in bacterial genetics to refer to the two flagellar-related proteins FlhB and FlhC. It is not a universally adopted designation; most studies treat FlhB and FlhC separately, with the canonical master regulator of flagellar gene expression described as FlhDC (or FlhD4C2 in some species). The two proteins play distinct roles in the biogenesis of bacterial flagella, spanning transcriptional control and the flagellar export apparatus.

FlhB is an integral inner membrane protein that forms part of the flagellar type III secretion system, also known as the flagellar export apparatus. It contains multiple transmembrane segments and a cytoplasmic domain that participates in substrate regulation. A key feature of FlhB is a proteolytic processing event within its cytoplasmic region, which is required for the switch from early to late flagellar substrates and for proper export specificity. FlhB interacts with other export components and helps govern the order in which flagellar proteins are secreted during assembly.

FlhC is the regulatory subunit of the FlhDC master regulator, partnering with FlhD to form a complex that activates transcription of class 2 flagellar genes. The FlhDC complex binds promoters across the flagellar regulon and initiates the transcriptional cascade that leads to flagellum construction. Regulation of FlhDC activity occurs at multiple levels, including transcription, proteolysis, and responses to environmental and cellular signals, enabling bacteria to coordinate flagellar production with growth conditions.

In common usage, FlhBC as a single entity is uncommon; the terms FlhB and FlhC are typically discussed separately. Together, they illustrate the two principal stages of flagellar biogenesis: transcriptional activation by FlhDC (incl. FlhC) and the assembly/export process mediated by FlhB.