Fig. 1.

Schematic of the hybrid two-component system (HTCS) family of signalling sensors.

A. Schematic of canonical two-component system. An environmental signal, such as a nutrient ligand, binds to the extracellular sensor domain (1), resulting in intracellular phosphorylation of a histidine residue on the phosphoacceptor domain (2). The phosphate is then transferred to an aspartic acid on the response regulator (3). The resulting activated response regulator affects cellular activity via an output domain (4), often by promoting transcription.

B. Schematic of an HTCS. HTCSs contain all of the domains typically found in the two proteins of a canonical two-component system encoded by a single gene. Additionally, they characteristically contain an output domain that consists of a helix–turn–helix DNA binding module of the AraC family (4).

C. Schematic of BT1754, a well-described HTCS in Bt, and its associated PUL (adapted from Sonnenburg et al., 2010). The periplasmic sensing domain binds monomeric fructose. BT1754 is required for the upregulation of the adjacent fructan utilization PUL and for efficient use of fructose-based carbohydrates.

D. Schematic of BT0366, another HTCS in Bt, and its adjacent PUL. We show BT0366 binding to a linear arabinan-derived hexameric oligosaccharide, one of several arabinan-derived ligands it has been shown to bind (Martens et al., 2011). The BT0366 gene sits within a PUL consisting of two divergently encoded gene cassettes with several genes shown to be involved in processing various forms of arabinan (Cartmell et al., 2011).