Histidine-aspartate phosphotransfer architectures.

(A) A canonical two-component signaling system consists of a histidine kinase and a response regulator, wherein a signal is transmitted by transfer of a phosphoryl group from a conserved histidine in the HisKA domain (teal oval) to a conserved aspartate in the REC domain in the response regulator (red rectangle). (B) The B. subtilis Spo0 pathway is a phosphorelay. Signal transduction is initiated by activation of one of the five sensor histidine kinases that are associated with this pathway. The phosphoryl group is transferred from the HisKA domain in the kinase to the Spo0F REC domain (blue rectangle), then to the phosphotransferase Spo0B (orange oval), and finally to the REC domain of Spo0A (green rectangle). Spo0F lacks an output domain; Spo0A has the domain architecture of a typical response regulator, including a REC domain and a DNA-binding domain. (C) The C. acetobutylicum Spo0 pathway has a direct phosphorylation architecture, wherein multiple sporulation kinases are capable of direct transfer of a phosphoryl group to Spo0A.