Two-component phosphotransfer schemes
Top: A basic prokaryotic two-component system with a sensor histidine kinase and a response regulator. H and D represent the conserved phospho-accepting histidine and aspartate residues involved in phosphorelay signaling.
Bottom: A multistep phosphorelay system involving a hybrid sensor kinase, with input, transmitter and receiver domains, a histidine-containing phosphotransfer protein and a response regulator.