Figure 1.

Schematic representation of the two-component and the multistep phosphorelay signaling systems. A, The prototypical two-component pathway uses a single phosphoryl transfer event between a His protein kinase and its cognate response regulator. B, The multistep His-to-Asp phosphorelay system in which a His-containing phosphotransfer protein serves as a phosphoryl acceptor and donor between the hybrid protein kinase and the response regulator. In yeast (Saccharomyces cerevisiae) osmosensing, the phosphorelay connects to an MAPK cascade (Wurgler-Murphy and Saito, 1997). In Arabidopsis cytokinin signaling, a response regulator directly regulates its target gene expression (Hwang and Sheen, 2001). The vertical bars represent transmembrane domains. H, His; D, Asp; P, phosphoryl group.