MINIREVIEW Keeping Signals Straight in Phosphorelay Signal Transduction†

Bacteria are highly adaptable organisms capable of growth on countless carbon and nitrogen sources and of occupying an inexhaustible variety of ecological niches. Any particular bac-terium possesses a subset of these capabilities that are encoded by a repertoire of genes normally kept unexpressed unless called upon. The key to adaptability in bacteria is their capacity to express only those genes for enzymes and pathways that they need for maximal growth in the environment in which they find themselves. This is achieved by their ability to recognize the composition of their environment by sensing signals emanating from it. One of the major mechanisms of signal recognition leading to specific gene expression is the two-component system and its more-complex variant, the phosphorelay (Fig. 1). Two-compo-nent systems consist of a signal recognition sensor kinase tha