A cascade of protein phosphorylation, initiated by autophosphorylation of the CheA protein, may be important in the signal transduction pathway of bacterial chemotaxis. A proteolytic fragment of CheA cannot autophosphorylate, but can still transfer phosphate to proteins that generate excitation and adaptation signals. The site of CheA phosphorylation is His 48; mutants altered at this position are non-chemotactic. Similar mechanisms of transient protein phosphorylation and phosphoryl group transfer seem to be involved in processing sensory data and in activating specific gene expression.