Two-component and phosphorelay signal transduction systems are the major me
ans by which bacteria recognize and respond to a variety of environmental s
timuli. Recent results have implicated these systems in the regulation of a
variety of essential processes including cell-cycle progression, pathogeni
city, and developmental pathways. Elucidation of the structures of the inte
racting domains is leading to an understanding of the mechanisms of molecul
ar recognition and phosphotransfer in these systems.