Introduction

Prior to 1986 it was assumed that protein phosphorylation played a relatively minor role in controlling the response of prokaryotes to environmental stimuli. Although many phosphoproteins had been detected by gel electrophoresis and a number of protein kinases had been purified, the physiological relevance of phosphorylation was not evident (Cozzone, 1984). The discovery of a role for phosphorylation in the response of bacteria to nitrogen status provided the first example of a defined stimulus–response pathway in which protein activity is controlled reversibly by phosphorylation and dephosphorylation (Ninfa & Magasanik, 1986). In the nitrogen regulation system two protein components are involved; one protein is a ‘sensor’ which has protein kinase activity and the other protein is a ‘regulator’ whose activity is responsive to phosphorylation. This pattern of ‘two-component regulation’ is surprisingly well conserved and serves to regulate a range of diverse processes in prokaryotes.

Bacteria are responsive to a wide variety of environmental signals including changes in nutrient concentration, osmolarity, temperature and the presence of other organisms. When DNA sequences of several genes involved in different signal transduction pathways were determined, it became obvious that many examples were representative of two families of proteins sharing amino acid similarities in common with the ‘sensor’ and ‘regulator’ proteins identified in the nitrogen regulation system (Nixon, Ronson & Ausubel, 1986; Winans et al., 1986; Ronson, Nixon & Ausubel, 1987b; Drummond & Wootton, 1987).