In Bacillus subtilis the Pho regulon is controlled by a sensor and regulator protein pair, PhoR and PhoP, that respond to phosphate concentrations. To facilitate studies of the Pho regulon, a strain with an altered PhoR protein was isolated by in vitro mutagenesis. The mutation in this strain (phoR12) leads to the production of a PhoR sensor kinase that, unlike the wild-type, is functionally active in phosphate-replete conditions. The lesion in phoRl2 was shown to be a single base change that results in an Arg to Ser substitution in a region of PhoR that is highly conserved in histidine sensor kinases. While a phoR-negative mutant was unable to induce the synthesis of cell wall teichuronic acid under phosphate-limited conditions, the phoR72 mutant showed a relative increase in teichuronic acid and a decrease in teichoic acid, even under phosphate-replete conditions. The latter suggests that some or all of the genes required for teichuronic acid synthesis are members of the Pho reg ulon.Keywords : phosphate stress, histidine sensor kinase, response regulator, alkaline phosphatase, signal transduction INTRODUCTIONPhosphorus is a major growth-limiting substrate in natural environments such as soil (Hulett, 1993). When the growth of Bacillus subtilis becomes limited by the availability of phosphorus, a programmed series of responses is induced that can ultimately lead to sporulation. Among the earliest of these responses are processes that reduce the cell's requirement for phosphorus, increase its affinity for the uptake of phosphate and lead to the production of extracellular hydrolytic enzymes that allow the cell to recover inorganic phosphate from organic sources such as teichoic acids (Grant, 1979;Eymann et al., 1996). If these conservation and scavenging mechanisms are effective, the developmental pathway leading to sporulation is reversed and the organism resumes vegetative growth. B. subtilis responds to phosphate stress by the coordinated induction of approximately 20 genes (Eymann et al., 1996). Prominent among these are genes of the Pho regulon (Seki et al., 1987;Hulett, 1993; Hulett et al., 1994a, b) , whose products include alkaline phosphatases (APase) , alkaline phosphodiesterase (APDase) , a highaffinity phosphate transporter and, as reported in this paper, proteins involved in teichuronic acid synthesis. a two-component, environment-sensing, signal-transduction system (Seki et al., 1987(Seki et al., ,1988. The components of this system, PhoP and PhoR, are equivalent, respectively, to the PhoB and PhoR proteins of Escherichia coli (Tommassen etal., 1982;Makino et al., 1985Makino et al., ,1986. In E. coli, PhoR is a membrane-spanning sensor protein with histidine protein kinase (HPKase) and phosphatase (PTase) activities Scholten & Tommassen, 1993). Activation at low phosphate concentrations leads to the production of PhoR -P which, in turn, acts as a substrate for the activation of the response regulator PhoB, required for the induction of the Pho regulon (Stock et al., 1989;Parkinson & Kofoid, 199...