Bacillus subtilis responds to phosphate starvation stress by inducing the PhoP and SigB regulons. While the PhoP regulon provides a specific response to phosphate starvation stress, maximizing the acquisition of phosphate (P i ) from the environment and reducing the cellular requirement for this essential nutrient, the SigB regulon provides nonspecific resistance to stress by protecting essential cellular components, such as DNA and membranes. We have characterized the phosphate starvation stress response of B. subtilis at a genome-wide level using DNA macroarrays. A combination of outlier and cluster analyses identified putative new members of the PhoP regulon, namely, yfkN (2,3 cyclic nucleotide 2-phosphodiesterase), yurI (RNase), yjdB (unknown), and vpr (extracellular serine protease). YurI is thought to be responsible for the nonspecific degradation of RNA, while the activity of YfkN on various nucleotide phosphates suggests that it could act on substrates liberated by YurI, which produces 3 or 5 phosphoribonucleotides. The putative new PhoP regulon members are either known or predicted to be secreted and are likely to be important for the recovery of inorganic phosphate from a variety of organic sources of phosphate in the environment.When Bacillus subtilis encounters phosphate starvation stress, it responds by inducing groups of genes that function to restrict the metabolic consequences of the limited supply of this essential nutrient. These groups of genes are collectively referred to as the phosphate (Pho) stimulon. The phosphate stimulon includes at least two well-described regulons, namely, the sigma B ( B ) general stress regulon and the phosphate starvation-specific PhoP regulon. When B. subtilis encounters phosphate starvation, genes of the SigB regulon are induced by the alternative sigma factor, B , and genes of the PhoP regulon are either induced or repressed by activated PhoP (namely, PhoPϳP). The B general stress regulon contains Ͼ100 genes (58, 64). These genes provide a nonspecific response to stress by encoding proteins that protect the DNA, membranes, and proteins from the damaging effects of stress. Proteins induced by B help the cell to survive potentially harmful environmental conditions, such as heat, osmotic, acid, or alkaline shock (6,21,23,26). This protective function is thought to be particularly important in maintaining the viability of nongrowing cells.The PhoP regulon currently consists of 34 members. Six operons (phoPR [56,60], phoB-ydhF [7,14], pstSAC-pstBApstBB [3,67], phoD-tatAD [7,19], resABCDE [10], and tuaABC DEFGH [40,72]) and five monocistronic genes (glpQ [7], phoA [30,31], tatCD [34], ykoL [60], and yttP [62]) are induced and two operons (tagAB and tagDEF (39) are repressed in response to phosphate starvation. phoA and phoB encode alkaline phosphatases (APases) which facilitate the recovery of inorganic phosphate (P i ) from organic sources (11, 30); phoD encodes a phosphodiesterase/APase, putatively involved in cell wall teichoic acid turnover, and is secreted exclus...

When Bacillus subtilis is subjected to phosphate starvation, the Pho regulon is activated by the PhoP-PhoR two-component signal transduction system to elicit specific responses to this nutrient limitation. The response regulator, PhoP, and its cognate histidine sensor kinase, PhoR, are encoded by the phoPR operon that is transcribed as a 2.7-kb bicistronic mRNA. The phoPR operon is transcribed from two A -dependent promoters, P 1 and P 2 . Under conditions where the Pho regulon was not induced (i.e., phosphate-replete conditions or phoR-null mutant), a low level of phoPR transcription was detected only from promoter P 1 . During phosphate starvation-induced transition from exponential to stationary phase, the expression of the phoPR operon was up-regulated in a phosphorylated PhoP (PhoPϳP)-dependent manner; in addition to P 1 , the P 2 promoter becomes active. In vitro gel shift assays and DNase I footprinting experiments showed that both PhoP and PhoPϳP could bind to the control region of the phoPR operon. The data indicate that while low-level constitutive expression of phoPR is required under phosphate-replete conditions for signal perception and transduction, autoinduction is required to provide sufficient PhoPϳP to induce other members of the Pho regulon. The extent to which promoters P 1 and P 2 are activated appears to be influenced by the presence of other sigma factors, possibly the result of sigma factor competition. For example, phoPR is hyperinduced in a sigB mutant and, later in stationary phase, in sigH, sigF, and sigE mutants. The data point to a complex regulatory network in which other stress responses and post-exponential-phase processes influence the expression of phoPR and, thereby, the magnitude of the Pho regulon response.

During phosphate starvation, Bacillus subtilis regulates genes in the PhoP regulon to reduce the cell's requirement for this essential substrate and to facilitate the recovery of inorganic phosphate from organic sources such as teichoic and nucleic acids. Among the proteins that are highly induced under these conditions is PstS, the phosphate-binding lipoprotein component of a high-affinity ABC-type phosphate transporter. PstS is encoded by the first gene in the pst operon, the other four members of which encode the integral membrane and cytoplasmic components of the transporter. The transcription of the pst operon was analysed using a combination of methods, including transcriptional reporter gene technology, Northern blotting and DNA arrays. It is shown that the primary transcript of the pst operon is processed differentially to maintain higher concentrations of PstS relative to other components of the transporter. The comparative studies have revealed limitations in the use of reporter gene technology for analysing the transcription of operons in which the messenger RNA transcript is differentially processed.

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