Activation of the PhoPR-Mediated Response to Phosphate Limitation Is Regulated by Wall Teichoic Acid Metabolism in Bacillus subtilis

Abstract

Phosphorous is essential for cell viability. To ensure an adequate supply under phosphate limiting conditions, bacteria induce a cohort of enzymes to scavenge for phosphate, and a high affinity transporter for its uptake into the cell. This response is controlled by a two-component signal transduction system named PhoBR in Escherichia coli and PhoPR in Bacillus subtilis. PhoR is a sensor kinase whose activity is responsive to phosphate availability. Under phosphate limiting conditions, PhoR exists in kinase mode that phosphorylates its cognate response regulator (PhoB, PhoP). When activated, PhoB∼P/PhoP∼P execute changes in gene expression that adapt cells to the phosphate limited state. Under phosphate replete conditions, PhoR exists in phosphatase mode that maintains PhoB/PhoP in an inactive, non-phosphorylated state. The mechanism by which phosphate availability is sensed and how it controls the balance between PhoR kinase and phosphatase activities has been studied in E. coli and B. subtilis. Two different mechanisms have emerged. In the most common mechanism, PhoR activity is responsive to phosphate transport through a PstSCAB/PhoU signaling complex that relays the conformational status of the transporter to PhoR. In the second mechanism currently confined to B. subtilis, PhoR activity is responsive to wall teichoic acid metabolism whereby biosynthetic intermediates can promote or inhibit PhoR autokinase activity. Variations of both mechanisms are found that allow each bacterial species to adapt to phosphate availability in their particular environmental niche.

Keywords: PhoBR Escherichia coli; PhoPR Bacillus subtilis; control of PhoR activity; phosphate limitation; phosphate transport mechanism; wall teichoic acid metabolism mechanism.

FIGURE 1

A model for controlling the balance between PhoR autokinase and phosphatase activities in E. coli generated by conformational changes in the PstSCAB phosphate transporter. The PhoR protein can exist in either autokinase mode during conditions of phosphate limitation (A) or in phosphatase mode during conditions of phosphate sufficiency (B). It is proposed that these alternate states are determined by different conformations of the PstSCAB transporter that are relayed to PhoR by PhoU to determine its activity. When PstSCAB is in a closed outward facing conformation (A) the PhoU adaptor protein is either unable to interact with PhoR (PhoU solid lines), or interacts with the PAS domain of PhoR to promote autokinase activity (PhoU broken lines) during conditions of phosphate limitation. The cognate PhoB response regulator is phosphorylated in this condition. However, when PstSCAB is in an open inward facing conformation (B) the PhoU adaptor protein interacts with the PAS domain of PhoR to promote phosphatase activity during conditions of phosphate sufficiency. The cognate PhoB response regulator is dephosphorylated in this condition. This diagram is an adaptation of the models and figures presented in Hsieh and Wanner (2010) and Vuppada et al. (2018).

FIGURE 2

A model for controlling the balance between PhoR autokinase and phosphatase activities in B. subtilis and B. spizizenii generated by intermediates in wall teichoic acid metabolism. In B. subtilis and B. spizizenii it is proposed that autokinase and phosphatase modes of PhoR activity are determined by WTA biosynthetic intermediates levels. (A) The metabolic pathway for WTA biosynthesis in B. spizizenii is shown with the final Lipid V product being composed of poly(ribitol phosphate) attached to a lipid carrier (jagged lines) by a linker composed of pyrophosphate, modified sugars (GlcNAc, ManNAc) and a single glycerol phosphate moiety. The genomic organization of the biosynthetic genes is shown in the box. It is proposed that the TarA or TarB enzyme product activates PhoR autokinase activity (arrows with broken lines to indicate a putative interaction). The level of activating intermediate will be determined by the balance between WTA and teichuronic acid synthesis in phosphate limited cells because of the reciprocal relationship between these two metabolic pathways. (B) The metabolic pathway for WTA biosynthesis in B. subtilis is shown with the final Lipid V product being composed of poly(glycerol phosphate). The genomic organization of the biosynthetic genes (in box) shows that the tagAB operon is repressed by activated PhoP∼P (-). It is proposed that the TagA or TagB enzyme product activates PhoR autokinase activity (arrows with broken lines). However, in B. subtilis Lipid V composed of poly(glycerol phosphate) can inhibit PhoR autokinase activity (shown in vitro, blue line with block). Thus the balance between PhoR autokinase and phosphatase activities will be determined by the level of the activating intermediates (TagA or TagB, black arrows), by the extent to which teichuronic acid synthesis is activated and WTA synthesis is repressed by PhoP∼P and by the level of the inhibitory Lipid V composed of poly(glycerol phosphate), blue line with block).