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Chapter 17 : General Stress Response in Bacillus subtilis and Related Gram-Positive Bacteria
Category: Microbial Genetics and Molecular Biology
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This chapter focuses on the partner-switching mechanism, showing how signaling versatility is achieved and outlining the questions about network function that remain to be answered. A short section at the end of the chapter summarizes the roles of general stress regulons in the physiology of gram-positive organisms that possess sB, or σB-like factors. In Bacillus subtilis, RsbW has two activities in unstressed cells: (i) as an anti-sigma factor it binds σB and prevents its association with RNA polymerase; and (ii) as a serine kinase it specifically phosphorylates and inactivates its own antagonist, the RsbV anti- anti-sigma. Three single-domain anti-anti- sigma factors of Mycobacterium tuberculosis have atypical N-terminal extensions rich in serine and threonine residues, allowing their interactions to be controlled by a unique mechanism that may affect σF activity. In many gram-positive bacteria σB is a master regulator of a general stress modulon that closely interacts with other global systems. Investigation of the roles of genes in the modulon can provide a unique perspective regarding the physiological changes that promote stress resistance. How σB signaling networks sense activating stresses remains an open question. Given the wide distribution of the recognized sensing modules and their association with different output domains, this question is relevant to many signaling pathways.
σB regulatory network in B. subtilis. (A) Genes encoding the principal network regulators are organized in two operons. Energy regulators (RsbQ and P) are indicated by light stippling; environmental regulators (RsbRA, S, T, U, and X) by dark; and common regulators (RsbV and W) by white. Promoters are denoted by the holoenzyme known (or likely) to recognize them. Other RsbR paralogs and the YtvA blue light sensor are encoded in scattered transcriptional units (not shown). (B) Model of the signaling network. Energy and environmental pathways converge on RsbV and RsbW, which directly regulate σB activity (see text). Horizontal arrows show conversion between RsbV and RsbV-P (with phosphate as stippled P). Full arrowheads indicate activating effects and T-headed lines inhibiting ones. PAS and N denote the regulatory domains of RsbP and RsbU, respectively. A cold stress input that bypasses these established pathways is less well understood. (C) Model of the environmental pathway that activates the RsbU phosphatase. The stressosome complex is represented by the RsbRA co-antagonist and RsbS antagonist. RsbRB, RC, RD, and YtvA are also present within the complex (not shown). Symbols are as in panel B.
Model of energy-stress signaling. The RsbP phosphatase and RsbW kinase provide two potential inputs to regulate the energy stress response (see text). Domains shown are involved in signaling; conversion, activating, and inhibiting symbols are as per Fig. 1 legend. RsbP input: the labels A and A’ indicate the hypothetical small molecule substrate and product of the RsbQ hydrolase; A’ is thought to bind the PAS domain of RsbP. PAS then activates the PP2C phosphatase domain by countering the negative effect of the coiled-coil. RsbW input: kinase activity increases in energy sufficient cells and decreases in starved cells, leading to diminished phosphorylation of the STAS domain of RsbV.
Model of environmental-stress signaling. RsbRA and RsbS form the core of the stressosome complex, which binds the RsbT kinase in an inactive state. For simplicity, the functional RsbRA dimer is represented here as a monomer, with its N-terminal nonheme globin domain (RNTD) and C-terminal STAS domain (RCTD) joined via a 13-residue α-helical linker (solid line). The smaller RsbS (S) has only a STAS domain, which directly binds RsbT (T). In this model, a stress signal results in a structural perturbation within RNTD. The perturbation is communicated via the α-helical linker to RCTD and thence to the adjacent S. These structural changes in the STAS domains of R and S allow their phosphorylation by T, which is then released to activate the RsbU phosphatase. In vivo the core of each stressosome consists of a mix of the RsbR family members RsbRA, RB, RC, RD, and YtvA, together with RsbS.
Comparison of sigB operons and functionally associated regulatory factors in gram-positive bacteria. Promoters are indicated by the holoenzyme form known or proposed to recognize them. Lm, L. monocytogenes; Sa, S. aureus; Bc, B. cereus; Mt, M. tuberculosis; Sc, S. coelicolor. The Lm operon is organized like its B. subtilis counterpart; dark stippling indicates known or suspected environmental regulators. Lm has additional RsbR paralogs and a YtvA blue light ortholog encoded elsewhere on the genome (not shown); these are presumed to form a stressosome complex with RsbRA and RsbS. Mt and Sc likewise have numerous potential regulators, but only those tested for a direct effect on sigma activity are included here (see text).