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Chapter 16 : The General Stress Response in Alphaproteobacteria
Category: Microbial Genetics and Molecular Biology
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Extensive studies with the model organisms Escherichia coli and Bacillus subtilis have led to the identification of two regulatory networks controlling general stress response. Genes encoding histidine kinases are found in the vicinity of phyR in several Alphaproteobacteria. However, the involvement of one of these histidine kinases in the PhyR cascade has not been demonstrated so far. Francez-Charlot et al. proposed that σEcfG or several members of the σEcfG family are responsible for transcription of stress-related genes. In several Alphaproteobacteria, a gene encoding a histidine kinase is found at the phyR locus. Signal perception in the σS and σB regulatory cascades has been described as highly complex, as a result of the necessity to integrate multiple signals; such complexity can be expected in Alphaproteobacteria too and may reflect an adaptation to the various environments in which the organisms live. In Rhizobium etli, the RpoE2/ σT/σEcfG homolog, RpoE4, was shown to be involved in oxidative and osmotic stresses. Genes encoding catalase, the DNA protection protein Dps or DNA repair enzymes, known to be crucial for σS-dependent resistance to oxidative stress, are controlled by the cascade in several species. All regulons contain several regulators whose functions are mainly unknown, such as kinases and response regulators of His-Asp phosphorelays, onecomponent systems, and sigma factors, such as RpoH.
(A) Conservation of the phyR - nepR - ecfG locus in selected members of the Alphaproteobacteria. Adapted from Gourion et al. ( 2008 ).
(B) Multiple sequence alignment of PhyR (top) and σEcfG (bottom) homologs. Conserved residues are highlighted in black (identical residues) and gray (similar residues). The regions important for σ factor function are indicated above. (B) Francez-Charlot et al. ( 2009 ).
Model of the core cascade controlling general stress response in Alphaproteobacteria. Under nonstressed conditions (left box) σEcfG is inhibited by the anti-sigma factor NepR. Upon sensing of a stress stimulus (right box) the corresponding histidine kinase activates PhyR by phosphorylation of the conserved aspartate residue in the C-terminal receiver domain (a kinase activating PhyR has not yet been identified experimentally; however, genes encoding histidine kinases are often genetically linked to phyR in Alphaproteobacteria). In its phosphorylated state, PhyR is able to sequester the anti-sigma factor NepR. This partner-switching mechanism allows the release of the ECF sigma factor σEcfG, leading to the activation of stress genes.
phyR and ecfG homologs present in the genomes of selected Alphaproteobacteria
Overview of stress-related target genes of the PhyR/NepR/σEcfG cascade