The role of the stringent response and Spx in stress response and thermotolerance development

Abstract

All organisms are frequently exposed to changing environmental conditions in their natural habitat, which can impose stress and threatens basic biological functions. To cope with such adverse conditions, cells have developed complex stress response systems which allow the sensing and integration of envi- ronmental stress signals and the regulation of appropriate responses. For example, a sudden temperature upshift and other proteotoxic conditions which facilitate the misfolding and aggregation of cellular pro- teins, activate the heat shock response. This global transcriptional response mediates a concerted up- regulation and accumulation of conserved chaperones and proteases, the protein quality control system, to restore and maintain protein homeostasis during stress. Another fast-acting bacterial stress response program, the stringent response, is activated upon amino acid starvation and many other stress signals. It is regulated by the second messenger nucleotide (p)ppGpp, which mediates the transcriptional repres- sion of ribosomal genes while activating stress response- and amino acid synthesis genes, but also causes the inhibition of translation, replication and interferes with other processes. In this thesis, the heat shock response of the Gram positive model organism Bacillus subtilis was studied. It could be demonstrated that the transcriptional regulator Spx is not only a central regulator of many heat shock response genes, but can also participate in the transcriptional down-regulation of rRNA and ribosomal protein genes, which were observed to be strongly down-regulated during many stress conditions. In addition, it could be demonstrated, that the stringent response mediated by (p)ppGpp is activated during the heat shock response. Increased (p)ppGpp levels conferred elevated heat stress re- sistance while the lack of (p)ppGpp renders cells more sensitive to stress. Remarkably, it appears that both (p)ppGpp and Spx are concurrently involved in the down-regulation of rRNA genes during heat stress. Furthermore, the results suggest that (p)ppGpp is involved in direct adjustments of translation during stress, which appears to be crucial for the protective role of the stringent response in the heat stress response. Together, the results suggest a model by which the heat shock response of B. subtilis not only involves the synthesis and accumulation of chaperones and proteases of the protein quality control system but also the concurrent curbing of the protein synthesis rate by (p)ppGpp to support protein homeostasis by reducing the load for the cellular protein quality control system.