The general stress response is regulated by the rpoS, a gene that encodes the aS in E. coli and other bacteria such as Shigella flexneri and Salmonella enterica serovar Typhimurium [15-17]. Although the regulation of the general stress response has been studied in a variety of microorganisms, the regulation mechanisms covered in this section will refer to E. coli, an organism in which these mechanisms have been well characterized. During rapid growth, microbial cells, not exposed to any particular stress, have hardly detectable levels of aS. Exposure of these cells to stress (e.g., entry into stationary phase, high osmolarity, high or low temperature) results in rapid aS accumulation to high levels, and subsequent expression of more than 50 genes involved in stress adaptation . The regulation of rpoS, which determines the cellular concentration of aS, occurs at multiple levels, including transcription, translation, and post-translational modifications (i.e., aS proteolysis), with the level of control being dependent on the type of stress affecting the cells [15,16,18]. In general, sudden exposure of bacteria to lethal stresses, which requires a rapid response (i.e., a shocking stress), involves aS
proteolysis-mediated regulation, while gradual exposure to stress usually requires stimulation of rpoS expression at the transcription or translation level [16,19].
Enhanced cellular accumulation of aS occurs during microbial growth in rich media, while cells are transitioning from late exponential phase to stationary phase [15,19]. At the transcriptional level, the two-component system, cAMP and its receptor protein, the catabolite regulatory protein (CRP), act as negative regulators of rpoS. Conversely, small molecules such as guanosine-3',5'-bispyrophosphate (ppGpp), homoserine lactone, and polyphosphate may enhance rpoS transcription [16,18]. Translational control involves a series of complex mechanisms in which stress conditions such as high osmolarity, low temperature, or entry into late exponential phase stimulate the translation of rpoS mRNA . It has been suggested that these stresses can play an important role in stabilizing the mRNA secondary structure, allowing its accessibility to ribosomes, and therefore enhancing its translation . Activation of rpoS mRNA translation requires the presence of Hfq, a small mRNA binding-protein that stabilizes the secondary structure of the polynucleotide. Translation of rpoS can also be enhanced by the stabilization of the mRNA with a small RNA fragment (DsrA RNA) in cells stressed by temperature downshifts . Control at the post-translational level involves regulation of the sigma-factor proteolysis rate. In cells growing exponentially, the levels of aS are very low because of its continuous proteolysis. Sudden stresses, including carbon starvation, shift to low pH, high temperature, and high osmolarity, prevent aS proteolysis and permit its accumulation in the cells to trigger the general stress response. Proteolysis of aS requires ClpXP protease, which is regulated by the RssB protein. The level of phosphorylation or dephosphorylation of RssB, influenced by the stresses already mentioned, determines its affinity for aS and the subsequent recognition of the aS-RssB complex by the ClpXP protease [13,16].
The activation of the general stress response, mediated by aS, results in the expression of stress-adaptive genes, including bolA (involved in controlling cell morphology), cfa (involved in cyclopropane fatty acid synthesis), uspB (involved in ethanol resistance), and katE and katG (encoding catalases), among many others [15,20]. Sensitivity of bacteria, defective in the rpoS gene, to a series of stresses such as heat shock, oxidative environment, starvation, acid, ethanol, and ultraviolet radiation provides additional, and indisputable, evidence of the role of aS in the control of the general stress response [21,22].
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