Starvation-inducible loci of Salmonella typhimurium: regulation and roles in starvation-survival

Four starvation-inducible loci (stiA, stiB, stiC, and stiE) of Salmonella typhimurium have been extensively characterized as to their genetic and physiologic regulation, and their roles in survival during prolonged simultaneous phosphate (P)-, carbon (C)- and nitrogen (N)-starvation (PCN-starvation). Strains of S. typhimurium LT-2, isogenic with the exception of lacking either the stiA, stiB or stiC locus, died off more quickly and survived at much reduced levels compared with their wild-type parent. When certain sti mutations were combined in the same strain, we found that viability of these cultures declined even more rapidly, and starvation-survival was affected to levels over-and-above the additive effects of each individual mutation, indicating an epistatic relationship between these loci. All four sti loci were, directly or indirectly, under negative control by the crp gene product (cAMP receptor protein, CRP). With the exception of stiB, all were similarly regulated by the cya gene product (i.e., cAMP). This suggests that CRP acts alone, or with a signal molecule other than cAMP, to cause repression of the stiB locus. In addition, all four loci are under positive regulation by the relA gene product (i.e., ppGpp) during C- or N-starvation, but not P-starvation. Since not all relA-dependent sti loci are induced during both C- and N-starvation, we propose that two separate ppGpp-dependent pathways function during C-starvation and N-starvation, respectively. Possible models for separate P-, C- and N-starvation-induction pathways are discussed.     

Cycles of famine and feast: the starvation and outgrowth strategies of a marineVibrio

Studies of starvation survival in non-differentiating bacteria have largely focused on physiological changes and regulatory aspects of a few master regulators such as the signal molecule ppGpp and the stationary phase alternative sigma factor, sigma S. Recent findings have implicated a series of novel key events for the entry as well as exit from starvation. The importance of alternative sigma factors other than sigma S is emerging. In addition, low molecular weight extracellular signals have been demonstrated to be essential for the induction and mediation of several adaptive responses. The importance of mRNA modification and stability for starvation survival as well as outgrowth is receiving renewedinterest. In this paper, we present the results obtained from studies of starvation survival and recovery ofVibrio sp. strain S14.     

Stress induced cross-protection against environmental challenges on prokaryotic and eukaryotic microbes

Prokaryotic and eukaryotic microbes thrive successfully in stressful environments such as high osmolarity, acidic or alkali, solar heat and u.v. radiation, nutrient starvation, oxidative stress, and several others. To live under these continuous stress conditions, these microbes must have mechanisms to protect their proteins, membranes, and nucleic acids, as well as other mechanisms that repair nucleic acids. The stress responses in bacteria are controlled by master regulators, which include alternative sigma factors, such as RpoS and RpoH. The sigma factor RpoS integrates multiple signals, such as the general stress response regulators and the sigma factor RpoH regulates the heat shock proteins. These response pathways extensively overlap and are induced to various extents by the same environmental stresses. In eukaryotes, two major pathways regulate the stress responses: stress proteins, termed heat shock proteins (HSP), which appear to be required only for growth during moderate stress, and stress response elements (STRE), which are induced by different stress conditions and these elements result in the acquisition of a tolerant state towards any stress condition. In this review, the mechanisms of stress resistance between prokaryotic and eukaryotic microbes will be described and compared.     

Glucose starvation stimulon of Escherichia coli: role of integration host factor in starvation survival and growth phase-dependent protein synthesis.

The DNA-binding protein IHF was found to be required for starvation survival and for the induction of 14 proteins of the glucose starvation stimulon. Many of these proteins have been shown previously to be general responders to diverse stress conditions. Overexpression of IHF during balanced growth was not sufficient to induce these proteins, but it resulted in an increased synthesis of rpoH-dependent heat shock proteins.     

The alternative sigma factor sigmaE controls antioxidant defences required for Salmonella virulence and stationary-phase survival

Bacteria must contend with conditions of nutrient limitation in all natural environments. Complex programmes of gene expression, controlled in part by the alternative sigma factors sigmaS (sigma38, RpoS) and sigmaH (sigma32, RpoH), allow a number of bacterial species to survive conditions of partial or complete starvation. We show here that the alternative sigma factor sigmaE (sigma24, RpoE) also facilitates the survival of Salmonella typhimurium under conditions of nutrient deprivation. Expression of the sigmaE regulon is strongly induced upon entry of Salmonella into stationary phase. A Salmonella mutant lacking sigmaE has reduced survival during stationary phase as well as increased susceptibility to oxidative stress. A Salmonella strain lacking both sigmaE and sigmaS is non-viable after just 24 h in stationary phase, but survival of these mutants is completely preserved under anaerobic stationary-phase conditions, suggesting that oxidative injury is one of the major mechanisms of reduced microbial viability during periods of nutrient deprivation. Moreover, the attenuated virulence of sigmaE-deficient Salmonella for mice can be largely restored by genetic abrogation of the host phagocyte respiratory burst, suggesting that the sigmaE regulon plays an important antioxidant role during Salmonella infection of mammalian hosts.     

Role of RpoH, a heat shock regulator protein, in Escherichia coli carbon starvation protein synthesis and survival.

Escherichia coli starvation proteins include several heat shock proteins whose induction by heat is controlled by the minor sigma factor, sigma 32. The level of sigma 32 increased in wild-type E. coli upon starvation, and three sigma 32-controlled heat shock proteins (DnaK, GroEL, and HtpG) were not induced during starvation in an isogenic delta rpoH strain, which is unable to synthesize sigma 32. Thus, sigma 32 plays a role in the induction of these proteins during both heat shock and starvation. The delta rpoH strain was more sensitive to starvation but could develop starvation-mediated cross protection against heat and oxidation.     

Regulation of RpoS by a novel small RNA: the characterization of RprA

Translational regulation of the stationary phase sigma factor RpoS is mediated by the formation of a double-stranded RNA stem-loop structure in the upstream region of the rpoS messenger RNA, occluding the translation initiation site. The interaction of the rpoS mRNA with a small RNA, DsrA, disrupts the double-strand pairing and allows high levels of translation initiation. We screened a multicopy library of Escherichia coli DNA fragments for novel activators of RpoS translation when DsrA is absent. Clones carrying rprA (RpoS regulator RNA) increased the translation of RpoS. The rprA gene encodes a 106 nucleotide regulatory RNA. As with DsrA, RprA is predicted to form three stem-loops and is highly conserved in Salmonella and Klebsiella species. Thus, at least two small RNAs, DsrA and RprA, participate in the positive regulation of RpoS translation. Unlike DsrA, RprA does not have an extensive region of complementarity to the RpoS leader, leaving its mechanism of action unclear. RprA is non-essential. Mutations in the gene interfere with the induction of RpoS after osmotic shock when DsrA is absent, demonstrating a physiological role for RprA. The existence of two very different small RNA regulators of RpoS translation suggests that such additional regulatory RNAs are likely to exist, both for regulation of RpoS and for regulation of other important cellular components.     

H-NS regulation of IraD and IraM antiadaptors for control of RpoS degradation

RpoS, the master sigma factor during stationary phase and under a variety of stress conditions, is regulated at multiple levels, including regulated degradation. Degradation is dependent upon ClpXP and the RssB adaptor protein. H-NS, a nucleoid-associated protein, affects the regulated degradation of RpoS; in the absence of H-NS, RpoS is stable. The mechanisms involved in this regulation were not known. We have found that H-NS inhibits the expression of iraD and iraM, the genes coding for two antiadaptor proteins that stabilize RpoS when overexpressed. The regulation by H-NS of iraM is independent from the previously demonstrated regulation by the PhoP/PhoQ two-component system. Moreover, differences in the behavior of several hns alleles are explained by a role for StpA, an H-NS-like protein, in the regulation of RpoS stability. This finding parallels recent observations for a role of StpA in regulation of RpoS stability in Salmonella.