Modulation of the heat shock response by one-carbon metabolism in Escherichia coli

Regulation of Serine, Glycine, and One-Carbon Biosynthesis

The biosynthesis of serine, glycine, and one-carbon (C1) units constitutes a major metabolic pathway in Escherichia coli and Salmonella enterica serovar Typhimurium. C1 units derived from serine and glycine are used in the synthesis of purines, histidine, thymine, pantothenate, and methionine and in the formylation of the aminoacylated initiator fMet-TRNAfMet used to start translation in E. coli and serovar Typhimurium. The need for serine, glycine, and C1 units in many cellular functions makes it necessary for the genes encoding enzymes for their synthesis to be carefully regulated to meet the changing demands of the cell for these intermediates. This review discusses the regulation of the following genes: serA, serB, and serC; gly gene; gcvTHP operon; lpdA; gcvA and gcvR; and gcvB genes. Threonine utilization (the Tut cycle) constitutes a secondary pathway for serine and glycine biosynthesis. L-Serine inhibits the growth of E. coli cells in GM medium, and isoleucine releases this growth inhibition. The E. coli glycine transport system (Cyc) has been shown to transport glycine, D-alanine, D-serine, and the antibiotic D-cycloserine. Transport systems often play roles in the regulation of gene expression, by transporting effector molecules into the cell, where they are sensed by soluble or membrane-bound regulatory proteins.

Mechanism of protonophores-mediated induction of heat-shock response in Escherichia coli

Background

Protonophores are the agents that dissipate the proton-motive-force (PMF) across E. coli plasma membrane. As the PMF is known to be an energy source for the translocation of membrane and periplasmic proteins after their initial syntheses in cell cytoplasm, protonophores therefore inhibit the translocation phenomenon. In addition, protonophores also induce heat-shock-like stress response in E. coli cell. In this study, our motivation was to investigate that how the protonophores-mediated phenomena like inhibition of protein translocation and induction of heat-shock proteins in E. coli were correlated.

Results

Induction of heat-shock-like response in E. coli attained the maximum level after about 20 minutes of cell growth in the presence of a protonophore like carbonyl cyanide m-chloro phenylhydrazone (CCCP) or 2, 4-dinitrophenol (DNP). With induction, cellular level of the heat-shock regulator protein sigma-32 also increased. The increase in sigma-32 level was resulted solely from its stabilization, not from its increased synthesis. On the other hand, the protonophores inhibited the translocation of the periplasmic protein alkaline phosphatase (AP), resulting its accumulation in cell cytosol partly in aggregated and partly in dispersed form. On further cell growth, after withdrawal of the protonophores, the previously accumulated AP could not be translocated out; instead the AP-aggregate had been degraded perhaps by an induced heat-shock protease ClpP. Moreover, the non-translocated AP formed binary complex with the induced heat-shock chaperone DnaK and the excess cellular concentration of DnaK disallowed the induction of heat-shock response by the protonophores.

Conclusion

Our experimental results suggested that the protonophores-mediated accumulation and aggregation of membrane proteins (like AP) in cell cytosol had signaled the induction of heat-shock proteins in E. coli and the non-translocated protein aggregates were possibly degraded by an induced heat-shock protease ClpP. Moreover, the induction of heat-shock response occurred by the stabilization of sigma-32. As, normally the DnaK-bound sigma-32 was known to be degraded by the heat-shock protease FtsH, our experimental results further suggested that the engagement of DnaK with the non-translocated proteins (like AP) had made the sigma-32 free and stable.

Genome-wide transcriptional responses of Escherichia coli K-12 to continuous osmotic and heat stresses

Osmotic stress is known to increase the thermotolerance and oxidative-stress resistance of bacteria by a mechanism that is not adequately understood. We probed the cross-regulation of continuous osmotic and heat stress responses by characterizing the effects of external osmolarity (0.3 M versus 0.0 M NaCl) and temperature (43 degrees C versus 30 degrees C) on the transcriptome of Escherichia coli K-12. Our most important discovery was that a number of genes in the SoxRS and OxyR oxidative-stress regulons were up-regulated by high osmolarity, high temperature, or a combination of both stresses. This result can explain the previously noted cross-protection of osmotic stress against oxidative and heat stresses. Most of the genes shown in previous studies to be induced during the early phase of adaptation to hyperosmotic shock were found to be also overexpressed under continuous osmotic stress. However, there was a poorer overlap between the heat shock genes that are induced transiently after high temperature shifts and the genes that we found to be chronically up-regulated at 43 degrees C. Supplementation of the high-osmolarity medium with the osmoprotectant glycine betaine, which reduces the cytoplasmic K(+) pool, did not lead to a universal reduction in the expression of osmotically induced genes. This finding does not support the hypothesis that K(+) is the central osmoregulatory signal in Enterobacteriaceae.

Mutants of Mycobacterium smegmatis unable to grow at acidic pH in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone

Mycobacterium smegmatis is able to grow and survive at acidic pH, and exhibits intracellular pH homeostasis under these conditions. In this study, the authors have identified low proton permeability of the cytoplasmic membrane, and high cytoplasmic buffering capacity, as determinants of intrinsic acid resistance of M. smegmatis. To identify genes encoding proteins involved in protecting cells from acid stress, a screening method was developed using the electrogenic protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). CCCP was used to suppress intrinsic acid resistance of M. smegmatis. The screen involved exposing cells to pH 5.0 in the presence of CCCP, and survivors were rescued at various time intervals on solid medium at pH 7.5. Cells capable of responding to intracellular acidification (due to CCCP-induced proton equilibration) will survive longer under these conditions than acid-sensitive cells. From a total pool of 5000 transposon (Tn611) insertion mutants screened, eight acid-sensitive M. smegmatis mutants were isolated. These acid-sensitive mutants were unable to grow at pH 5.0 in the presence of 1-5 microM CCCP, a concentration not lethal to the wild-type strain mc2155. The DNA flanking the site of Tn611 was identified using marker rescue in Escherichia coli, and DNA sequencing to identify the disrupted locus. Acid-sensitive mutants of M. smegmatis were disrupted in genes involved in phosphonate/phosphite assimilation, methionine biosynthesis, the PPE multigene family, xenobiotic-response regulation and lipid biosynthesis. Several of the acid-sensitive mutants were also defective in stationary-phase survival, suggesting that overlapping stress protection systems exist in M. smegmatis.

Inhibition of Escherichia coli growth by acetic acid: a problem with methionine biosynthesis and homocysteine toxicity

The mechanism by which methionine relieves the growth inhibition of Escherichia coli K-12 that is caused by organic weak acid food preservatives was investigated. In the presence of 8 mM acetate the specific growth rate of E. coli Frag1 (in MacIlvaine's minimal medium pH 6.0) is reduced by 50%. Addition of methionine restores growth to 80% of that observed in untreated controls. Similar relief was seen with cultures treated with either benzoate or propionate. Mutants with an elevated intracellular methionine pool were almost completely resistant to the inhibitory effects of acetate, suggesting that the methionine pool becomes limiting for growth in acetate-treated cells. Measurement of the intracellular concentrations of pathway intermediates revealed that the homocysteine pool is increased dramatically in acetate-treated cells, suggesting that acetate inhibits a biosynthetic step downstream from this intermediate. Supplementation of the medium with homocysteine inhibits the growth of E. coli cells. Acetate inhibition of growth arises from the depletion of the intracellular methionine pool with the concomitant accumulation of the toxic intermediate homocysteine and this augments the effect of lowering cytoplasmic pH.

A novel mechanism for upregulation of the Escherichia coli K-12 hmp (flavohaemoglobin) gene by the 'NO releaser', S-nitrosoglutathione: nitrosation of homocysteine and modulation of MetR binding to the glyA-hmp intergenic region

The flavohaemoglobin gene, hmp, of Escherichia coli is upregulated by nitric oxide (NO) in a SoxRS-independent manner. We now show that hmp expression is also upregulated by S-nitrosoglutathione (GSNO, widely used as an NO releaser) and sodium nitroprusside (SNP, which is a NO+ donor). Elevated homocysteine (Hcy) levels, achieved either by adding Hcy extracellularly or using metE mutants, decreased hmp expression. Conversely, metC mutants (defective in Hcy synthesis) had higher levels of hmp expression. Mutations in metR abolished hmp induction by GSNO and SNP, and hmp expression became insensitive to Hcy. We propose that the previously documented modulation by Hcy of MetR binding to the glyA-hmp intergenic regulatory region regulates hmp transcription. Although two MetR binding sites are present in this region, only the higher affinity site proximal to hmp is required for hmp induction by GSNO and SNP. GSNO and SNP react with Hcy in vitro under physiologically relevant conditions of pH and temperature generating S-nitrosohomocysteine, although in the latter case this would be co-ordinated to the Fe in SNP as a stable species. The free S-nitrosocysteine generated in the reaction with GSNO breaks down to release NO more readily than via homolysis of GSNO. As GSNO and SNP upregulate hmp similarly, the NO released in the former case on reaction with homocysteine cannot be involved in hmp regulation.

Role of an Escherichia coli stress-response operon in stationary-phase survival

The phage shock protein operon (pspABCE) of Escherichia coli is strongly expressed in response to stressful environmental conditions, such as heat shock, ethanol treatment, osmotic shock, and filamentous phage infection. We show that bacteria lacking the pspABC genes exhibit a substantial decrease in the ability to survive prolonged incubation in stationary phase under alkaline conditions (pH 9). The psp mutant bacteria grow approximately as well as wild-type strains in the alkaline medium, and stationary-phase survival of the psp mutants improves substantially at pH values closer to the optimal growth range (pH 6-8). In late stationary-phase (1- to 2-day-old) cells, the operon can be strongly induced under certain conditions, and PspA can become one of the most highly expressed bacterial proteins. The combination of stationary-phase starvation and alkaline pH is likely to place a severe strain on the maintenance of endogenous energy sources, and, consistent with these effects, we find that psp expression is also induced by uncouplers of oxidative phosphorylation and other agents that interfere with energy production. The death rate of psp mutants in stationary phase is accelerated by the presence of wild-type bacteria in the same culture, suggesting that the psp operon may play a significant role in enabling E. coli to compete for survival under nutrient- or energy-limited conditions.

The role of H-NS in one carbon metabolism

The H-NS protein of Escherichia coli regulates the expression of genes involved in many general processes such as osmoregulation and virulence. More recently, H-NS was shown to exert an effect on ilvIH gene expression in conjunction with the leucine responsive regulatory protein (Lrp). We show that H-NS is involved in the transcriptional regulation of the kbl/tdh operon, which is also Lrp regulated. Insertional inactivation of the hns gene results in two-fold derepression of the kbl/tdh operon. This level of expression is sufficient to suppress the auxotrophic requirements imposed by a glyA mutation. We show that expression of the kbl/tdh operon is temperature controlled and that this control is not mediated through H-NS action as has been shown for some other temperature controlled genes.

Adaptation of Escherichia coli to the uncoupler of oxidative phosphorylation 2,4-dinitrophenol

Escherichia coli was found to adapt to the uncoupler of oxidative phosphorylation 2,4-dinitrophenol. The rates of synthesis of 53 proteins were increased following exposure to 2,4-dinitrophenol. Adaptation was accelerated when the cofactor pyrroloquinoline quinone was provided in the growth medium.