Rapid response to osmotic upshift by osmoregulated genes in Escherichia coli and Salmonella typhimurium.

The rapid in vivo response of both Escherichia coli and Salmonella typhimurium osmoregulated genes to an osmotic upshift was analyzed in detail by using chromosomal operon fusions. Within 10 min after the addition of 0.3 M NaCl to the culture medium, the differential rates of expression of both an S. typhimurium proU-lac fusion and a proP-lac fusion increased by 180- and 17-fold respectively, while an E. coli ompC-lac fusion increased by 3.4-fold. For all three stimulated promoters, the increased rate of expression was maintained until about 40 min after the osmotic upshift. Thereafter, proU expression continued at a steady-state rate that was 27-fold higher than that of the control, while proP and ompC expression fell to 1.4- and 2-fold of the control rates, respectively. In contrast, expression of an E. coli ompF-lac fusion decreased twofold within 2.5 min. For proU, the length of the lag phase, which preceded the onset of the rapid response, increased with the degree of osmotic upshift, above a threshold of 0.2 M NaCl; the onset of the rapid proU response also preceded the resumption of growth. The rapid response phase, which was first quantitated for proU, proP, ompC, and ompF in this study, is an important component of the osmoregulation of these promoters. The addition of the osmoprotectant glycine betaine at the time of osmotic upshift decreased both the length of the rapid response and the subsequent steady-state of expression of proU.     

The role of antioxidant systems in response of bacteria Escherichia coli to heat shock]

Shifting the temperature from 30 to 45 degrees C in an aerobic Escherichia coli culture inhibited the expression of the antioxidant genes katG, katE, sodA, and gor. The expression was evaluated by measuring beta-galactosidase activity in E. coli strains that contained fusions of the antioxidant gene promoters with the lacZ operon. Heat shock inhibited catalase and glutathione reductase, lowered the intracellular level of glutathione, and increased its extracellular level. It also suppressed the growth of mutants deficient in the katG-encoded catalase HPI, whereas the sensitivity of the wild-type and sodA sodB mutant cells to heat shock was almost the same. In the E. coli culture adapted to growth at 42 degrees C, the content of both intracellular and extracellular glutathione was two times higher than in the culture grown at 30 degrees C. The temperature-adapted cells grown aerobically at 42 degrees C showed an increased ability to express the fused katG-lacZ genes.     

Purification and reconstitution of an osmosensor: transporter ProP of Escherichia coli senses and responds to osmotic shifts

The ProP protein of Escherichia coli is an osmoregulatory H+-compatible solute cotransporter. ProP is activated by an osmotic upshift in both whole cells and membrane vesicles. We are using biochemical and biophysical techniques to explore the osmosensory and catalytic mechanisms of ProP. We now report the purification and reconstitution of the active transporter. Protein purification was facilitated by the addition of six histidine (His) codons to the 3' end of proP. The recombinant gene was overexpressed from the E. coli galP promoter, and ProP-(His)6 was shown to be functionally equivalent to wild-type ProP by enzymatic assay of whole cells. ProP-(His)6, purified by Ni2+ (NTA) affinity chromatography, cross-reacted with antibodies raised against the ProP protein. ProP-(His)6 was reconstituted into Triton X-100 destabilized liposomes prepared with E. coli phospholipid. The reconstituted transporter mediated proline accumulation only if (1) a membrane potential was generated by valinomycin-mediated K+ efflux and (2) the proteoliposomes were subjected to an osmotic upshift (0.6 M sucrose). Activity was also stimulated by DeltapH. Pure ProP acts, in the proteoliposome environment, as sensor, transducer, and respondent to a hyperosmotic shift. It is the first such osmosensor to be isolated.     

Role of heat shock protein DnaK in osmotic adaptation of Escherichia coli.

Escherichia coli can adapt and recover growth at high osmolarity. Adaptation requires the deplasmolysis of cells previously plasmolyzed by the fast efflux of water promoted by osmotic upshift. Deplasmolysis is essentially ensured by a net osmo-dependent influx of K+. The cellular content of the heat shock protein DnaK is increased in response to osmotic upshift and does not decrease as long as osmolarity is high. The dnaK756(Ts) mutant, which fails to deplasmolyze and recover growth, does not take up K+ at high osmolarity; DnaK protein is required directly or indirectly for the maintenance of K+ transport at high osmolarity. The temperature-sensitive mutations dnaJ259 and grpE280 do not affect the osmoadaptation of E. coli at 30 degrees C.     

Escherichia coli shows two types of behavioral responses to osmotic upshift.

Behavioral responses to osmotic upshift were characterized by temporal assays of free-swimming cells of Escherichia coli. Small osmotic upshifts (200 to 300 mosM) elicited tumble responses which were chemotaxis dependent, while large osmotic upshifts (400 to 500 mosM) elicited stopping followed by pseudotumbling which was chemotaxis independent.     

Protein synthesis is required for in vivo activation of polysialic acid capsule synthesis in Escherichia coli K1.

The kinetics of in vivo expression of the polysialosyl (K1) capsular antigen in Escherichia coli has been studied. Growth of E. coli K1 strains at 15 degrees C prevents K1 polysaccharide synthesis (F. A. Troy and M. A. McCloskey, J. Biol. Chem. 254:7377-7387, 1979). Synthesis is reactivated in cells grown at 15 degrees C after upshift to 37 degrees C. The early expression and resultant morphology of K1 capsular antigen was monitored in temperature upshift experiments by using electron microscopy. Morphological stabilization of the capsule was achieved by treatment of cells with an antiserum specific for the alpha, 2-8-linked polysialosyl antigen. The kinetics of K1 capsule expression in growing cells was measured by bacteriophage adsorption with phage K1F, which required the K1 capsule for binding. The results of temperature upshift experiments showed that capsule first appeared on the cell surface after 10 min. Subsequent bacteriophage binding increased linearly with time until a fully encapsulated state was reached 45 min after upshift. The initiation of K1 capsule appearance was dependent on protein synthesis and the addition of chloramphenicol before temperature upshift prevented any expression of the K1 antigen. Chloramphenicol reduced the rate of K1 synthesis when added after temperature upshift. We conclude from these results that protein synthesis is a prerequisite for activation of capsule expression in vivo, but not for subsequent elongation of polysialosyl chains.     

Transcriptional regulation of katE in Escherichia coli K-12

Escherichia coli produces two distinct species of catalase, hydroperoxidases I and II, which differ in kinetic properties and regulation. To further examine catalase regulation, a lacZ fusion was placed into one of the genes that is involved in catalase synthesis. Transductional mapping revealed the fusion to be either allelic with or very close to katE, a locus which together with katF controls the synthesis of the aerobically inducible hydroperoxidase (hydroperoxidase II). katE was expressed under anaerobic conditions at levels that were approximately one-fourth of those found in aerobically grown cells and was found to be induced to higher levels in early-stationary-phase cells relative to levels of exponentially growing cells under both anaerobic and aerobic conditions. katE was fully expressed in air and was not further induced when the growth medium was sparged with 100% oxygen. Expression of katE was unaffected by the addition of hydrogen peroxide or by the presence of additional lesions in oxyR or sodA, indicating that it is not part of the oxyR regulon. When katF::Tn10 was introduced into a katE::lacZ strain, beta-galactosidase synthesis was largely eliminated and was no longer inducible, suggesting that katF is a positive regulator of katE expression.     

A novel, non-invasive promoter probe vector: cloning of the osmoregulated proU promoter of Escherichia coli K12

We have constructed a novel promoter probe plasmid pSB40, containing a unique lac-alpha-tetracycline marker gene tandem, which allows for both positive and negative selection of active promoters. Promoters cloned in pSB40 can be readily mobilized as EcoRI cassettes. Using this vector we have performed a non-invasive analysis of the E. coli chromosome for promoters regulated by osmotic upshift. Only one such promoter, subsequently identified as part of the proU operon, was isolated. A sequence of 253 bp, sufficient to mediate osmotic regulation of the proU promoter, was defined. This E. coli promoter was normally regulated in Salmonella typhimurium, Klebsiella and Citrobacter but not in Shigella. A proU-luxAB fusion plasmid was constructed and used to monitor in vivo real-time kinetics of proU induction following osmotic upshock.     

The role of antioxidant systems in the response of Escherichia coli to acetamidophenol and some antibiotics

The role of glutathione and other antioxidant systems in the response of Escherichia coli to acetamidophenol (paracetamol), rifampicin, and chloramphenicol was studied. The exposure of aerobically growing E. coli cells to acetamidophenol diminished the intracellular level of glutathione by 40% and the reduced-to-oxidized glutathione ratio in the cells by 50%, while it enhanced the expression of the antioxidant genes soxS and sodA by 2.7 and 1.8 times, respectively. Glutathione-deficient cells were more susceptible to acetamidophenol than were normal cells. All this suggests that acetamidophenol induces a mild oxidative stress in E. coli cells. The oxidative stress induced by rifampicin was still less pronounced, whereas chloramphenicol-treated E.coli cells exhibited no signs of oxidative stress at all.     

A peroxide/ascorbate-inducible catalase from Haemophilus influenzae is homologous to the Escherichia coli katE gene product.

Bacterial catalases are induced by exposure to peroxide (e.g., Escherichia coli katG) or entry into stationary phase (e.g., E. coli katE). To study regulatory systems in Haemophilus influenzae, we complemented an E. coli rpoS mutant, which is unable to induce katE in stationary phase, with a plasmid library of H. influenzae Rd- chromosomal DNA. Nineteen complementing clones with a catalase-positive phenotype were obtained and characterized after screening about 10(5) transformants. All carried the same structural gene for an H. influenzae catalase. The DNA sequence of this gene, called hktE, encodes a 508-amino-acid polypeptide with strong homology to eukaryotic catalases and E. coli katE. However, hktE is regulated like E. coli katG, with catalase activity increasing 10-fold and hktE mRNA levels increasing 4-fold upon exposure to ascorbic acid, which serves to generate hydrogen peroxide. Mutations in the known global regulatory genes of H. influenzae--crp, cya, and sxy--do not affect the inducibility of hktE. The hktE gene maps to a 225-kb segment of the H. influenzae chromosome in a region encoding resistance to spectinomycin.     

Expression analysis and characterization of the mutant of a growth-phase- and starvation-regulated monofunctional catalase gene from Xanthomonas campestris pv. phaseoli

Analysis of the Xanthomonas campestris pv. phaseoli (Xp) catalase profile using an activity gel revealed at least two distinct monofunctional catalase isozymes denoted Kat1 and Kat2. Kat1 was expressed throughout growth, whereas Kat2 was expressed only during the stationary phase of growth. The nucleotide sequence of a previously isolated monofunctional catalase gene, Xp katE, was determined. The deduced amino acid sequence of Xp KatE showed a high percentage identity to an atypical group of monofunctional catalases that includes the well-characterized E. coli katE. Expression of Xp katE was growth phase-dependent but was not inducible by oxidants. In addition, growth of Xp in a carbon-starvation medium induced expression of the gene. An Xp katE mutant was constructed, and analysis of its catalase enzyme pattern showed that Xp katE coded for the Kat2 isozyme. Xp katE mutant had resistance levels similar to the parental strain against peroxide and superoxide killing at both exponential and stationary phases of growth. Interestingly, the level of total catalase activity in the mutant was similar to that of the parental strain even in stationary phase. These results suggest the existence of a novel compensatory mechanism for the activity of Xp catalase isozymes.     

Polarized light scattering for rapid observation of bacterial size changes.

The effect of changing growth conditions on the diameter of rod-shaped bacteria was studied in vivo with the use of polarized light scattering. The value of a ratio of scattering matrix elements was measured as a function of scattering angle at various times after nutritional "upshift" for two strains of Escherichia coli cells. The peak locations of the scattering function were calibrated against the diameter for rod-shaped bacteria. The peaks moved toward smaller angles as a function of time after upshift, indicating that the diameter was increasing. Under special conditions, substantial peak shifts occurred within a few minutes of growth condition change, indicating a rapid onset of growth in diameter. The rate of increase of the diameters after upshift was obtained from the angular shift of peak location. This rate was approximately 14 nm/min for E. coli K12 and approximately 9 nm/min for E. coli B/r at 37 degrees C. The rate of diameter increase is smaller at lower temperatures. Experiments with Bacillus megaterium showed that any diameter change after nutritional upshift at 37 degrees C is limited to at most a very small increase, at least for the strain and medium tested.     

The role of the carboxyl terminal alpha-helical coiled-coil domain in osmosensing by transporter ProP of Escherichia coli

Concentrative uptake of osmoprotectants via transporter ProP contributes to the rehydration of Escherichia coli cells that encounter high osmolality media. A member of the major facilitator superfamily, ProP is activated by osmotic upshifts in whole bacteria, in cytoplasmic membrane vesicles and in proteoliposomes prepared with the purified protein. Soluble protein ProQ is also required for full osmotic activation of ProP in vivo. ProP is differentiated from structural and functional homologues by its osmotic activation and its C-terminal extension, which is predicted to form an alpha-helical coiled-coil. A synthetic polypeptide corresponding to the C-terminus of ProP (ProP-p) formed a dimeric alpha-helical coiled-coil. A derivative of transporter ProP lacking 26 C-terminal amino acids was expressed but inactive. A derivative harbouring amino acid changes K460I, Y467I and H495I (each at the core, coiled-coil 'a' position) required a larger osmotic upshift for activation than did the wild type transporter. The same changes extended, stabilized and altered the oligomeric state of the coiled-coil formed by ProP-p. Amino acid change R488I (also at the 'a' position) further increased the magnitude of the osmotic upshift required to activate ProP, reduced the activity attained and rendered ProP activation transient. Unexpectedly, replacement R488I destabilized the coiled-coil formed by ProP-p. The activity and osmotic activation of ProP were even more strongly attenuated by helix-destabilizing change I474P. These data demonstrate that the carboxyl terminal domain of ProP can form a homodimeric alpha-helical coiled-coil with unusual properties. They implicate the C-terminal domain in the osmotic activation of ProP.     

Osmotic adaptation of Escherichia coli with a negligible proton motive force in the presence of carbonyl cyanide m-chlorophenylhydrazone.

It has been reported that Escherichia coli is able to grow in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) when ATP is produced by glycolysis (N. Kinoshita et al., J. Bacteriol. 160:1074-1077, 1984). We investigated the effect of CCCP on the osmotic adaptation of E. coli growing with glucose. When E. coli growing in rich medium containing CCCP was transferred to medium containing sucrose, its growth stopped for a while and then started again. This lag time was negligible in the absence of CCCP. The same results were obtained when the osmolarity was increased by N-methylglucamine-maleic acid. In addition to adapting itself to the hyperosmotic rich medium, E. coli adapted itself to hyperosmolarity in a minimal medium containing CCCP, again with a lag time. Hyperosmotic shock decreased the internal level of potassium ion rather than causing the accumulation of external potassium ion in the presence of CCCP. The internal amount of glutamic acid increased in cells growing in hyperosmotic medium in the presence and absence of CCCP. Large elevations in levels of other amino acids were not observed in the cells adapted to hyperosmolarity. Trehalose was detected only in hyperosmosis-stressed cells in the presence and absence of CCCP. These results suggest that E. coli can adapt to changes in the environmental osmolarity with a negligible accumulation of osmolytes from the external milieu but that the accumulation may promote the adaptation.     

Factors reducing and promoting the effectiveness of proline as an osmoprotectant in Escherichia coli K12

Proline accumulation in Escherichia coli is mediated by three proline porters. Proline catabolism is effected by proline porter I (PPI) and proline/delta 1-pyrroline carboxylate dehydrogenase. Proline did not accumulate cytoplasmically when E. coli was subjected to osmotic stress in minimal salts medium. Although PPI is induced when proline is provided as carbon or nitrogen source, its activity decreased following growth of the bacteria in minimal salts medium of high osmotic strength. Proline dehydrogenase was induced by proline in low or high osmotic strength media. Proline porter II (PPII) was both activated and induced in osmotically stressed bacteria, though the dependencies of the two responses on medium osmolarity differed. Osmotic downshift during the transport measurement decreased the uptake of proline, serine and glutamine by bacteria cultured in media of high osmotic strength. Thus, while osmotic upshift caused specific activation of PPII, osmotic downshift caused a non-specific reduction in amino acid uptake. Glycine betaine inhibited the uptake of [14C]proline via PPII and PPIII but not via PPI. The dependence of that inhibition on glycine betaine concentration was similar when PPII was uninduced, induced or activated by osmotic stress, or induced by amino acid limited growth. Thus PPII and PPIII, not PPI, contribute to the mechanism of osmoprotection by proline and glycine betaine. The tendency for exogenous proline to accumulate in the cytoplasm of bacteria exposed to osmotic stress would, however, be countered by increased proline catabolism.     

Osmotic shock: a mechanosensitive channel blocker can prevent release of cytoplasmic but not periplasmic proteins

When over-expressed in the cytoplasm of Escherichia coli, carboxylesterase Est55 of Geobacillus stearothermophilus was found to be released from cells upon osmotic shock. Comparing two osmotic shock protocols showed that release of Est55 was abolished in the absence of mechanosensitive channel MscL by one method but not the other. The discrepancy extended to several previously reported cytoplasmic proteins released by osmotic shock, including: EF-Tu, thioredoxin, and DnaK in E. coli. Stepwise analyses of parameters between these two protocols revealed that the use of mechanical pipetting instead of gentle dilution of cells prior to exposure to hypotonic solution abolished the effect of MscL. Furthermore, while this phenomenon of release of certain cytoplasmic proteins was sustained in all three wild type strains of E. coli, presence of gadolinium was able to serve as an MscL channel blocker and prevented release of Est55 and EF-Tu in the process. An optimized protocol of osmotic shock was developed from this study to provide a more reliable assessment of location of proteins in E. coli. This method allowed release of authentic periplasmic MalE and beta-lactamase proteins comparable to that by EDTA-lysozyme treatment.     

Induction of oxidative stress by high hydrostatic pressure in Escherichia coli

Using leaderless alkaline phosphatase as a probe, it was demonstrated that pressure treatment induces endogenous intracellular oxidative stress in Escherichia coli MG1655. In stationary-phase cells, this oxidative stress increased with the applied pressure at least up to 400 MPa, which is well beyond the pressure at which the cells started to become inactivated (200 MPa). In exponential-phase cells, in contrast, oxidative stress increased with pressure treatment up to 150 MPa and then decreased again, together with the cell counts. Anaerobic incubation after pressure treatment significantly supported the recovery of MG1655, while mutants with increased intrinsic sensitivity toward oxidative stress (katE, katF, oxyR, sodAB, and soxS) were found to be more pressure sensitive than wild-type MG1655. Furthermore, mild pressure treatment strongly sensitized E. coli toward t-butylhydroperoxide and the superoxide generator plumbagin. Finally, previously described pressure-resistant mutants of E. coli MG1655 displayed enhanced resistance toward plumbagin. In one of these mutants, the induction of endogenous oxidative stress upon high hydrostatic pressure treatment was also investigated and found to be much lower than in MG1655. These results suggest that, at least under some conditions, the inactivation of E. coli by high hydrostatic pressure treatment is the consequence of a suicide mechanism involving the induction of an endogenous oxidative burst.