Living with water stress: evolution of osmolyte systems

A physiological role for DNA supercoiling in the osmotic regulation of gene expression in S. typhimurium and E. coli

The proU locus encodes an osmotically inducible glycine betaine transport system that is important in the adaptation to osmotic stress. We present evidence that DNA supercoiling plays a key role in the osmotic induction of proU transcription. An increase in extracellular osmolarity increases in vivo DNA supercoiling, and the expression of proU is highly sensitive to these changes. Furthermore, topA mutations can mimic an increase in osmolarity, facilitating proU expression even in media of low osmolarity in which it is not normally expressed. Selection for trans-acting mutations that affect proU expression has yielded only mutations that alter DNA supercoiling, either in topA or a new genetic locus, osmZ, which strongly influences in vivo supercoiling. Mutations in osmZ are highly pleiotropic, affecting expression of a variety of chromosomal genes including ompF, ompC, fimA, and the bgl operon, as well as increasing the frequency of site-specific DNA inversions that mediate fimbrial phase variation.

Betaine aldehyde dehydrogenase polymorphism in spinach: genetic and biochemical characterization

Spinach (Spinacia oleracea L.) has a major chloroplastic isozyme of betaine aldehyde dehydrogenase (BADH) and a minor cytosolic one. Among a diverse collection of spinach accessions, three electrophoretic banding patterns of chloroplastic BADH were found: two were single banded and one was triple banded. Genetic analysis of these patterns indicated that chloroplastic BADH is encoded by a single, nuclear gene with two alleles, designated slow (S) and fast (F), and that products of these alleles can hybridize to form either homodimers or a heterodimer. The S allele was by far the most common among the accessions examined. Native and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the SS and FF homodimers differ in charge but not molecular weight.

Adaptation for growth at various saline concentrations by the archaebacterium Methanosarcina thermophila

We report the ability of Methanosarcina thermophila TM-1 to adapt and grow in media containing NaCl concentrations of 0.005 to 1.2 M. When adapted to marine NaCl concentrations, this species ceased to produce the heteropolysaccharide outer layer typically formed by species of nonmarine origin. concomitant with this adaptation, M. thermophila ceased to grow as multicellular aggregates and existed solely in single-cell form. The sodium ion concentration was critical for the adaptation process, although magnesium ion appeared to contribute to the cell wall stability of single cells. The results suggest that these archaebacteria possess regulatory systems that enable them to adapt to environments with a wide range of saline concentrations.

Osmotic effectors in kidneys of xeric and mesic rodents: corticomedullary distributions and changes with water availability

Urea, sodium, the methylamines glycine betaine and glycerophosphorylcholine (GPC), and the polyols sorbitol and myo-inositol are reported to be the major osmolytes in kidneys of laboratory mammals. These were measured (millimoles per kilogram wet weight) in kidney regions and urines of three species of wild rodents with different dehydration tolerances: the pocket mouse Perognathus parvus (xeric), vole Microtus montanus (mesic), and deer mouse Peromyscus m. gambeli (intermediate). In animals kept without water for 4-6 days, sodium, urea, betaine and GPC + choline were found in gradients increasing from cortex to outer to inner medulla in all species, with Perognathus having the highest levels. Sorbitol was high in the inner medulla but low in the cortex and outer medulla; inositol was highest in the outer medulla. Totals of methylamines and methylamines plus polyols in the medulla showed high linear correlations (positive) with urea and with sodium values. Whole medullae were analyzed at several time points in Microtus and Peromyscus subject to water diuresis followed by antidiuresis. In 102 h diuresis in Microtus, all osmolytes decreased except inositol; however, only urea, sodium and sorbitol reached new steady states within 24 h. Urea returned to initial values in 18 h antidiuresis, while other osmolytes required up to 90 h. In Peromyscus, all osmolytes except the polyols declined in diuresis (max. 78 h test period). During antidiuresis, urea and GPC + choline rose to initial values in 18 h, with sodium and betaine requiring more time. In plots of both species combined, total methylamines + polyols correlated linearly (positive) with sodium, and GPC + choline with urea. Estimates of tissue concentrations suggest that total methylamines + polyols can account for intracellular osmotic balance in all species in antidiuresis and that sufficient concentrations of methylamines may be present to counteract perturbing effects of urea on proteins.

Preservation of ATP in Hypersaline Environments

High concentrations of particulate ATP were found in the anoxic brines of the Orca Basin and East Flower Garden, Gulf of Mexico. Other measurements indicative of growth and respiration suggested that the microbial community in the brines was inactive, but somehow the ATP associated with the cells persisted. Conceivably, when cells growing just above the interface sank into the brine, the increased osmotic stress could elicit an osmoregulatory response resulting in increased ATP. It was also possible that hydrolytic enzymes were inactivated, resulting in the preservation of ATP. Experiments in which a culture of marine bacteria was suspended in menstrua of different salinities comparable to those found across the Orca Basin interface revealed that as salinity increased, ATP increased three- to sixfold. Within 24 h the ATP fell to its initial level and remained at that concentration for 3 days, at which time the experiment was terminated. In contrast, the control suspensions, at a salinity of 28% (grams per liter) had 1/10th of the initial ATP concentration when the experiment was ended. Cells were also exposed to killing UV irradiation, enabling us to demonstrate with absolute certainty that cellular ATP could be preserved. At the end of the experiment, the viable component of the population was reduced by orders of magnitude by UV irradiation, but the ATP levels of the cells suspended in brine did not decrease. In certain environments it appears that the conventional analytical tools of the microbial ecologist must be interpreted with caution.

Effects of urea and trimethylamine-N-oxide on enzyme activity and stability

The interactions of urea, trimethylamine-N-oxide (TMAO), and related solutes on a number of enzymes were examined. Urea inhibited enzymatic activity and accelerated the thermal inactivation of catalase, whereas TMAO activated some enzymes but inhibited others. The effects of urea and of TMAO, whether parallel or in opposition, were exerted independently. Thus, in those cases where TMAO increases enzymatic activity, it did so to the same relative degree, whether or not urea was present. TMAO markedly decreased the rate of thermal inactivation of catalase, indicating that it does favor compact protein structures. The assumption that TMAO factors compaction of protein structure, whereas urea has the contrary effect, does not lead to the expectation that TMAO must always oppose the effect of urea on enzymatic activity, since the most compact form of an enzyme may not always be the most active form.

Osmoprotective activity for Escherichia coli in mammalian renal inner medulla and urine. Correlation of glycine and proline betaines and sorbitol with response to osmotic loads

Escherichia coli are protected against hypertonic NaCl by human urine. We have shown that this is due in part to the presence of glycine betaine and proline betaine. Several investigators have proposed that betaines and sorbitol are concentrated in the cells of the renal inner medulla where they exert a protective role against urea and extracellular osmotic forces. E. coli was used in the present studies as an "osmosensor" to detect osmoprotective activity in mammalian tissues. The greatest activity was found in extracts of renal inner medulla and to a lesser extent in the renal outer medulla and cortex of several mammalian species. Liver extracts were more active than other nonrenal tissues. Bacterial osmoprotective activity and concentration of glycine betaine in the renal inner medulla of rabbits were found to correlate closely with urinary osmolarity. Concentrations of sorbitol were found to be also increased in the renal inner medulla during osmotic stress, but this compound is not osmoprotective for E. coli. Glycine and proline betaine were recovered in urine of rabbits and were increased in those given high osmotic loads. Only small amounts of proline betaine were recovered in the renal inner medulla. The source from which proline betaine is derived is unknown.

Stabilization of phosphofructokinase during air-drying with sugars and sugar/transition metal mixtures

Phosphofructokinase (PFK) purified from rabbit skeletal muscle is fully inactivated after air-drying and rehydration. The addition of trehalose, maltose, or sucrose to the enzyme solution prior to rapid drying results in a recovery of almost 70% of the original activity, whereas about 30% is recovered during slow drying. Similar stabilization is seen with up to 200 mM lactose, but at higher concentrations the sugar comes out of solution during drying, and there is a dramatic drop in the activity recovered. Glucose at concentrations up to 500 mM is ineffective at protecting air-dried PFK. Addition of ionic zinc to enzyme-sugar mixtures prior to drying greatly enhances the stabilization imparted by the above sugars, but zinc alone affords no protection. Several other organic solutes (proline, glycine, trimethylamine N-oxide, glycerol, and myo-inositol) that afford cryoprotection to PFK, an effect enhanced by the addition of zinc, do not stabilize the enzyme during air-drying, even if zinc is present. The addition of ionic copper, cobalt, or nickel to trehalose-PFK solution prior to rapid drying results in a large increase in the activity recovered, and the presence of cadmium or manganese leads to a minor increase. Magnesium and calcium are ineffective in this respect. During slow drying, the presence of cadmium or calcium leads to increased preservation, magnesium and manganese have no influence on stabilization, and copper and nickel inactive the enzyme.

Betaine Transport Imparts Osmotolerance on a Strain of Lactobacillus acidophilus

Unlike most Lactobacillus acidophilus strains, a specific strain, L. acidophilus IFO 3532, was found to grow in rich medium containing 1 M sodium acetate, KCl, or NaCl. This strain could also grow with up to 1.8 M NaCl or 3 M nonelectrolytes (fructose, xylose, or sorbitol) added. Thus, this strain was tolerant to osmotic pressures up to 2.8 osM. A search for an intracellular solute which conferred osmoprotection led to the identification of glycine betaine (betaine). Betaine was accumulated to high concentrations in cells growing in MRS medium supplemented with 1 M KCl or NaCl. Uptake of [ 14 C]betaine by L. acidophilus 3532 cells suspended in buffer was stimulated by increasing the medium osmotic pressure with 1 M KCl or NaCl. The accumulated betaine was not metabolized further; transport was relatively specific for betaine and was dependent on an energy source. Other lactobacilli, more osmosensitive than strain 3532, including L. acidophilus strain E4356, L. bulgaricus 8144, and L. delbrueckii 9649, showed lower betaine transport rates in response to an osmotic challenge than L. acidophilus 3532. Experiments with chloramphenicol-treated L. acidophilus 3532 cells indicated that the transport system was not induced but appeared to be activated by an increase in osmotic pressure.

Dimethylthetin can substitute for glycine betaine as an osmoprotectant molecule for Escherichia coli

Glycine betaine is believed to be the most active naturally occurring osmoprotectant molecule for Escherichia coli and other bacteria. It is a dipolar ion possessing a quaternary ammonimum group and a carboxylic acid group. To examine the molecular requirements for osmoprotective activity, dimethylthetin was compared with glycine betaine. Dimethylthetin is identical to glycine betaine except for substitution of dimethyl sulfonium for the quaternary nitrogen group. Dimethylthetin was found to be about equally as effective as glycine betaine in permitting E. coli to grow in hypertonic NaCl, and both compounds were recovered almost completely from bacterial cells grown in the presence of hypertonic NaCl. 3-Dimethylsulfonioproprionate, an analog of dimethylthetin observed in marine algae, and 3-Dimethylsulfonio-2-methylproprionate were found to be less active. Dimethylthetin may prove useful as a molecular probe to study betaine metabolism and as a model for the development of antibacterial agents.

OSMOTIC ADJUSTMENT IN MARINE EUKARYOTIC ALGAE: THE ROLE OF INORGANIC IONS, QUATERNARY AMMONIUM, TERTIARY SULPHONIUM AND CARBOHYDRATE SOLUTES: I. DIATOMS AND A RHODOPHYTE

The unicellular marine algae, Phaeodactylum tricornutum Bohlin, Cyclotella cryptica Reimann and Cyclotella meneghiniana Kützing (Bacillariophyceae) and Porphyridium aerugineum Geitler (Rhodophyceae) synthesized and accumulated glycine betaine and proline in response to increases of the NaCl concentration (150 to 1000 mol m^-3 NaCl) of the growth medium. C. cryptica and C. meneghiniana also synthesized and accumulated homarine (N-methyl picolinic acid betaine). Both P. tricornutum and P. aerugineum synthesized increasing amounts of intracellular glycerol and P. aerugineum also formed the heteroside, floridoside [O-α-D-galactopyranosyl (1 → 2)-glycerol], in response to the elevated salinities. No major low molecular weight carbohydrates were found in Cyclotella. Sucrose was not detected in the algal extracts. Only P. tricornutum synthesized the tertiary sulphonium compound, β-dimethylsulphoniopropionate (DMSP), and the quantity of this solute in the alga was dependent on the amount of NaCl in the medium. Intracellular K⁺ concentrations in the algae were three to six times greater than those of Na⁺ . Increases of the salinity of the media led to the uptake and accumulation of K⁺ by the cells, and smaller increases of Na⁺ and Cl^-1 and loss of intracellular NO₃ ^- . The inorganic cations Na⁺ and K⁺ , with their accompanying anions, and the estimated organic solutes could largely account for the osmotic balance of P. tricornutum and P. aerugineum.

Role and regulation of glycerophosphorylcholine in rat renal papilla

Glycerophosphorylcholine (GPC)--an organic solute which is considered to be involved in cellular osmoregulation in the renal medulla--was determined by means of an enzymatic assay in various zones of the rat kidney and in papillary tubule suspensions. In antidiuresis, GPC content in cortex, outer medulla and papillary tip was 0.64, 14.6, and 108.9 mmol/kg fresh weight, respectively. Significant concentrations of GPC could not be detected in the urine or in the peripheral plasma. The sharp increase in GPC concentration from cortex to papillary tip was partially abolished by the induction of diuresis by either waterloading or furosemide. These manoeuvres, however, did not change cortical GPC content. Papillary tubule suspensions prepared from hydropenic rats contained only slightly less GPC per g protein than whole papillae from antidiuretic animals. Incubation of tubules over 120 min did not lead to a significant loss of GPC which is in accordance with the low activity of GPC degrading enzymes in papillary tissue. The results confirm the intracellular localization of GPC and provide further evidence that this substance plays a substantial role in the osmoregulation of renal papillary cells.

Some emerging principles underlying the physical properties, biological actions, and utility of vitrification solutions

Vitrification solutions are aqueous cryoprotectant solutions which do not freeze when cooled at moderate rates to very low temperatures. Vitrification solutions have been used with great success for the cryopreservation of some biological systems but have been less successful or unsuccessful with other systems, and more fundamental knowledge about vitrification solutions is required. The purpose of the present survey is to show that a general understanding of the physical behavior and biological effects of vitrification solutions, as well as an understanding of the conditions under which vitrification solutions are required, is gradually emerging. Detailed nonequilibrium phase diagram information in combination with specific information on the tolerance of biological systems to ice and to cryoprotectant at subzero temperatures provides a quantitative theoretical basis for choosing between vitrification and freezing. The vitrification behavior of mixtures of cryoprotective agents during cooling is predictable from the behavior of the individual agents, and the behavior of individual agents is gradually becoming predictable from the details of their molecular structures. Progress is continuing concerning the elucidation of mechanisms and cellular sites of toxicity and mechanisms for the reduction of toxicity. Finally, important new information is rapidly emerging concerning the crystallization of previously vitrified cryoprotectant solutions during warming. It appears that vitrification tendency, toxicity, and devitrification all depend on subtle variations in the organization of water around dissolved substances.

Selection of mutations that alter the osmotic control of transcription of the Salmonella typhimurium proU operon

We isolated 60 independent mutations, designated osmX, in Salmonella typhimurium that result in constitutive expression of the normally osmoregulated proU operon. Each of the osmX mutations is closely linked to the proU locus and cis-dominant over the osmX+ allele in diploid strains. These results suggest that the mutations are probably in the 5' transcriptional control region of the proU operon. Our failure to obtain either recessive or unlinked mutations that altered the osmotic control of transcription of the proU operon suggests that transcriptional regulation of the gene is not under the negative control of a repressor protein that is dispensable for cell viability. We discuss possible models for the mechanism of osmotic regulation of transcription of the proU operon.

Transformations of arsenic in the marine environment

It is ten years since arsenobetaine was first isolated from the western rock lobster Palinurus cygnus. Subsequently this naturally-occurring arsenical has been found in many species of marine animals contributing to the human diet. The identification of arsenic-containing ribofuranosides in algae and the production of dimethylarsinoylethanol from their anaerobic decomposition has allowed speculation on arsenic metabolism in marine organisms and has suggested a possible route to arsenobetaine from oceanic arsenate.

Are ninhydrin-positive substances volume-regulatory osmolytes in rat renal papillary cells?

1. A study has been made of the concentrations and contents of ninhydrin-positive substances (n.p.s.), presumed to be predominantly but not exclusively amino acids, in the cells of rat renal papillary slices incubated in variously modified Krebs phosphate-bicarbonate Ringer solution. 2. When the medium osmolality was increased from 710 (control) to 2000 mosmol/kg H2O by additional NaCl and urea, the steady-state cellular n.p.s. concentration rose from 42.3 +/- 0.6 (mean +/- S.E. of mean; n = 36) to 105 +/- 2 (n = 68) mmol/l (glycine equivalent). Cell fluid content fell from 5.11 +/- 0.09 (n = 36) to 4.16 +/- 0.11 (n = 68) microliter/mg solute-free dry weight. Hence cell n.p.s. content increased from 211 +/- 4 (n = 36) to 421 +/- 10 (n = 68) nmol/mg solute-free dry weight. 3. A comparable loss of cell fluid was observed when urea was replaced by sucrose or sorbitol. No increase in cell n.p.s. occurred, and there was a marked cell Na+-for-K+ exchange. 4. The extent of the increase in cell n.p.s. in the presence of 2000 mosmol/kg H2O (NaCl + urea) was sensitive to the presence of external anions in the sequence acetate less than Cl- less than NO3- less than or equal to SCN-. 5. Cell n.p.s. concentration increased progressively as the medium osmolality was increased by the addition of urea, but Na+ at a concentration above 330 mmol/l had an inhibitory effect. The increase in n.p.s. concentration was also significantly reduced in hyperosmotic media in which Na+ was replaced by choline. 6. The increase in cell n.p.s. content due to hyperosmotic NaCl + urea was completely inhibited by pre-incubation in control medium containing trimethylamine N-oxide. 7. On transference of slices from control to hyperosmotic media (NaCl + urea) the steady-state increase in cell n.p.s. concentration was complete within 20 min and followed a time course similar to that for cell fluid loss. The n.p.s. concentration and cell fluid content returned to control levels, with similar time courses, following re-immersion in control medium. 8. Efflux of alpha-amino[1-14C]isobutyric acid (AIB) from slices pre-loaded in control medium containing 1 mmol AIB/l was slightly but significantly slower into AIB-free hyperosmotic NaCl + urea than into AIB-free control medium. The rate of efflux was greatly increased by the presence of hyperosmotic sucrose or very high Na+ (935 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of aldose reductase and sorbitol in renal inner medullary cells by elevated extracellular NaCl

Aldose reductase [aldehyde reductase 2; alditol:NAD(P)+ 1-oxidoreductase, EC 1.1.1.21] catalyzes conversion of glucose to sorbitol. Although its activity is implicated in the progression of ocular and neurological complications of diabetes, the normal function of the enzyme in most cells is unknown. Both aldose reductase activity and substantial levels of sorbitol were previously reported in renal inner medullary cells. In this tissue, the extracellular NaCl concentration normally is high and varies considerably depending on the urine concentration. We report here on a line of renal medullary cells in which medium that is high in NaCl greatly increases both aldose reductase activity and intracellular sorbitol. In these tissue culture cells (and presumably also in the renal inner medulla), the intracellular sorbitol helps balance the osmotic pressure of elevated extracellular NaCl and thus prevents cellular dehydration.

Functional alcohol dehydrogenase mutants of Saccharomyces cerevisiae conferring temperature-conditional allyl alcohol resistance

Selection for allyl alcohol resistance in respiratory incompetent yeast is a highly specific method for isolating functional mutations at ADH1, the gene coding for the cytoplasmic alcohol dehydrogenase, ADHI. Because of the nature of this selection scheme, the ADHI activity of such mutants is retained, but the kinetic characteristics of the enzymes are altered. The high specificity for targeting functional mutations at this locus suggested that selection for enzyme variants with more subtle phenotypic effects might be possible. Here, we describe functional ADHI mutants that are temperature-conditional in their allyl alcohol resistance. Haploid cells of one of these mutants grow well on plates at 10 mM allyl alcohol at 19 degrees, but not at 37 degrees, the restrictive temperature. A second mutant grows well at 10 mM at 37 degrees, but its growth is restricted at 19 degrees. What distinguishes these mutants from other temperature-sensitive mutants is that the temperature-conditional growth phenotypes described here must be due to interactions between allyl alcohol levels and ADHI functional properties and cannot be due to lability of the enzyme at the restrictive temperature. This system shows promise for the investigation of functional enzyme variants that differ by only one or two amino acid residues but have significant temperature- and substrate-conditional effects on growth phenotypes in both the haploids and the diploids.

Buoyant density variation during the cell cycle in microorganisms

The behavior of cell buoyant density during the cell cycle has been determined for a number of different cell types, including bacteria, yeast, and mammalian cells. Mean buoyant density was extremely constant and independent of cell age during the cell cycle of the bacterium Escherichia coli, the fission yeast Schizosaccharomyces cerevisiae, the protozoan Amoebae proteus, cells from suspension cell cultures of mouse lymphoma and myeloma, and Chinese hamster ovary cells. In all of these cases, the buoyant densities of these cells were very narrowly distributed, with coefficients of variation of 0.1 to 0.3%. In contrast, buoyant density was variable in cells with thick cell walls and high buoyant densities. Density varied markedly during the cell cycle of the budding yeast Schizosaccharomyces cerevisiae and of the bacterium Streptococcus faecium. The average buoyant densities of cells in exponentially growing cultures of E. coli or Schizosaccharomyces pombe were also independent of growth rate of the cultures. Experiments with E. coli have established that cell buoyant density is controlled by the osmoregulatory system. Although the regulatory mechanisms for this control are unknown, the results suggest that the same or similar mechanisms regulate buoyant density in all of the cells that do not have unduly heavy cell walls and, therefore, these regulatory mechanisms were either conserved during evolution or reflect the convergent evolution found for organic osmolytes.

Molecular cloning of an osmoregulatory locus in Escherichia coli: increased proU gene dosage results in enhanced osmotolerance

The proU locus in Escherichia coli encodes an important osmoregulatory function which mediates the growth-promoting effect of L-proline and glycine betaine in high-osmolarity media. This locus was cloned, in contiguity with a closely linked Tn10 insertion, onto a multicopy plasmid directly from the E. coli chromosome. For a given level of osmotic stress, the magnitude of osmoresponsive induction of a single-copy proU::lac fusion was reduced in strains with multiple copies of the proU+ genes; in comparison with haploid proU+ strains, strains with the multicopy proU+ plasmids also exhibited enhanced osmotolerance in media supplemented with 1 mM L-proline or glycine betaine. Experiments involving subcloning, Tn1000 mutagenesis, and interplasmid complementation in a deletion mutant provided evidence for the presence at this locus of two cistrons, both of which are necessary for the expression of ProU function. We propose the designations proU for the gene originally identified by the proU224::Mu d1(lac Ap) insertion and proV for the gene upstream (that is, counterclockwise) of proU.

Cryoprotection of phosphofructokinase with organic solutes: characterization of enhanced protection in the presence of divalent cations

Phosphofructokinase (PFK) purified from rabbit skeletal muscle is fully inactivated after being frozen in liquid nitrogen for 30 s and thawed. The addition of 500 mM trehalose, sucrose, or proline to the enzyme solution prior to freezing results in a recovery of over 70% of the original activity after thawing. Slightly less stabilization is imparted by maltose and 4-hydroxyproline whereas glucose, glycine, inositol, and glycerol at concentrations up to 500 mM are relatively ineffective at protecting PFK. With 50 mM trimethylamine-N-oxide, almost 50% of the prefreeze activity is recovered, and this same level of cryoprotection is noted at concentrations up to 500 mM. The addition of ionic zinc to enzyme-organic solute mixtures prior to freezing greatly enhances the cryoprotection imparted by all of the solutes tested. This effect is not simply due to the summation of the individual cryoprotective capacities of zinc and the organic solute because in many instances a great degree of cryoprotection is noted when each component is present at a concentration at which, by itself, it is totally ineffective. In the presence of a constant 50 mM organic solute concentration, freeze-thaw stabilization of PFK is increased as the concentration of zinc is increased. When the zinc concentration is held constant (0.6 mM) and organic solute concentration varied, the maximum cryoprotection, in most cases, is noted with less than 50 mM organic solute. At higher solute concentrations the degree of enhancement decreases such that with 500 mM organic solute the addition of zinc results in only a slight increase in protection. The addition of ionic copper, cadmium, nickel, and cobalt to trehalose-PFK solutions prior to freezing also increases the percentage of activity recovered after thawing. Magnesium, manganese, and calcium are ineffective in this respect.

Osmotic regulation of transcription: induction of the proU betaine transport gene is dependent on accumulation of intracellular potassium

The proU locus, which encodes a high-affinity betaine transport system, and the kdp operon, which encodes a potassium transport system, are the principal osmoresponsive genes in Escherichia coli and Salmonella typhimurium. The kdp operon is known to be induced in response to changes in cell turgor. We have investigated the control of proU expression and shown that it differs from that of kdp in a number of fundamental ways. Rather than responding to changes in turgor, proU expression is principally determined by the intracellular accumulation of potassium ions. Potassium and betaine were shown to play distinct osmoprotective roles. Potassium serves as the principal osmoprotectant and is accumulated in response to low-level osmotic stress to restore turgor. As external osmolarity is increased to a level at which the corresponding increase in internal potassium concentrations is potentially deleterious to enzyme function, betaine (when available) is accumulated in preference to potassium. The different mechanisms of proU and kdp regulation reflect the different physiological roles of these two osmoprotectants.

Binding protein dependent transport of glycine betaine and its osmotic regulation in Escherichia coli K12

Glycine betaine, which functions as an osmoprotectant, is accumulated to high intracellular concentrations in Escherichia coli at high osmolarity. We demonstrate the presence of a high-affinity, binding protein dependent transport system for glycine betaine, which is encoded by the proU region. We show the osmotically regulated synthesis of a 32 kDa periplasmic protein that is a glycine betaine binding protein with a KD of 1.4 microM. ProU-mediated glycine betaine transport is osmotically stimulated at the level of gene expression. The osmolarity of the medium also regulates the activity of the transport system, while binding of glycine betaine to its binding protein is independent of the osmolarity. We also find a second glycine betaine transport system that is dependent on proP and exhibits a lower substrate affinity. Like ProU, this system is regulated at two levels: both gene expression and the activity of the transport system are osmotically stimulated. Using lambda plac Mu-generated lacZ operon and gene fusions, we find that expression of the proU region is osmotically regulated at the level of transcription. We cloned a part of the proU region together with the phi(proU-lacZ)hyb2 gene fusion into a multicopy plasmid and show that the DNA sequences required in cis for osmotic regulation are present on the plasmid.

Solute effects on mitochondria from an elasmobranch (Raja erinacea) and a teleost (Pseudopleuronectes americanus)

Varying osmolarity with sucrose/KCl media resulted in similar effects on the oxidation of glutamate by mitochondria isolated from the livers of an elasmobranch, Raja erinacea, and a teleost, Pseudopleuronectes americanus. In both species trimethylamine oxide (TMAO) inhibited mitochondrial oxidation of glutamate. Urea penetrated the inner mitochondrial membrane of both species and equilibrated with a ratio ureai/ureao of unity. Urea had little effect on the oxidation of glutamate in both species at concentrations as high as 760 mM. Addition of urea (urea/TMAO, 2:1) did not overcome the detrimental effects of TMAO in the mitochondria of either species. In the case of the elasmobranch, the osmolarity of the urea/TMAO media giving the optimal rate of respiration was hypoosmotic with respect to the intracellular osmolarity. The rate of glutamate oxidation steadily declined as osmolarity increased above this value. Assuming the osmotic profile obtained with the urea/TMAO (2:1) medium resembled most closely the in vivo situation, higher rates of oxidation or organic solutes at low osmolarity would help deplete the cell of these solutes and could contribute to cell volume regulation during hypoosmotic stress. It is suggested that two broad classes of intracellular solutes can be defined based on their effects on mitochondrial respiration. Solutes such as K+, C1-, and TMAO penetrate the inner mitochondrial membrane slowly or not at all. Increasing concentrations of these solutes result in lower rates of oxidation. This capacity may be important in regulating intracellular levels of organic solutes during osmotic stress. Solutes such as urea rapidly penetrate the cell and inner mitochondrial membrane reducing the mitochondrial volume changes associated with osmotic stress. The known detrimental effects of urea on protein structure may prevent its exclusive use as an intracellular osmotic effector.

Does monosodium glutamate cause flushing (or merely "glutamania")?

Monosodium glutamate is widely regarded as the provocative agent in the "Chinese restaurant syndrome," of which flushing is regarded as part of the reaction. Six subjects were monitored by laser Doppler velocimetry for changes in facial cutaneous blood flow during challenge with monosodium glutamate and its cyclization product, pyroglutamate. Additionally, records of patients challenged with monosodium glutamate in the laboratory were reviewed. No flushing was provoked among the twenty-four people tested, eighteen of whom gave a positive history of Chinese restaurant syndrome flushing. These results indicate that monosodium glutamate-provoked flushing, if it exists at all, must be rare. Monosodium glutamate and its cyclization product, pyroglutamate, may provoke edema and associated symptoms.

Regulation of envelope protein composition during adaptation to osmotic stress in Escherichia coli

Adaptation to osmotic stress alters the amounts of several specific proteins in the Escherichia coli K-12 envelope. The most striking feature of the response to elevated osmolarity was the strong induction of a periplasmic protein with an Mr of 31,000. This protein was absent in mutants with lambda plac Mu insertions in an osmotically inducible locus mapping near 58 min. The insertions are likely to be in proU, a locus encoding a transport activity for the osmoprotectants glycine betaine and proline. Factors affecting the extent of proU induction were identified by direct examination of periplasmic proteins on sodium dodecyl sulfate gels and by measuring beta-galactosidase activity from proU-lac fusions. Expression was stimulated by increasing additions of salt or sucrose to minimal medium, up to a maximum at 0.5 M NaCl. Exogenous glycine betaine acted as an osmoregulatory signal; its addition to the high-osmolarity medium substantially repressed the expression of the 31,000-dalton periplasmic protein and the proU-lac+ fusions. Elevated osmolarity also caused the appearance of a second periplasmic protein (Mr = 16,000), and severe reduction in the amounts of two others. In the outer membrane, the well-characterized repression of OmpF by high osmolarity was observed and was reversed by glycine betaine. Additional changes in membrane composition were also responsive to glycine betaine regulation.

Choline-glycine betaine pathway confers a high level of osmotic tolerance in Escherichia coli

Glycine betaine and its precursors choline and glycine betaine aldehyde have been found to confer a high level of osmotic tolerance when added exogenously to cultures of Escherichia coli at an inhibitory osmotic strength. In this paper, the following findings are described. Choline works as an osmoprotectant only under aerobic conditions, whereas glycine betaine aldehyde and glycine betaine function both aerobically and anaerobically. No endogenous glycine betaine accumulation was detectable in osmotically stressed cells grown in the absence of the osmoprotectant itself or the precursors. A membrane-bound, O2-dependent, and electron transfer-linked dehydrogenase was found which oxidized choline to glycine betaine aldehyde and aldehyde to glycine betaine at nearly the same rate. It displayed Michaelis-Menten kinetics; the apparent Km values for choline and glycine betaine aldehyde were 1.5 and 1.6 mM, respectively. Also, a soluble, NAD-dependent dehydrogenase oxidized glycine betaine aldehyde. It displayed Michaelis-Menten kinetics; the apparent Km values for the aldehyde, NAD, and NADP were 0.13, 0.06, and 0.5 mM, respectively. The choline-glycine betaine pathway was osmotically regulated, i.e., full enzymic activities were found only in cells grown aerobically in choline-containing medium at an elevated osmotic strength. Chloramphenicol inhibited the formation of the pathway in osmotically stressed cells.