Osmoregulation in Azospirillum brasilense: glycine betaine transport enhances growth and nitrogen fixation under salt stress

Addition of glycine betaine (1 mM) stimulated aerobic growth of Azospirillum brasilense Sp 7 in the presence of 0.3 M-NaC1. The nitrogenase activity of whole cells was particularly sensitive to salt stress, being almost totally inhibited in the presence of the same concentration of salt. Added glycine betaine strongly enhanced nitrogen fixation activity under salt stress. Under such conditions, maximal nitrogenase activity was obtained at a p(02),value (1 kPa) that inhibits nitrogen fixation activity in the absence of salt. We demonstrated the presence of a high affinity transport system for glycine betaine, with an apparent K(m), of 10 μM. The osmolarity of the medium regulated the activity of the transport system. The maximal transport rates were 4 and 20 nmol min(-l) (mg protein)(-l) in cells grown in low-salt and high-salt medium, respectively. A high intracellular concentration of glycine betaine (480 mM) was observed only at a high osmolarity (0.3 M-NaC1). Glycine betaine uptake was significantly reduced in osmotically shocked cells and a glycine betaine binding activity was detected in the crude periplasmic shock fluid. This suggests a transport mechanism involving a periplasmic glycine betaine binding protein. A. brasilense was unable to use the transported glycine betaine as a carbon- or nitrogen-source, in low- or high-salt medium. Intracellular glycine betaine was not catabolized.

Fructose uptake in Sinorhizobium meliloti is mediated by a high-affinity ATP-binding cassette transport system

By transposon mutagenesis, we have isolated a mutant of Sinorhizobium meliloti which is totally unable to grow on fructose as sole carbon source as a consequence of its inability to transport this sugar. The cloning and sequencing analysis of the chromosomal DNA region flanking the TnphoA insertion revealed the presence of six open reading frames (ORFs) organized in two loci, frcRS and frcBCAK, transcribed divergently. The frcBCA genes encode the characteristic components of an ATP-binding cassette transporter (FrcB, a periplasmic substrate binding protein, FrcC, an integral membrane permease, and FrcA, an ATP-binding cytoplasmic protein), which is the unique high-affinity (K(m) of 6 microM) fructose uptake system in S. meliloti. The FrcK protein shows homology with some kinases, while FrcR is probably a transcriptional regulator of the repressor-ORF-kinase family. The expression of S. meliloti frcBCAK in Escherichia coli, which transports fructose only via the phosphotransferase system, resulted in the detection of a periplasmic fructose binding activity, demonstrating that FrcB is the binding protein of the Frc transporter. The analysis of substrate specificities revealed that the Frc system is also a high-affinity transporter for ribose and mannose, which are both fructose competitors for the binding to the periplasmic FrcB protein. However, the Frc mutant was still able to grow on these sugars as sole carbon source, demonstrating the presence of at least one other uptake system for mannose and ribose in S. meliloti. The expression of the frcBC genes as determined by measurements of alkaline phosphatase activity was shown to be induced by mannitol and fructose, but not by mannose, ribose, glucose, or succinate, suggesting that the Frc system is primarily targeted towards fructose. Neither Nod nor Fix phenotypes were impared in the TnphoA mutant, demonstrating that fructose uptake is not essential for nodulation and nitrogen fixation, although FrcB protein is expressed in bacteroids isolated from alfalfa nodulated by S. meliloti wild-type strains.

Characterization of a Snorhizobium meliloti ATP-binding cassette histidine transporter also involved in betaine and proline uptake

The symbiotic soil bacterium Sinorhizobium meliloti uses the compatible solutes glycine betaine and proline betaine for both protection against osmotic stress and, at low osmolarities, as an energy source. A PCR strategy based on conserved domains in components of the glycine betaine uptake systems from Escherichia coli (ProU) and Bacillus subtilis (OpuA and OpuC) allowed us to identify a highly homologous ATP-binding cassette (ABC) binding protein-dependent transporter in S. meliloti. This system was encoded by three genes (hutXWV) of an operon which also contained a fourth gene (hutH2) encoding a putative histidase, which is an enzyme involved in the first step of histidine catabolism. Site-directed mutagenesis of the gene encoding the periplasmic binding protein (hutX) and of the gene encoding the cytoplasmic ATPase (hutV) was done to study the substrate specificity of this transporter and its contribution in betaine uptake. These mutants showed a 50% reduction in high-affinity uptake of histidine, proline, and proline betaine and about a 30% reduction in low-affinity glycine betaine transport. When histidine was used as a nitrogen source, a 30% inhibition of growth was observed in hut mutants (hutX and hutH2). Expression analysis of the hut operon determined using a hutX-lacZ fusion revealed induction by histidine, but not by salt stress, suggesting this uptake system has a catabolic role rather than being involved in osmoprotection. To our knowledge, Hut is the first characterized histidine ABC transporter also involved in proline and betaine uptake.

Presence of a gene encoding choline sulfatase in Sinorhizobium meliloti bet operon: choline-O-sulfate is metabolized into glycine betaine

Glycine betaine is a potent osmoprotectant accumulated by Sinorhizobium meliloti to cope with osmotic stress. The biosynthesis of glycine betaine from choline is encoded by an operon of four genes, betICBA, as determined by sequence and mutant analysis. The betI and betC genes are separated by an intergenic region containing a 130-bp mosaic element that also is present between the betB and betA genes. In addition to the genes encoding a presumed regulatory protein (betI), the betaine aldehyde dehydrogenase (betB), and the choline dehydrogenase (betA) enzymes also found in Escherichia coli, a new gene (betC) was identified as encoding a choline sulfatase catalyzing the conversion of choline-O-sulfate and, at a lower rate, phosphorylcholine, into choline. Choline sulfatase activity was absent from betC but not from betB mutants and was shown to be induced indifferently by choline or choline-O-sulfate as were the other enzymes of the pathway. Unlike what has been shown in other bacteria and plants, choline-O-sulfate is not used as an osmoprotectant per se in S. meliloti, but is metabolized into glycine betaine. S. meliloti also can use this compound as the sole carbon, nitrogen, and sulfur source for growth and that depends on a functional bet locus. In conclusion, choline-O-sulfate and phosphorylcholine, which are found in higher plants and fungi, appear to be substrates for glycine betaine biosynthesis in S. meliloti.

Choline and glycine betaine uptake in various strains of Rhizobia isolated from nodules of Vicia faba var. major and Cicer arietinum l.: modulation by salt, choline, and glycine betaine

Two strains of Rhizobia isolated from nodules of Vicia faba var. major and one strain isolated from nodules of Cicer arietinum L. were characterized for salt resistance. The presence of 1 mM glycine betaine or choline in a minimal medium with added NaCl had a beneficial role on the growth of the three strains. Both molecules were found to be taken up by cells obtained at low osmolarity, and whereas glycine betaine uptake activity was stimulated significantly in cells grown in the presence of 0.15 M NaCl, choline uptake activity was strongly inhibited by salt in all tested strains. However, in cells grown with exogenous choline,the uptake inhibition exerted by salt was relieved, mainly in the strain isolated from nodules of C. arietinum L. On the basis of kinetics determinations, in control cells as well as in salt-stressed cells, only high-affinity activities were observed for glycine betaine and choline(apparent Kms between 3 and 18 micro;M). Periplasmic proteins that bound glycine betaine or choline were identified. In nondenaturing conditions, these proteins extracted from the various strains showed different electrophoretic mobility with always a less negative entire charge than the analogous proteins from Rhizobium meliloti.

A prominent role for glucosylglycerol in the adaptation of Pseudomonas mendocina SKB70 to osmotic stress

The mechanism of osmoadaptation in a salt-tolerant (1.2 M NaCl) bacterial isolate identified as Pseudomonas mendocina (N. J. Palleroni, M. Doudoroff, R. Y. Stanier, R. E. Solanes, and R. Mandel, J. Gen. Microbiol. 60:215-231, 1970) was investigated. In response to osmotic stress, this species accumulated a number of compatible solutes, the intracellular levels of which depended on both the osmolarity and the ionic composition of the growth medium. Glucosylglycerol [alpha-D-glucopyranosyl-alpha-(1-->2)-glycerol], N-acetylglutaminylglutamine amide, and L-alpha-glutamate were the major compatible solutes accumulated via de novo biosynthesis. Trehalose was also accumulated, but only in cells grown in the presence of high concentrations of sulfate or phosphate ions. Glycine betaine was accumulated only when supplied exogenously to cells grown at high osmolarity, and its accumulation caused a significant depletion of the intracellular pools of glucosylglycerol and glutamate. Glucosylglycerol was also found to accumulate in the type strains of P. mendocina and P. pseudoalcaligenes. This is the first report demonstrating the pivotal role of glucosylglycerol in osmoadaptation in a nonphotosynthetic microorganism.

Characterization of an osmoregulated periplasmic glycine betaine-binding protein in Azospirillum brasilense sp7

Azospirillum brasilense is able to use glycine betaine as a powerful osmoprotectant; the uptake of this compound is strongly stimulated by salt stress, but significantly reduced by cold osmotic shock. Non-denaturing PAGE in the presence of [methyl-14C] glycine betaine and autoradiography demonstrated the presence of one glycine betaine-binding protein (GBBP) in periplasmic shock fluid obtained from high-osmolarity-grown cells. The binding activity was absent in periplasmic fractions from cells grown at low osmolarity. SDS-PAGE analysis showed that the osmotically inducible GBBP has an apparent molecular weight of 32,000. The isoelectric point was between 5.9 and 6.6, as determined by isoelectric focusing. This protein bound glycine betaine with high affinity (KD of 3 microM), but had no affinity for either other betaines (proline betaine, gamma-butyrobetaine, pipecolate betaine, trigonelline, homarine) or related compounds (choline, glycine betaine aldehyde, glycine and proline). Optimum binding activity occurred at pH 7.0 to 7.5, and was not altered whether or not the binding assays were done at low or high osmolarity. Immunoprecipitation and Western blotting showed that immunoadsorbed anti-GBBP antibody from E coli cross-reacted with the GBBP produced by A brasilense cells grown at high osmolarity.

Effects of Salt Stress on Amino Acid, Organic Acid, and Carbohydrate Composition of Roots, Bacteroids, and Cytosol of Alfalfa (Medicago sativa L.)

Ethanol-soluble organic acid, carbohydrate, and amino acid constituents of alfalfa (Medicago sativa) roots and nodules (cytosol and bacteroids) have been identified by gas-liquid chromatography and high performance liquid chromatography. Among organic acids, citrate was the predominant compound in roots and cytosol, with malonate present in the highest concentration in bacteroids. These two organic acids together with malate and succinate accounted for more than 85% of the organic acid pool in nodules and for 97% in roots. The major carbohydrates in roots, nodule cytosol, and bacteroids were (descending order of concentration): sucrose, pinitol, glucose, and ononitol. Maltose and trehalose appeared to be present in very low concentrations. Asparagine, glutamate, alanine, gamma-aminobutyrate, and proline were the major amino acids in cytosol and bacteroids. In addition to these solutes, serine and glutamine were well represented in roots. When alfalfa plants were subjected to 0.15 m sodium chloride stress for 2 weeks, total organic acid concentration in nodules and roots were depressed by more than 40%, whereas lactate concentration increased by 11, 27, and 94% in cytosol, roots, and bacteroids, respectively. In bacteroids, lactate became the most abundant organic acid and might contribute partly to the osmotic adjustment. On the other hand, salt stress induced a large increase in the amino acid and carbohydrate pools. Within the amino acids, proline showed the largest increase, 11.3-, 12.8-, and 8.0-fold in roots, cytosol, and bacteroids, respectively. Its accumulation reflected an osmoregulatory mechanism not only in roots but also in nodule tissue. In parallel, asparagine concentration was greatly enhanced; this amide remained the major nitrogen solute and, in bacteroids, played a significant role in osmoregulation. On the contrary, the salt treatment had a very limited effect on the concentration of other amino acids. Among carbohydrates, pinitol concentration was increased significantly, especially in cytosol and bacteroids (5.4- and 3.4-fold, respectively), in which this cyclitol accounted for more than 35% of the total carbohydrate pool; pinitol might contribute to the tolerance to salt stress. However, trehalose concentration remained low in both nodules and roots; its role in osmoregulation appeared unlikely in alfalfa.

Identification of an osmotically induced periplasmic glycine betaine-binding protein from Rhizobium meliloti

The effect of salt stress on glycine betaine-binding activity has been investigated in periplasmic fractions released from Rhizobium meliloti 102F34 by cold osmotic shock. Binding activity was monitored by three techniques: equilibrium dialysis, filter procedure, and detection of 14C ligand-protein binding by direct non-denaturing polyacrylamide gel electrophoresis (PAGE) followed by autoradiography. The three methods demonstrated the existence of a strong glycine betaine-binding activity, but only in periplasmic fractions from cells grown at high osmolarity. The non-denaturing PAGE of such periplasmic shock fluids mixed with [methyl-14C]glycine betaine showed only one radioactive band, indicating the involvement of one glycine betaine-binding protein. To determine the possible implication of this binding protein in glycine betaine uptake, transport activity was measured with cells submitted to cold osmotic shock. No significant decrease of transport activity was noticed. This lack of effect could be explained by the small quantity of periplasmic proteins released as judged by the low activity of phosphodiesterase, a periplasmic marker enzyme, observed in the shock fluid. The specificity of binding was analysed with different potential competitors: other betaines such as gamma-butyrobetaine, proline betaine, pipecolate betaine, trigonelline and homarine, or amino acids like glycine and proline, did not bind to the glycine betaine-binding protein, whereas glycine betaine aldehyde and choline were weak competitors. Optimum pH for binding was around 7.0, but approx. 90% of the glycine betaine-binding activity remained at pH 6.0 or 8.0. The calculated binding affinity (KD) was 2.5 microM. Both glycine betaine-binding activity and affinity were not significantly modified whether or not the binding assays were done at high osmolarity. A 32 kDa osmotically inducible periplasmic protein, identified by SDS-PAGE, apparently corresponds to the glycine betaine-binding protein.

Intracellular accumulation of potassium and glutamate specifically enhances survival of Escherichia coli in seawater

The high resistance of Escherichia coli grown in saline media to seawater was suppressed by an osmotic down-shock. The shock released several molecules into the medium, including potassium, glutamate, and glycine betaine when cells were previously grown in the presence of this osmolyte. Incubation of such sensitized cells in a solution containing K+ (80 mM) and glutamate (50 mM) at pH 7.4 restored their resistance to seawater up to a level close to that observed initially. The protective effect was partly due to the rapid accumulation of K+; a significant exponential relationship between intracellular concentration of K+ and resistance to seawater was observed. Glutamate was accumulated more slowly and progressively completed the action of K+. These data emphasize the specific influence of potassium glutamate on osmotically stressed E. coli cells. They confirm that regulation of osmotic pressure and, probably, of intracellular pH strongly enhances survival of E. coli in seawater. Osmotic fluctuations in waters carrying enteric bacteria from intestines to seawater, together with variations in their K+ and amino acid contents, could modify the ability of cells to survive in marine environments. These results demonstrate the need to strictly control conditions (K+ content, temperature) used to wash cells before their transfer to seawater microcosms. They suggest that the K+ and glutamate contents of media in which E. coli cells are transported to the sea can influence their subsequent survival in marine environments.

Survival in seawater of Escherichia coli cells grown in marine sediments containing glycine betaine

Considering both the protective effect of glycine betaine (GB) on enteric bacteria grown at high osmolarity and the possible presence of GB in marine sediments, we have analyzed the survival, in nutrient-free seawater, of Escherichia coli cells incubated in sediments supplemented with GB or not supplemented and measured the efficiency of GB uptake systems and the expression of proP and proU genes in both seawater and sediments. We did this by using strains harboring proP-lacZ and proU-lacZ operon or gene fusions. We found that the uptake of GB and the expression of both proP and proU were very weak in seawater. The survival ability of cells in seawater supplemented with GB was a linear function of GB concentration, although the overall protection by the osmolyte was low. In sediments, proP expression was weak and GB uptake and proU expression were variable, possibly depending on the availability of organic nutrients. In a sediment with a high total organic carbon content, GB uptake was very high and proU expression was enhanced; cells previously incubated in this sediment showed a higher resistance to decay in seawater. GB might therefore play a significant role in the long-term maintenance of enteric bacterial cells in some marine sediments.

Uptake of glycine betaine and its analogues by bacteroids of Rhizobium meliloti

Bacteroids isolated from alfalfa nodules induced by Rhizobium meliloti 102F34 transported glycine betaine at a constant rate for up to 30 min. Addition of sodium salts greatly increased the uptake activity, whereas other salts or non-electrolytes had less effect. The apparent Km for glycine betaine uptake was 8.3 microM and V was about 0.84 nmol min-1 (mg protein)-1 in the presence of 200 mM-NaCl which gave maximum stimulation of the transport. Supplementing bacteroid suspensions with various energy-yielding substrates, or ATP, did not increase glycine betaine uptake rates. The uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), and the respiratory inhibitor potassium cyanide strongly inhibited glycine betaine uptake, but arsenate was totally inactive. Glycine betaine transport showed considerable structural specificity: choline, proline betaine, gamma-butyrobetaine and trigonelline did not competitively inhibit the system, although choline and proline betaine were transported by bacteroids. Both a high-affinity activity and a low-affinity activity were found for choline uptake. These osmoprotective compounds might have a significant role in the maintenance of nitrogenase activity in bacteroids subjected to salt stress.

Characterization of three choline transport activities in Rhizobium meliloti: modulation by choline and osmotic stress

Choline has both a nutritional and osmoregulatory role in Rhizobium meliloti (T. Bernard, J. A. Pocard, B. Perroud, and D. Le Rudulier, Arch. Microbiol. 143:359-364, 1986). In view of this fact, choline transport was studied in R. meliloti 102F34 to determine how the rate of choline uptake is modulated. The effects of the cultural conditions on the kinetics of transport are presented. A high-affinity activity and a low-affinity activity were found in cells grown in minimal medium. The addition of 0.3 M NaCl or other osmolytes to the medium resulted in a marked decrease in the high-affinity activity, whereas the low-affinity activity remained fairly constant. Furthermore, results from osmotic upshock and downshock experiments indicate that the response of the cell to high osmolarity is rapid; hence, the mechanism of regulation by salt likely does not involve gene induction. A second high-affinity transport activity was induced by choline itself. Like the constitutive low-affinity transport activity, this activity was not greatly altered when the cells were grown in media of elevated osmotic strength. We conclude that although all three kinetically distinct transport systems are efficient at low osmolarity, only the induced high- and low-affinity activities are important for osmoregulation. The characteristics of the three transport activities from R. meliloti are compared with those from other bacterial species that use choline for growth and/or osmoregulation.

Osmotic control of glycine betaine biosynthesis and degradation in Rhizobium meliloti

Intracellular accumulation of glycine betaine has been shown to confer an enhanced level of osmotic stress tolerance in Rhizobium meliloti. In this study, we used a physiological approach to investigate the mechanism by which glycine betaine is accumulated in osmotically stressed R. meliloti. Results from growth experiments, 14C labeling of intermediates, and enzyme activity assays are presented. The results provide evidence for the pathway of biosynthesis and degradation of glycine betaine and the osmotic effects on this pathway. High osmolarity in the medium decreased the activities of the enzymes involved in the degradation of glycine betaine but not those of enzymes that lead to its biosynthesis from choline. Thus, the concentration of the osmoprotectant glycine betaine is increased in stressed cells. This report demonstrates the ability of the osmolarity of the growth medium to regulate the use of glycine betaine as a carbon and nitrogen source or as an osmoprotectant. The mechanisms of osmoregulation in R. meliloti and Escherichia coli are compared.

Glycine betaine transport in Escherichia coli: osmotic modulation

Exogenous glycine betaine highly stimulates the growth rate of various members of the Enterobacteriaceae, including Escherichia coli, in media with high salt concentrations (D. Le Rudulier and L. Bouillard, Appl. Environ. Microbiol. 46:152-159, 1983). In a nitrogen- and carbon-free medium, glycine betaine did not support the growth of E. coli either on low-salt or high-salt media. This molecule was taken up by the cells but was not catabolized. High levels of glycine betaine transport occurred when the cells were grown in media of elevated osmotic strength, whereas relatively low activity was found when the cells were grown in minimal medium. A variety of electrolytes, such as NaCl, KCl, NaH2PO4, K2HPO4, K2SO4, and nonelectrolytes like sucrose, raffinose, and inositol triggered the uptake of glycine betaine. Furthermore, in cells subjected to a sudden osmotic upshock, glycine betaine uptake showed a sixfold stimulation 30 min after the addition of NaCl. Part of this stimulation might be a consequence of protein synthesis. The transport of glycine betaine was energy dependent and occurred against a concentration gradient. 2,4-Dinitrophenol almost totally abolished the glycine betaine uptake. Azide and arsenate exerted only a small inhibition. In addition, N,N'-dicyclohexylcarbodiimide had a very low inhibitory effect at 1 mM. These results indicated that glycine betaine transport is driven by the electrochemical proton gradient. The kinetics of glycine betaine entry followed the Michaelis-Menten relationship, yielding a Km of 35 microM and a Vmax of 42 nmol min-1 mg of protein-1. Glycine betaine transport showed considerable structural specificity. The only potent competitor was proline betaine when added to the assay mixtures at 20-fold the glycine betaine concentration. From these results, it is proposed that E. coli possesses an active and specific glycine betaine transport system which is regulated by the osmotic strength of the growth medium.

Molecular Biology of Osmoregulation

The drought of 1983 resulted in some 10 billion dollars in agricultural losses and has focused attention on the vulnerability of our major crops to this devastating form of environmental stress. This article is concerned with the molecular biology of a new class of genes, called osm (osmotic tolerance) genes, that protect bacteria like Escherichia coli against osmotic stress and may work in a similar manner in plants and animals. Osm genes govern the production of a class of molecules, such as betaine and proline, that protect the cell and its constituents against dehydration. These osmoprotectant molecules have been known for many years to accumulate in plants but have only recently been shown to have potent antistress activity for bacteria.

Osmoregulation in Klebsiella pneumoniae: enhancement of anaerobic growth and nitrogen fixation under stress by proline betaine, gamma-butyrobetaine, and other related compounds

Exogenous proline betaine ( stachydrine or N- dimethylproline ) or gamma-butyrobetaine (gamma-trimethylaminobutyrate), at a concentration as low as 1 mM, were found to stimulate the growth rate of Klebsiella pneumoniae, wild type M5A1 , in media of inhibitory osmotic strength (0.8 M NaC1). Simultaneously, nitrogen fixation by whole cells, a process particularly sensitive to osmotic stress, was strongly enhanced by these compounds. However, in the absence of sodium chloride, both the growth and nitrogen fixation were not affected by the addition of the methylammonium derivatives in the medium. The sensitivity of the nitrogen fixation to osmotic stress was used as a bioassay to evaluate the potentiality of osmoprotective compound in relation to the number of methyl groups on the nitrogen atom of glycine, proline, and gamma-aminobutyrate. Experiments with sarcosine ( monomethylglycine ), dimethylglycine, and glycine betaine ( trimethylglycine ), or experiments with mono- and di- methylproline or gamma-mono-, gamma-di, gamma-tri- methylaminobutyrate , indicated that the greatest stress tolerance was always obtained with the more N-methylated compounds.

Glycine betaine, an osmotic effector in Klebsiella pneumoniae and other members of the Enterobacteriaceae

Osmoregulation was examined in members of the Enterobacteriaceae. Exogenous glycine betaine at a concentration as low as 1 mM was found to stimulate the growth rate of Escherichia coli, Salmonella typhimurium, and Klebsiella pneumoniae in media of inhibitory osmotic strength. The stimulation was shown to be independent of any specific solutes, electrolytes, or nonelectrolytes. Therefore, the stimulatory effect of glycine betaine was a consequence of high osmotic potential. This effect was found to be far greater than the proline effect previously observed in S. typhimurium. Whereas nitrogen fixation by K. pneumoniae is completely inhibited under conditions of osmotic stress, nitrogenase activity could be partially restored by the addition of exogenous glycine betaine to the culture medium. Furthermore, glycine betaine in combination with proline, especially proline produced internally at a high level because of regulatory mutations affecting proline biosynthesis, strongly stimulated nitrogen fixation activity during osmotic stress. Glycine betaine was accumulated by the cells, and the amount taken up was correlated with the osmolarity of the medium. These findings are discussed in relation to the possible mechanisms by which glycine betaine might cause enhanced osmotolerance.

Nitrogen fixation in Klebsiella pneumoniae during osmotic stress. Effect of exogenous proline or a proline overproducing plasmid

Osmotic stress, imposed by 0.5 M NaCl or other electrolytes and non-electrolytes, caused over a 100-fold reduction in the whole-cell nitrogen fixation activity of Klebsiella pneumoniae, wild-type strain M5A1. This reduction of nitrogen fixation activity could be reversed by the addition of proline to the culture medium at 0.5 mM concentration. With 0.5 M NaCl, in the presence of proline, nitrogenase activity was 47-fold greater than in the absence of proline. A mutation, originally isolated in Salmonella typhimurium, which resulted in proline over-production and enhanced osmotolerance, was transferred into K. pneumoniae by F' conjugation. Intracellular proline, synthesized at high levels because of the mutation, had similar stimulatory effects on nitrogen fixation under osmotic stress as proline provided exogenously. In the overproducing strain, the cellular level of proline is elevated as much as 125-fold during stress over that seen in the control strain. To determine the mechanism of stimulation of nitrogen fixation by proline during stress, the biosynthesis of nitrogenase polypeptides was studied. Net nitrogenase biosynthesis and the biosynthesis of other unidentified peptides, is strongly inhibited during osmotic stress; proline reverses the inhibition. The role of proline in enhancing nitrogen fixation during osmotic stress is discussed.