Initial Organic Products of Fixation of [N]Dinitrogen by Root Nodules of Soybean (Glycine max)

Nitrogen Assimilation and Transport by Ex Planta Nitrogen-Fixing Bradyrhizobium diazoefficiens Bacteroids Is Modulated by Oxygen, Bacteroid Density and l-Malate

Symbiotic nitrogen fixation requires the transfer of fixed organic nitrogen compounds from the symbiotic bacteria to a host plant, yet the chemical nature of the compounds is in question. Bradyrhizobium diazoefficiens bacteroids were isolated anaerobically from soybean nodules and assayed at varying densities, varying partial pressures of oxygen, and varying levels of l-malate. Ammonium was released at low bacteroid densities and high partial pressures of oxygen, but was apparently taken up at high bacteroid densities and low partial pressures of oxygen in the presence of l-malate; these later conditions were optimal for amino acid excretion. The ratio of partial pressure of oxygen/bacteroid density of apparent ammonium uptake and of alanine excretion displayed an inverse relationship. Ammonium uptake, alanine and branch chain amino acid release were all dependent on the concentration of l-malate displaying similar K0.5 values of 0.5 mM demonstrating concerted regulation. The hyperbolic kinetics of ammonium uptake and amino acid excretion suggests transport via a membrane carrier and also suggested that transport was rate limiting. Glutamate uptake displayed exponential kinetics implying transport via a channel. The chemical nature of the compounds released were dependent upon bacteroid density, partial pressure of oxygen and concentration of l-malate demonstrating an integrated metabolism.

On-line rapid purification of [13N]N2 gas for visualization of nitrogen fixation and translocation in nodulated soybean

Accurate analysis of N fixation in leguminous crops requires determination of N utilization within an intact plant; however, most approaches require tissue disassembly. We developed a simple and rapid technique to generate high-purity and high-yield [13N]N2 gas and obtained real-time images of N fixation in an intact soybean plant. The purification efficiency was ∼81.6% after decay correction. Our method provides accurate signals of N fixation and allows free changes to the tracer gas composition to suit different experimental designs.

Use of gaseous 13NH3 administered to intact leaves of Nicotiana tabacum to study changes in nitrogen utilization during defence induction

Nitrogen-13 (t(1/2) 9.97 m), a radioactive isotope of nitrogen, offers unique opportunities to explore plant nitrogen utilization over short time periods. Here we describe a method for administering (13)N as gaseous (13)NH(3) to intact leaves of Nicotiana tabacum L. (cv Samsun), and measuring the labelled amino acids using radio high-performance liquid chromatography (HPLC) on tissue extract. We used this method to study the effects of defence induction on plant nitrogen utilization by applying treatments of methyl jasmonate (MeJA), a potent defence elicitor. MeJA caused a significant increase relative to controls in key [(13)N]amino acids, including serine, glycine and alanine by 4 h post-treatment, yet had no effect on (13)NH(3) incorporation, a process that is primarily under the control of the glutamine synthatase/glutamate synthase pathway (GS/GOGAT) in cellular photorespiration. We suggest that the reconfiguration of nitrogen metabolism may reflect induction of non-photorespiratory sources of nitrogen to better serve the plant's defences.

Nitrogen assimilation in alfalfa: isolation and characterization of an asparagine synthetase gene showing enhanced expression in root nodules and dark-adapted leaves

Asparagine, the primary assimilation product from N2 fixation in temperate legumes and the predominant nitrogen transport product in many plant species, is synthesized via asparagine synthetase (AS; EC 6.3.5.4). Here, we report the isolation and characterization of a cDNA and a gene encoding the nodule-enhanced form of AS from alfalfa. The AS gene is comprised of 13 exons separated by 12 introns. The 5' flanking region of the AS gene confers nodule-enhanced reporter gene activity in transformed alfalfa. This region also confers enhanced reporter gene activity in dark-treated leaves. These results indicate that the 5' upstream region of the AS gene contains elements that affect expression in root nodules and leaves. Both AS mRNA and enzyme activity increased approximately 10- to 20-fold during the development of effective nodules. Ineffective nodules have strikingly reduced amounts of AS transcript. Alfalfa leaves have quite low levels of AS mRNA and protein; however, exposure to darkness resulted in a considerable increase in both. In situ hybridization with effective nodules and beta-glucuronidase staining of nodules from transgenic plants showed that AS is expressed in both infected and uninfected cells of the nodule symbiotic zone and in the nodule parenchyma. RNA gel blot analysis and in situ hybridization results are consistent with the hypothesis that initial AS expression in nodules is independent of nitrogenase activity.

Some processes related to nitrogen fixation in nodulated legumes

We have summarized information in four areas of the broad topic of legume- Rhizobium symbiosis. These include: carbon substrates provided to nodule bacteroids by the host, assimilation of fixed nitrogen by the host, O 2 metabolism in legume nodules and involvement of H 2 in nodule metabolism. Although nodules contain a variety of carbon substrates, both biochemical and genetic evidence indicate that C4 dicarboxylates are the major carbon substrates that support N 2 fixation in nodules. The biochemical pathways for utilization of products of N 2 fixation are fairly well understood but relatively little is known about the regulation of the assimilation of fixed nitrogenous compounds at the gene level. Ureides are primary nitrogenous compounds exported from nodules of the tropical legumes. Because the catabolism of these products may involve the hydrolysis of urea by nickel-dependent urease, the possible importance of nickel as a trace element in the nutrition of legumes is raised. The O 2 supply to nodule bacteroids is regulated by a barrier to free-O 2 diffusion and by leghaemoglobin. Progress has been made in understanding of the molecular genetics and biochemistry of leghaemoglobin but little is known about the mechanisms that control the physical barrier to O 2 diffusion. Legume nodules contain mechanisms for the disposition of peroxide and free radicals of oxygen. The importance of these systems as protective mechanisms for the O 2 -labile nitrogenase is discussed. Some strains of Rhizobium form nodules which recycle the H 2 produced as a byproduct of N 2 fixation. The genes necessary for H 2 oxidation have been cloned and transferred within and among species of Rhizobium . The advantages and disadvantages of H 2 recycling in legume nodules are discussed.

Expression pattern of uricase II gene during root nodule development in Phaseolus vulgaris

A Phaseolus vulgaris uricase II cDNA clone has been isolated and sequenced. Comparison on the nucleotide level between this clone and the soybean uricase II clone revealed 88.8% identity. The in situ hybridization technique was used to follow the expression pattern in developing root nodules of Phaseolus vulgaris. The uricase II transcripts were localized only in the uninfected cells of the central tissue and mainly in the periphery of the cell. Uricase II mRNA is first detected in nodules 12 days after infection. A maximum level of transcripts is reached in 21-day-old nodules, followed by a considerable reduction in 28-day-old nodules.

Glutamate synthase in Medicago sativa L. Occurrence and properties of FD-dependent enzyme in plant cell fraction during root nodule development

In the plant cell fraction of Medicago sativa (L. cv Europe) nodules, glutamate synthase is active with reduced Fd, MV, NADH and NADPH as an electron donor. Up to 25 to 30 days after inoculation, the activities of Fd-dependent glutamate synthase (EC 1.4.1.7), the most active form of the enzyme, NADH-dependent (EC 1.4.1.14) and NADPH-dependent (EC 1.4.1.13) glutamate synthases increase about 2-fold followed by a relatively constant level per gram fresh weight of nodules. The activities of glutamate synthases with different electron carriers increase constantly about 30-fold after 46 days of inoculation by total fresh weight of nodules per plant. These nodule glutamate synthase activities with Fd, NADH or NADPH represent 30% relative to those of root glutamate synthases per plant with the respective electron donor. Fd-glutamate synthase in nodule plant fraction is a protein molecule immunochemically distinct from pyridine nucleotide-glutamate synthases. MV-linked enzyme activity is associated with Fd-glutamate synthase. The Fd-glutamate synthase has a subunit molecular mass of 68.2 kDa, and it exhibits a high affinity for spinach Fd as an electron carrier. The increase in Fd-glutamate synthase activity during nodule development is accompanied by a rise in the enzyme protein content. The total activity of different forms of glutamate synthase in vitro ensures a higher level than the rate of ammonia production during N2 fixation in bacteroids of Medicago sativa nodules.

Azolla-Anabaena Relationship : XIII. Fixation of [N]N(2)

The major radioactive products of the fixation of [(13)N]N(2) by Azolla caroliniana Willd.-Anabaena azollae Stras. were ammonium, glutamine, and glutamate, plus a small amount of alanine. Ammonium accounted for 70 and 32% of the total radioactivity recovered after fixation for 1 and 10 minutes, respectively. The presence of a substantial pool of [(13)N]N(2)-derived (13)NH(4) (+) after longer incubation periods was attributed to the spatial separation between the site of N(2)-fixation (Anabaena) and a second, major site of assimilation (Azolla). Initially, glutamine was the most highly radioactive organic product formed from [(13)N]N(2), but after 10 minutes of fixation glutamate had 1.5 times more radiolabel than glutamine. These kinetics of radiolabeling, along with the effects of inhibitors of glutamine synthetase and glutamate synthase on assimilation of exogenous and [(13)N]N(2)-derived (13)NH(4) (+), indicate that ammonium assimilation occurred by the glutamate synthase cycle and that glutamate dehydrogenase played little or no role in the synthesis of glutamate by Azolla-Anabaena.

Isozymes of Glutamine Synthetase in Phaseolus vulgaris L. and Phaseolus lunatus L. Root Nodules

The glutamine synthetase (GS) isozymes in the plant fraction of nodule extracts from 62 cultivars of Phaseolus vulgaris L. and one cultivar of Phaseolus lunatus L. were analyzed by polyacrylamide gel electrophoresis. All P. vulgaris nodule extracts displayed two GS activity bands: a nodule-specific band (GS(n1)) and a band (GS(n2)) similar to the single band (GS(r)) present in root extracts. In nodule extracts of P. lunatus, the GS(n1) band was detected, but the GS(n2) band was barely detectable. In contrast to P. vulgaris, the GS(n2) band and the GS(r) band of P. lunatus appeared to be different. The electrophoretic mobility of the GS(n1) band in P. vulgaris was governed by both the plant cultivar and the development stage of the nodule. In nodule extracts of P. vulgaris and P. lunatus, the zone of GS(n1) activity coincided with six to nine distinct protein bands as revealed after treatment of gels, which had previously been stained for GS activity, with Coomassie blue. All these protein bands were shown to consist of polypeptides of identical molecular weight (approximately 47,000 daltons) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results indicate that P. vulgaris continuously generates isozymes of GS(n1) of increasing electrophoretic mobility during the course of nodule development.

Fixation of [(13)N]N 2 and transfer of fixed nitrogen in the Anthoceros-Nostoc symbiotic association

The initial product of fixation of [(13)N]N2 by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [(13)N]N2 after incubation times of 0.5-10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N2-derived NH 4 (+) by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous (13)NH 4 (+) into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N2-derived NH 4 (+) . This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [(13)N]N2 in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N2-derived NH 4 (+) and that NH 4 (+) was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N2 was lost to the suspension medium, it appears that transfer of NH 4 (+) from symbiont to host tissue was very efficient in this extracellular symbiotic association.

Assimilation of (13)NH 4 (+) by Anthoceros grown with and without symbiotic Nostoc

The pathways of assimilation of ammonium by pure cultures of symbiont-free Anthoceros punctatus L. and the reconstituted Anthoceros-Nostoc symbiotic association were determined from time-course (5-300 s) and inhibitor experiments using (13)NH 4 (+) . The major product of assimilation after all incubation times was glutamine, whether the tissues were cultured with excess ammonium or no combined nitrogen. The (13)N in glutamine was predominantly in the amide-nitrogen position. Formation of glutamine and glutamate by Anthoceros-Nostoc was strongly inhibited by either 1mM methionine sulfoximine (MSX) or 1 mM exogenous ammonium. These data are consistent with the assimilation of (13)NH 4 (+) and formation of glutamate by the glutamine synthetase (EC 6.3.1.2)-glutamate synthase (EC 1.4.7.1) pathway in dinitrogen-grown Anthoceros-Nostoc. However, in symbiont-free Anthoceros, grown with 2.5 mM ammonium, formation of glutamine, but not glutamate, was decreased by either MSX or exogenous ammonium. These results indicate that during short incubation times ammonium is assimilated in nitrogenreplete Anthoceros by the activities of both glutamine synthetase and glutamate dehydrogenase (EC 1.4.1.2). In-vitro activities of glutamine synthetase were similar in nitrogen-replete Anthoceros and Anthoceros-Nostoc, indicating that the differences in the routes of glutamate formation were not based upon regulation of synthesis of the initial enzyme of the glutamine synthetase-glutamate synthase pathway. When symbiont-free Anthoceros was cultured for 2 d in the absence of combined nitrogen, total (13)NH 4 (+) assimilation, and glutamine and glutamate formation in the presence of inhibitors, were similar to dinitrogen-grown Anthoceros-Nostoc. The routes of immediate (within 2 min) glutamate formation and ammonium assimilation in Anthoceros were apparently determined by the intracellular levels of ammonium; at low levels the glutamine synthetase-glutamate synthase pathway was predominant, while at high levels independent activities of both glutamine synthetase and glutamate dehydrogenase were expressed.

Appearance of a novel form of plant glutamine synthetase during nodule development in Phaseolus vulgaris L

The activities of glutamine synthetase (GS), nitrogenase and leghaemoglobin were measured during nodule development in Phaseolus vulgaris infected with wild-type or two non-fixing (Fix(-)) mutants of Rhizobium phaseoli. The large increase in GS activity which was observed during nodulation with the wild-type rhizobial strain occurred concomitantly with the detection and increase in activity of nitrogenase and the amount of leghaemoglobin. Moreover, this increase in GS was found to be due entirely to the appearance of a novel form of the enzyme (GSn1) in the nodule. The activity of the form (GSn2) similar to the root enzyme (GSr) remained constant throughout the experiment. In nodules produced by infection with the two mutant strains of Rhizobium phaseoli (JL15 and JL19) only trace amounts of GSn1 and leghaemoglobin were detected.

Ammonia Assimilation in the Roots of Nitrate- and Ammonia-Grown Hordeum Vulgare (cv Golden Promise)

(15)N kinetic labeling studies were performed on seedlings of Hordeum vulgare L. var. Golden Promise growing under steady state conditions. Patterns of label incorporation in the pools of nitrogen compounds of roots fed [(15)N]ammonium were compared with computer-simulated labeling curves. The data were found to be quantitatively consistent with a three-compartment model in which ammonium is assimilated solely into the amide-N of glutamine. Labeling data from roots fed [(15)N]nitrate were also found to be at least qualitatively consistent with the assimilation of ammonia into glutamine. Methionine sulfoximine almost completely blocked the incorporation of (15)N label into the amino acid pools of barley roots fed [(15)N]nitrate. These observations suggest that ammonia assimilation occurs solely via the glutamine synthetase/glutamate synthase pathway in both nitrate- and ammonia-grown barley roots.

Isolation and Immunochemical Characterization of Plant Glutamine Synthetase in Alfalfa (Medicago sativa L.) Nodules

Host plant glutamine synthetase (GS) has been purified 100-fold from N(2)-fixing alfalfa (Medicago sativa L.) nodules by a new procedure involving preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as a final step. An SDS-polypeptide fraction corresponding to plant GS was identified and consisted of two major polypeptides of 40,000 to 45,000 molecular weight. Antibodies to the SDS-polypeptide fraction were raised in mice by intraperitoneal injection, and antisera were collected as ascitic fluid. Crude extracts of soluble protein from the plant fraction of nodules were resolved by SDS-PAGE and then subjected to electrophoresis in the second dimension into antibody-containing agarose gel. A single immunochemically active protein species was observed using this crossed immunoelectrophoresis method, even though both major GS SDS-polypeptides were apparently resolved in the first (SDS-PAGE) dimension. Plant GS protein in crude nodule extracts was quantitated immunochemically by comparison with immunoprecipitin arcs of similarly treated amounts of pure antigen. Using this technique, it was determined that plant GS was present at 150 micrograms per gram fresh weight or 1.2% of total plant soluble protein in N(2)-fixing alfalfa nodules.Results suggest that alfalfa nodule plant GS consists of two major subunit polypeptides, but only a single immunochemically active native protein was observed. The crossed immunoelectrophoresis procedure described here should be generally applicable for immunochemical detection of lower abundance components of crude plant extracts.

Uninfected Cells of Soybean Root Nodules: Ultrastructure Suggests Key Role in Ureide Production

In soybean root nodules, which export recently fixed nitrogen mainly as the ureides allantoin and allantoic acid, cells uninfected by rhizobia undergo a pronounced ultrastructural differentiation not shown by the infected cells, including enlargement of the microbodies and proliferation of smooth endoplasmic reticulum. Since some of the enzymes contributing to ureide synthesis occur in these subcellular components in root nodule preparations, the uninfected cells may participate in ureide synthesis and thus play an essential role in the symbiosis between host and bacterium.

Root Nodule Enzymes of Ammonia Assimilation in Alfalfa (Medicago sativa L.) : DEVELOPMENTAL PATTERNS AND RESPONSE TO APPLIED NITROGEN

Nitrogenase-dependent acetylene reduction activity of glasshouse-grown alfalfa (Medicago sativa L.) decreased rapidly in response both to harvesting (80% shoot removal) and applied NO(3) (-) at 40 and 80 kilograms N per hectare. Acetylene reduction activity of harvested plants grown on 0 kilogram N per hectare began to recover by day 15 as shoot regrowth became significant. In contrast, acetylene reduction activity of all plants treated with 80 kilograms NO(3) (-)-N per hectare and harvested plants treated with 40 kilograms NO(3) (-)-N per hectare remained low for the duration of the experiment. Acetylene reduction of unharvested alfalfa treated with 40 kilograms N per hectare declined to an intermediate level and appeared to recover slightly by day 15. Changes in N(2)-fixing capacity were accompanied by similar changes in levels of nodule soluble protein.After an initial lag of 24 hours, specific activities of alfalfa nodule glutamine synthetase, NADH-glutamate synthase, and NAD-glutamate dehydrogenase (oxidative amination) decreased similar to but less rapidly than acetylene reduction activity. Increased specific activities of these nodule enzymes occurred as acetylene reduction activity increased and shoot growth resumed. The observed rates of glutamine synthetase and glutamate synthase were sufficient to assimilate ammonia produced via symbiotic N(2) fixation. Nodule NADH-dependent glutamate dehydrogenase (reductive amination) specific activity was not associated with changes in acetylene reduction activity.The data indicate that host plant glutamine synthetase and NADH-glutamate synthase function to assimilate symbiotically fixed N and that NADH-dependent glutamate dehydrogenase may function in ammonia assimilation during senescence in alfalfa nodules.

Ammonia Assimilation in Alnus glutinosa and Glycine max: SHORT-TERM STUDIES USING [N]AMMONIUM

The pattern of assimilation of NH(4) (+) by Alnus glutinosa, a N(2)-fixing, nonleguminous angiosperm, was examined. Detached nodules, roots, and nodulated roots of intact plants were exposed to (13)NH(4) (+) for up to 15 minutes. Glutamine was the most highly labeled compound at all times; the only other compound labeled significantly was glutamate. Similar results were obtained after incubating soybean (L. merr) nodules and roots with (13)NH(4) (+). These observations and the results of pulse-labeling and inhibitor studies with nodules of Alnus were distinctly different from those predicted for the assimilation of NH(4) (+) via glutamine synthetase and glutamate synthase and suggest that glutamate dehydrogenase may play a major role in the assimilation of exogenously supplied NH(4) (+).

Regulation of glutamine synthetase in the blue-green alga Anabaena L-31

In N2-grown cultures of Anabaena L-31, in which protein synthesis was prevented by chloramphenicol, presence of NH+4 caused a drastic decrease of glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2) activity indicating NH+4-mediated inactivation or degradation of the enzyme. The half-life of glutamine synthetase was more than 24 h, whereas that of nitrogenase (reduced ferredoxin:dinitrogen oxidoreductase (ATP-hydrolysing), EC 1.18.2.1) was less than 4 h, suggesting that glutamine synthetase may not act as positive regulator of nitrogenase synthesis in Anabaena. Glutamine synthetase purified to homogeneity was subject to cumulative inhibition by alanine, serine and glycine. The amino acids, however, exhibited partial antagonism in this behaviour. Glyoxylate, an intermediate in photorespiration, virtually prevented the amino acid inhibition. Kinetic studies revealed inhibition of the enzyme activity by high Mg2+ concentration under limiting glutamate level and by high glutamate in limiting Mg2+. Maximum enzyme activity occurred when the ratio of glutamate to free Mg2+ was 0.5 to 1.0. The results demonstrate that the enzyme is subject to multiple regulation by various metabolites involved in nitrogen assimilation.

Development of nodules of Glycine max infected with an ineffective strain of Rhizobium japonicum

Bacteroids in ineffective (nitrogenase negative) nodules of Glycine max, infected with Rhizobium japonicum 61-A-24, as compared to those in effective nodules are characterized by reduced specific activities of alanine dehydrogenase to 15%, of 3-hydroxybutyrate dehydrogenase to 50%, and an increase of glutamine synthetase to 400%. In the plant cytoplasm of ineffective nodules, glutamine synthetase activity is reduced to 10-30%, glutamate dehydrogenase to 50-70%, and the aspartate aminotransferase and alanine aminotransferase are enhanced to 120-200%, depending on the age of the nodules. The total pool of soluble amino acids is reduced to 52 μmol per g nodule fresh weight, as compared to 186 μmol in effective nodules, with a replacement of asparagine (42 mol% of the amino acids) by an unknown amino compound. This compound is absent in nitrogenase, repressed and derepressed, free-living Rhizobium japonicum cells and in the uninfected root tissue. In nitrogenase derepressed, as compared to the repressed free-living cells of Rhizobium japonicum 61-A-101, arginine shows the most obvious change with a reduction to less than one tenth. The ultrastructure of the ineffective nodule is different from the effective organ even in the early stages. The membrane envelopes of the infection vacuoles are decomposing in heavily infected cells within 18 to 20 d after infection. In lightly infected cells very large vacuoles develop with only a few bacteroids inside. No close associations of cristae-rich mitochondria with amyloplasts are observed as in effective nodules. The uninfected cells keep their large starch granules even 40 d after infection. Some poly-β-hydroxybutyrate accumulation in the bacteroids is observed but only in the early stages, and it is almost absent in old nodules (40 d). At this age the infected cells are obviously compressed by uninfected cells, whereas in effective nodules with nitrogenase activity and leghaemoglobin formation, the infected cells have a much higher osmotic pressure than the neighbouring uninfected cells.

Isolation and metabolism of Vigna unguiculata root nodule protoplasts

Axenic cultures of bacteroid-containing protoplasts were isolated from root nodules of Vigna unguiculata L. Walp. Dimensions of the protoplasts were 35 to 135 μm long x 35 to 95 μm wide. Yields were about 30 to 50 mg dry weight per gram fresh weight of nodules. About 5x10(8) protoplasts packed into 1 ml of basal medium under the influence of gravity. When incubated in hypertonic, nitrogen-free media, freshly isolated protoplasts began to reduce acetylene to ethylene after a lag period of 24 to 48 h. Various additions to the basal medium showed that the system possessed functional glycolytic and tricarboxylic acid pathways. Endogenous application of various intermediary metabolites stimulated both acetylene reduction and respiration, though not often equally. As acetylene reduction, but not respiration, was inhibitable by both asparagine and glutamine, the system appears suitable for the study of mechanisms controlling symbiotic nitrogen fixation.