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PubMed1973

Germination of Azotobacter vinelandii cysts: sequence of macromolecular synthesis and nitrogen fixation.

Azotobacter vinelandii cysts undergo conversion to vegetative cells in Burk's nitrogen-free medium utilizing glucose, sucrose, or acetate. In 1% glucose, this overall process was complete in 8 hr and consisted of a germination and an outgrowth phase. Respiration, ribonucleic acid, and protein synthesis began soon after the addition of the germinant, and these processes proceeded at rates characteristic of the germination. The rates of respiration and synthesis increased sharply between 4 and 5 hr, the beginning of the outgrowth, at which time deoxyribonucleic acid synthesis and nitrogen fixation began. Respiration, macromolecular synthesis and nitrogen fixation continued at high rates until the emergence of vegetative cells from the cyst coats.

Acetates↗
PubMed1984

Organization and regulation of nitrogen fixation genes of Rhodopseudomonas capsulata.

The nitrogen fixation (nif) genes of Rhodopseudomonas capsulata appear to be much more widely dispersed than those of the well-studied Klebsiella pneumoniae. In both organisms the genes coding for the structural components of the nitrogenase complex, nifHDK, comprise a single unit of transcription. Only in the photosynthetic bacterium, however, are there multiple copies of these genes. The extra copies are normally silent but they can be activated by mutation, detected as nif+ pseudorevertants of nif- deletions of the expressed nifHDK genes. Glutamine is required for the repression of nif gene transcription in R. capsulata.

Chromosome Mapping↗
PubMed1995

Induction of complex intracytoplasmic membranes related to nitrogen fixation in Azoarcus sp. BH72.

We report the discovery of novel subcellular structures related to bacterial nitrogen fixation in the strictly respiratory diazotrophic bacterium Azoarcus sp. BH72, which was isolated as an endophyte from Kallar grass. Nitrogenase is derepressed under microaerobic conditions at O2 concentrations in the micromolar range. With increasing O2 deprivation, bacteria can develop into a hyperinduced state, which is characterized by high specific rates of respiration and efficient nitrogen fixation at approximately 30 nM O2. Ultrastructural analysis of cells in the course of hyperinduction revealed that complex intracytoplasmic membrane systems are formed, which consist of stacks of membranes and which are absent under standard nitrogen-fixing conditions. The iron protein of nitrogenase was highly enriched on these membranes, as evidenced by immunohistochemical studies. Membrane deficiency in NifH/K- mutants, a deletion mutant in the nifK gene and the character of NH+4-grown cells suggested, in concert with the membrane localization of nitrogenase, that these structures are specialized membranes related to nitrogen fixation. We propose the term 'diazosomes' for them. Development of intracytoplasmic membranes coincides with the appearance of a high-molecular-mass form of the iron protein of nitrogenase, which was detectable in membrane fractions. Mutational analysis, and determination of the N-terminal amino acid sequence indicate that the nifH gene product is covalently modified by a mechanism probably different from adenosine diphosphoribosylation. Development of diazosomes in nitrogen-fixing cells can be induced in pure cultures and in co-culture with a fungus isolated from the rhizosphere of Kallar grass.

Blotting, Western↗
PubMedDate not supplied

[Study of associate nitrogen fixation in pasture ryegrass].

A direct proof of intensification of associative nitrogen-fixation activity and considerable increase of intake of biological nitrogen by the pasture ryegrass plants, when using low doses of ammonium-carbonate compounds (ACC) as compared with ammonium nitrate (AN) has been obtained with the help of labelled nitrogen 15N. The isotope method confirms a possibility of biological nitrogen-fixation intensification in the mentioned culture when using synthetic plant growth regulator of a new generation Triman-1. More efficient use of ACC and AN by plant has been established.

Agrochemicals↗
PubMedDate not supplied

[Nitrogen fixation--new fenomenon in rodent feeding].

We have conducted a series of experiments in order to test hypothesis about the possible use of nitrogen fixation by small phytophagous mammals as a way to supplement their nitrogen balance. Two groups of red voles Clethrionomys rutilis were kept on rations with different protein nitrogen content. Three days later, the rate of nitrogen fixation was determined in different regions of the digestive tract. Nitrogen-fixing activity has been detected in the GI tract of all studied voles, specifically in those regions, where symbiocenoses were present; it was particularly high in animals of the experimental group under the conditions of dietary nitrogen limitation. A mixed culture of bacteria capable of nitrogen fixation has been isolated from the digestive tract of red voles.

Animals↗
PubMed1976

Hydrogen evolution: A major factor affecting the efficiency of nitrogen fixation in nodulated symbionts.

Nitrogenase-dependent hydrogen evolution from detached legume nodules and from reaction mixtures containing cell-free nitrogenase has been well established, but the overall effect of hydrogen evolution on the efficiency of nitrogen fixation in vivo has not been critically assessed. This paper describes a survey which revealed that hydrogen evolution is a general phenomenon associated with nitrogen fixation by many nodulated nitrogen-fixing symbionts. An evaluation of the magnitude of energy loss in terms of the efficiency of electron transfer to nitrogen, via nitrogenase, in excised nodules suggested that hydrogen production may severely reduce nitrogen fixation in many legumes where photosynthate supply is a factor limiting fixation. With most symbionts, including soybeans, only 40-60% of the electron flow to nitrogenase was transferred to nitrogen. The remainder was lost through hydrogen evolution. In situ measurements of hydrogen evolution and acetylene reduction by nodulated soybeans confirmed the results obtained with excised nodules. In an atmosphere of air, a major portion of the total electron flux available for the reduction of atmospheric nitrogen by either excised nodules or intact nodulated plants was utilized in the production of hydrogen gas. Some non-leguminous symbionts, such as Alnus rubra, and a few legumes (i.e., Vigna sinensis) apparently have evolved mechanisms of minimizing net hydrogen production, thus increasing their efficiency of electron transfer to nitrogen. Our results indicate that the extent of hydrogen evolution during nitrogen reduction is a major factor affecting the efficiency of nitrogen fixation by many agronomically important legumes.

Journal Article↗
PubMed1978

Nitrogen fixation (acetylene reduction) by epiphytes of freshwater macrophytes.

The involvement of epiphytic microorganisms in nitrogen fixation was investigated in a shallow freshwater pond near Ithaca, N.Y. The acetylene reduction technique was used to follow diel and seasonal cycles of nitrogen fixation by epiphytes of Myriophyllum spicatum. Acetylene-reducing activity was maximal between noon and 6 p.m., but substantial levels of activity relative to daytime rates continued through the night. Experiments with the seasonal course of activity showed a gradual decline during the autumn months and no activity in January or February. Activity commenced in May, with an abrupt increase to levels between 0.45 and 0.95 nmol of ethylene formed per mg (dry weight) of plant per h. Through most of the summer months, mean rates of acetylene reduction remained between 0.15 and 0.60 nmol/mg (dry weight) per h. It was calculated from diel and seasonal cycles that, in the pond areas studied, epiphytes were capable of adding from 7.5 to 12.5 mug of N per mg of plant per year to the pond. This amount is significant relative to the total amount of nitrogen incorporated into the plant. Blue-green algae (cyanobacteria), particularly Gloeotrichia, appeared to bear prime responsibility for nitrogen fixation, but photosynthetic bacteria of the genus Rhodopseudomonas were isolated from M. spicatum and shown to support high rates of acetylene reduction.

Journal Article↗
PubMed1996

Symbiotic nitrogen fixation does not require adenylylation of glutamine synthetase I in Rhizobium meliloti.

Symbiotic nitrogen fixation is accompanied by a shift of Rhizobium nitrogen metabolism from ammonium assimilation to ammonium export, which probably involves genetic or metabolic regulation of glutamine synthetase activity. In free-living Rhizobium meliloti glutamine synthetase I (GSI) is regulated post-translationally by reversible adenylylation in response to ammonium addition. Moreover, full expression of the GSI gene glnA requires the transcriptional activator, NtrC. A glnA1 mutant synthesizing a non-adenylylatable GSI produces normal nitrogen-fixing nodules on alfalfa: GSI adenylylation is dispensable for symbiotic nitrogen fixation. This is rationalized by the observation that less GS protein is present in R. meliloti bacteroids than in free-living bacterial cells.

Adenosine Phosphosulfate↗
PubMed2004

Nitrogen fixation in wheat provided by Klebsiella pneumoniae 342.

In this report, all of the criteria necessary for the demonstration of nitrogen fixation in wheat (Triticum aestivum L.), the world's most important crop, are shown upon inoculation with a nitrogen-fixing bacterium, Klebsiella pneumoniae 342 (Kp342). Kp342 relieved nitrogen (N) deficiency symptoms and increased total N and N concentration in the plant. Nitrogen fixation was confirmed by 15N isotope dilution in the plant tissue and in a plant product, chlorophyll. All of these observations were in contrast to uninoculated plants, plants inoculated with a nitrogen-fixing mutant of Kp342, and plants inoculated with dead Kp342 cells. Nitrogenase reductase was produced by Kp342 in the intercellular space of the root cortex. Wild-type Kp342 and the nifH mutant colonized the interior of wheat roots in equal numbers on a fresh weight basis. The nitrogen fixation phenotype described here was specific to cv. Trenton. Inoculation of cvs. Russ or Stoa with Kp342 resulted in no relief of nitrogen deficiency symptoms.

Base Sequence↗
PubMed1969

The relationship between nitrogen fixation and the production of HD from D2 by cell-free extracts of soya-bean nodule bacteroids.

1. Cell-free extracts prepared from soya-bean nodule bacteroids produced HD from D(2) in the presence of dithionite, an ATP-generating system and nitrogen. 2. Crude extracts of bacteroids or of Azotobacter vinelandii showed some background D(2) exchange when any one of these was omitted. 3. Partial purification of bacteroid extracts diminished this background activity and gave increased D(2) exchange and nitrogen fixation. 4. Although increasing pN(2) stimulated both reactions, the apparent K(m) (N(2)) for nitrogen fixation was much higher than the apparent K(m) (N(2)) for D(2) exchange when partially purified bacteroid extracts were used. 5. Carbon monoxide was a competitive inhibitor of nitrogen fixation by partially purified bacteroid extracts, but D(2) exchange was inhibited in a non-competitive fashion. 6. These results are discussed in relation to the possible existence of enzyme-bound intermediates of nitrogen fixation.

Adenosine Triphosphate↗
PubMed1997

Regulation of nitrogen fixation in Azospirillum brasilense.

The regulation of nitrogen fixation in Azospirillum brasilense is very complicated, and it responds to exogenous fixed nitrogen or a change of oxygen concentration. This regulation occurs at both transcriptional and posttranslational levels. Unlike regulation seen in Klebsiella pneumoniae, transcription of nifA does not require NTRB/NTRC in A. brasilense and the expression of nifHDK is controlled by posttranslational regulation of NIFA activity. Addition of NH4+ or a shift from microaerobic to anaerobic conditions also causes a rapid loss of nitrogenase activity in A. brasilense. This posttranslational regulation of nitrogenase activity involves the DRAT/DRAG regulatory system, which is similar to that of Rhodospirillum rubrum. Both DRAT and DRAG activities are regulated in vivo, but the mechanisms for their regulation are unknown.

Azospirillum brasilense↗
PubMed2003

Effects of drought stress on legume symbiotic nitrogen fixation: physiological mechanisms.

Drought stress is one of the major factors affecting nitrogen fixation by legume-rhizobium symbiosis. Several mechanisms have been previously reported to be involved in the physiological response of symbiotic nitrogen fixation to drought stress, i.e. carbon shortage and nodule carbon metabolism, oxygen limitation, and feedback regulation by the accumulation of N fixation products. The carbon shortage hypothesis was previously investigated by studying the combined effects of CO2 enrichment and water deficits on nodulation and N2 fixation in soybean. Under drought, in a genotype with drought tolerant N2 fixation, approximately four times the amount of 14C was allocated to nodules compared to a drought sensitive genotype. It was found that an important effect of CO2 enrichment of soybean under drought was an enhancement of photo assimilation, an increased partitioning of carbon to nodules, whose main effect was to sustain nodule growth, which helped sustain N2 rates under soil water deficits. The interaction of nodule permeability to O2 and drought stress with N2 fixation was examined in soybean nodules and led to the overall conclusion that O2 limitation seems to be involved only in the initial stages of water deficit stresses in decreasing nodule activity. The involvement of ureides in the drought response of N2 fixation was initially suspected by an increased ureide concentration in shoots and nodules under drought leading to a negative feedback response between ureides and nodule activity. Direct evidence for inhibition of nitrogenase activity by its products, ureides and amides, supported this hypothesis. The overall conclusion was that all three physiological mechanisms are important in understanding the regulation of N2 fixation and its response of to soil drying.

Carbon Dioxide↗
PubMed2002

The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum).

Production of grain legumes is severely reduced in salt-affected soils because their ability to form and maintain nitrogen-fixing nodules is impaired by both salinity and sodicity (alkalinity). Genotypes of chickpea, Cicer arietinum, with high nodulation capacity under stress were identified by field screening in a sodic soil in India and subsequently evaluated quantitatively for nitrogen fixation in a glasshouse study in a saline but neutral soil in the UK. In the field, pH 8.9 was the critical upper limit for most genotypes studied but genotypes with high nodulation outperformed all others at pH 9.0-9.2. The threshold limit of soil salinity for shoot growth was at ECe 3 dS m(-1), except for the high-nodulation selection for which it was ECe 6. Nodulation was reduced in all genotypes at salinities above 3 dS m(-1) but to a lesser extent in the high-nodulation selection, which proved inherently superior under both non-saline and stress conditions. Nitrogen fixation was also much more tolerant of salinity in this selection than in the other genotypes studied. The results show that chickpea genotypes tolerant of salt-affected soil have better nodulation and support higher rates of symbiotic nitrogen fixation than sensitive genotypes.

Algorithms↗
PubMed1977

Comparison of Nitrogen Fixation Activity in Tall and Short Spartina alterniflora Salt Marsh Soils.

A comparison of the N(2) fixers in the tall Spartina alterniflora and short S. alterniflora marsh soils was investigated. Zero-order kinetics and first-order kinetics of acetylene reduction were used to describe the activity of the N(2) fixers in marsh soil slurries. It was found that the V(max) values were approximately 10 times greater for the N(2) fixers in the tall Spartina than in the short Spartina marsh when raffinose was used as the energy source. In addition, the (K(s) + S(n)) values were approximately 4 to 15 times lower for the N(2) fixers in the tall Spartina than in short Spartina marsh. First-order kinetics of nitrogen fixation for several substrates indicate that the N(2) fixers in the tall Spartina marsh were two to seven times more active than those in the short Spartina marsh. Ammonium chloride (25 mug/ml) did not inhibit nitrogen fixation in the tall Spartina marsh, but there was a 50% inhibition in nitrogen fixation in the short Spartina marsh. On the other hand, sodium nitrate inhibited nitrogen fixation almost 100% at 25 mug/ml in both soil environments. Amino nitrogen (25 to 100 mug/ml) had little or no effect on nitrogen fixation. The results indicate that the N(2) fixers in the tall Spartina marsh were physiologically more responsive to nutrient addition than those in the short Spartina marsh. This difference in the two populations may be related to the difference in daily tidal influence in the respective areas and thus provide another explanation for the enhanced S. alterniflora production in the creek bank soil system.

Journal Article↗
PubMed1977

Regulation of symbiotic nitrogen fixation in root nodules of alfalfa (Medicago sativa) infected with Rhizobium meliloti.

Symbiotic nitrogen fixation of Rhizobium meliloti bacteroids in Medicago sativa root nodules was suppressed by several inorganic nitrogen sources. Amino acids like glutamine, glutamic acid and aspartic acid, which can serve as sole nitrogen sources for the unnodulated plant did not influence nitrogenase activity of effective nodules, even at high concetrations. Ammonia and nitrate suppressed symbiotic nitrogen fixation in vivo only at concentrations much higher than those needed for suppression of nitrogenase activity in free living nitrogen fixing bacteria. The kinetics of suppression were slow compared with that of free living nitrogen fixing bacteria. On the other hand, nitrite, which acts as a direct inhibitor of nitrogenase, suppressed very quickly and at low concentrations. Glutamic acid and glutamine enhanced the effect of ammonia dramatically, while the suppression by nitrate was enhanced only slightly.

Ammonia↗