Biological Nitrogen Fixation in Pachyrhizus ahipa (Wedd.) Parodi

The patterns of plant growth and N₂ fixation capability in Pachyrhizusahipa (Wedd) Parodi inoculated with Bradyrhizobium ‘PachyrhizusSpec 1’ strains (Lipha Tech) were investigated in a zero-Nculture system under greenhouse conditions The P ahipa plantis day-neutral with respect to reproductive development Competitionoccurred between the two storage organs (legume and tuber) andprevented high tuber yield in P ahipa The symbiotic effectivenessof the association was high, as the profuse nodulation providedthe inoculated plants with adequate amounts of N Nodules werepresent throughout the cycle of P ahipa The change in rate ofN₂ fixation (RNF) and relative growth rate (RGR) was almostparallel during ontogenesis The developmental pattern of N₂fixation activity revealed that 65% of total N₂ fixation occurredafter N began to accumulate in the reproductive (pod wall plusseed) tissue During pod filling allocation of N compounds tothe seeds exceeded N₂ fixation, the pod walls being the primarysource of redistributed N, followed by the leaves. Pachyrhizus ahipa (Wedd) Parodi, ahipa, tuber crop, dinitrogen fixation, dry matter, N partitioning, reproductive growth     

An Isotopic Study of the Fixation of Nitrogen associated with Nodulated Plants of Alnus, Myrica, and Hippophae

Nodulated plants of Alnus glutinosa, Myrica gale, and Hippophaërhamnoides, the root systems of which had been exposed to excessfree ¹⁵N, showed substantial enrichment in fixed ¹⁵N contentin all parts of the plant, but particularly in the root nodules.The data resemble closely those obtained by the present andprevious authors with legumes under comparable conditions, andsupport the conclusion already drawn from experiments of traditionaltype that the nodules of these non-legumes are similar to thoseof legumes in their function and relation to the rest of theplant. Nodulated plants of Alnus and Myrica continued to fixfree nitrogen concurrently with the uptake of combined nitrogenwhen the latter was supplied in the rooting medium in amountsunlikely to be exceeded in the field. Isotopic tests on detachednodules of one of the species (Alnus glutinosa) showed thatfixation continued, and though much reduced as compared withthat shown by attached nodules it considerably exceeded thatexperienced by the present and previous authors with detachedlegume nodules, on the basis of fixation per unit of total nodulenitrogen. It is probable that detached Alnus nodules presentconvenient material for the further study of various aspectsof the fixation process.     

Environmental Influences on CAM Activity of Cissus quadrangularis

Cissus quadrangularis, Vitacea, has a succulent stem and showsCrassulacean Acid Metabolism (CAM). Environmental control ofCAM is shown with respect to water supply, day temperature,and light intensity. C. quadrangularis, under moderate drought,has a high capacity for dark CO₂ fixation and high efficiencyof water use (nocturnal transpiration ratio = 8). Severely droughtedplants of C. quadrangularis still show dark CO₂ fixation althoughat a reduced rate, which allows a maintenance level metabolism.On the other hand, high temperature and light intensity promoteCAM activity in well-watered plants resulting in luxuriant andfast growth. Data indicate that under natural conditions CAMshould effectively contribute to the successful adaptation ofC. quadrangularis to its environment.     

Partitioning and Utilization of Carbon and Nitrogen in Nodulated Roots and Nodules of Chickpea (Cicer arietinum) Grown at Two Moisture Levels

The partitioning and utilization of carbon (C) and nitrogen(N) in nodulated roots and nodules of chickpea (Cicer arietinumL.) was studied at two moisture levels at 10-d intervals 40–140d after sowing (DAS). More C was used in respiration and lessin growth of nodulated roots and nodules under water stresscompared to controls during all growth stages except at theearly vegetative stage. Similarly, less nitrogen was investedin dry matter of both nodules and nodulated roots under stress,except during the vegetative stage where more nitrogen was used.Calculated over the entire growth period, as much as 14 and20% of the total nitrogen and 3 and 4% of the total carbon fixedby the plant was lost to the rooting medium under controlledand stressed conditions, respectively. The efficiency of nitrogen fixation with respect to net C utilizationwas maximal during seed filling under both control and stressconditions However, the efficiency of nitrogen fixation wasalways greater under drier conditions. Carbon, chickpea (Cicer arietinum L.), nitrogen, nitrogen fixation, partitioning     

Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) II. Effects of Inhibitors, Uncouplers and an Artificial Electron Mediator on the Inhibition of 14CO2 Fixation by Anaerobiosis

In "air-grown" Chroomonas sp. cells, low concentrations of DCMU(less than 0.1 µM) could prevent the inhibition of ¹⁴CO₂fixation by anaerobiosis under light-saturating conditions (morethan 40 W.m^–2), with phenazine methosulfate showing asimilar effect. Antimycin A, carbonyl cyanide m-chlorophenylhydrazone(CCCP), and N,N'-dicyclohexylcarbodiimide strongly inhibitedanaerobic photosynthesis at concentrations which did not significantlyinhibit the rate under 2% O₂ at high light intensity (200 W.m^–2),although 0.2 µM CCCP stimulated the rate under 2% O₂ tosome extent. On the other hand, KCN inhibited the rate muchmore strongly under 2% O₂ than N₂, although it inhibited therate very strongly at concentrations above 5 µM both underN₂ and 2% O₂. These results suggest that the inhibition of photosynthetic¹⁴CO₂ fixation by anaerobiosis in this alga result from ATPdeficiency caused by over-reduction of electron carriers ofthe cyclic electron flow and that oxygen can prevent the over-reduction.Cyclic electron flow seems to be necessary to provide additionalATP for CO₂ reduction under anaerobic conditions, although itseems to be less necessary under aerobic conditions. (Received July 21, 1983; Accepted January 23, 1984)     

Pathway of dark inorganic carbon fixation in two species of diatoms: influence of light regime and regulator factors on diel variations

Planktonic algae submitted to vertical mixing with a short periodicitycommute many times a day from low to high irradiance levels.To study the influence of this light periodicity, two diatoms,Skeletonema coslatum and Nitzschia turgiduloides, were cultivatedunder alternating conditions of 2 h light/2 h dark (2 h/2 h),simulating vertical mixing in the natural environment. Two otherlight regimes were used: continuous light (CL) and alternatecycles of 12 h light/12 h dark (12 h/12 h). Products synthesizedin the dark by S.costmum during 60 s incubation for 2 h/2 hculture or during 5 min for 12 h/12 h culture were determined.They were essentially sugars, malate, aspartate and glyceratefor 2 h/2 h cells and 12 h/12 h cells taken at the beginningof the light period. In contrast, 12 h/12 h cells taken duringthe darkness or in the middle of the light period and set inthe dark synthesized only amino acids. Our results corroborateprevious reports on dark CO₂ fixation via phosphoenolpyruvatecarboxykinase (PEPCKase, enzyme allowing the fixation of CO₂on PEP and the synthesis of amino acids) with involvement ofa substrate synthesized during the light period, but demonstratethat incorporation also occurs by the C-3 pathway (pathway responsiblefor the major CO₂ fixation in the light) in the very early stagesof the dark period. Another important result highlighted bythis study is the appreciable rate of dark fixation: on average6.7, 8.3 and 12.7% of photosynthesis at saturating photon fluxdensity for N.turgiduloides cultivated under 2 h/2 h, CL and12 h/12 h regime respectively and nearly 12% for S.costatumin the 2 h/2 h light regime. Variation of dark fixation wasinvestigated as a function of hour in the two species. Skeletonemacostatum cells submitted to the 2 h/2 h cycle show a constantrate of light-independent assimilation throughout the day. Bycontrast, both N.turgiduloides grown under the 12 h/12 h or2 h/2 h regime and S.costatum cultured under the 12 h/12 h cycleundergo fluctuations in the rate of dark fixation over the light/darkcycle. The mean dark fixation rate is controlled by the lengthof the photoperiod or the frequency of light fluctuations, dependingon species. We argue that this phenomenon must be taken intoconsideration in primary production calculations. Dependingon whether they are synthesized at the beginning or at the endof the light period, products are somewhat different and therate of fixation varies. This leads us to suggest that the pathwayof dark fixation may be regulated by at least two factors: amountof available substrate and enzyme (RuBPCase and PEPCKase) activityand/or amount.     

The Photometabolism of Acetate by Chlorella pyrenoidosa

Chlorella pyrenoidosa can utilize sodium acetate as a carbonsource for growth in the light. Growth proceeds under aerobicconditions both in the presence and in the absence of carbondioxide, but under anaerobic conditions only in its presence.The assimilation of acetate does not result from oxidation tocarbon dioxide followed by photosynthetic fixation because theproducts of ¹⁴C-acetate assimilation are different from theproducts of ¹⁴CO₂ fixation in the presence of unlabelled acetate. In aerobic conditions 10-⁶ M DCMU induces a pattern of acetateassimilation in the light similar to that in the dark. Thus,in the presence of DCMU in the light, less acetate carbon isincorporated into cells, particularly into lipids, polysaccharide,and protein, and more is released as carbon dioxide than inits absence. The effect of 4 x 10^-3 M MFA on acetate assimilationin the presence of 10^-6 M DCMU is the same in light and dark.Acetate assimilation is unaffected by desaspidine and sodiumbisulphite. The mean generation time of C. pyrenoidosa growing on acetatein the light under aerobic conditions is 20 hours. When 10^-5M DCMU is added the mean generation time is 60 hours, the sameas that for Chlorella growing on acetate in the dark. The activityof the enzymes of the glyoxylate cycle, isocitrate lyase (E.C.4.1.3.1.)and malate synthetase (E.C.4.1.3.2.) is repressed in the light,but activity of both enzymes increases markedly when DCMU isadded.     

Reduced Phosphorus Requirement of a Mutant Azolla-Anabaena Symbiotic N2-fixing Complex

The use of the photo-autotrophic nitrogen-fixing water fernAzolla as an effective source of organic nitrogen in tropicalpaddy fields has been limited by a high phosphorus requirement.Azolla species with a minimum of 1.5 to 2.0 mM phosphate (P)requirement, under controlled conditions, are known. A local Azolla species requiring at least 1.5 mM sodium phosphatefor a normal rate of multiplication and N₂ fixation was exposedto N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The resultingmutant population had a significantly lower P requirement, butwas auxotrophic for glutamine with an extremely reduced glutaminesynthetase (GS) activity. An L-methionine-DL-sulphoximine (MSX)-resistant(MSX^r) Azolla population, having an approximately 1.5 timeshigher GS activity than that of the wild type (WT) parent organism,was cultured and subjected to MNNG-induced mutation for lowP requirement while putting MSX as a control in the mutant selectionmedium. The resulting population of mutant Azolla was a normalprototroph with a P requirement as low as 0.75 mM for its ‘WTparent-like’ usual growth and N₂ fixation. Key words: Azolla, phosphorus requirement, mutation     

The Effects of Exogenous Amino Acid on Acetylene Reduction Activity of Vicia faba L. cv. Fiord

The feed back control mechanism proposed to explain the inhibitionof N₂ fixation by N was investigated using Vicia faba cv. Fiord.Plants were grown under controlled conditions without mineralN in coarse river sand. Asparagine was supplied to plants activelyfixing N₂ by absorption through cut roots and via a wick ordirect injection into the stem just above the bottom leaf. Responsesin N₂ fixation were measured by acetylene reduction (AR). Feedingplants with [¹⁴C]-labelled asparagine showed that the amidewas taken up when exogenously applied. Asparagine (10 mM) suppliedby the above procedures resulted in a 50-70% inhibition of ARby 48 h. Glutamine produced a similar effect. The cut root methodallowed higher levels of these amides to be supplied but theinhibition observed with 10 mM asparagine was only increasedslightly with higher levels of the amide. The antibiotic Securopenprevented bacterial contamination of root solutions of asparagineand glutamine and had no effect on nodule activity. It is concludedthat accumulation of asparagine of glutamine or the resultantincrease in the pool of soluble N in the plant cause a feedbackeffect on the activity of nitrogenase.Copyright 1993, 1999 AcademicPress Vicia faba, faba bean, asparagine, inhibition of N₂ fixation     

Relationship between Inhibition of CO2 Fixation and Glutamine Synthetase Inactivation in Lemna gibba L. Treated with L-Methionine-D,L-Sulphoximine (MSO)

The rate of net CO₂ fixation in Lemna gibba L. was decreasedto 50% by 100–150 min incubation in the presence of 0•5mol m^–3 L-methionine-D,L-Sulphoximine (MSO), an irreversibleinhibitor of glutasnine synthetase (GS). The pattern of inhibitionwas similar in both 21% O₂ and 2% O₂. The inhibition was accompaniedby increased intracellular levels. Incubation with 10 mol m^–3 under the same conditions, but without MSO, resulted in even higher levels but the rate of CO₂ fixation was unaffected. Additions of glutamine, glutamate, glycine or serine delayedthe MSO-induced inhibition of CO₂ fixation. The same amino acidsdelayed the inactivation of GS by MSO. Thus inhibition of CO₂ fixation by MSO in Lemna is neither causedby elevated levels nor closely related to photorespiration. Possibly, MSO causes shortage of amino-N formaintenance of the functional integrity of the photosyntheticapparatus. Key words: Methionine sulphoximine, CO₂, fixation, Lemna     

The nifH-Like (frxC) Gene Is Involved in the Biosynthesis of Chlorophyll in the Filamentous Cyanobacterium Plectonema boryanum

The frxC gene found in the DNA of the liverwort chloroplastencodes a protein of unknown function. The deduced amino acidsequence shows significant homology to that of the nitrogenaseFe-protein encoded by the nifH gene. We previously identifiedthe frxC and nifH genes in the filamentous cyanobacterium Plectonemaboryanum. We describe here the isolation of targeted mutantsof frxC (YFC1004) and nifH (YFH201) which were generated byinsertion of a kanamycin-resistance gene into the structuralportion of the respective genes. As expected, YFH201 cannotgrow under nitrogen-fixing conditions. However, YFC1004 growsas well as the wild type does under nitrogen-fixing, photoautotrophicand chemoheterotrophic conditions, indicating that the FrxCprotein is essential neither for nitrogen fixation nor for majorenergytransduction systems, such as photosynthesis and respiration.YFC1004 synthesizes chlorophyll (Chi) normally in the lightbut not in the dark, and it accumulates a precursor to Chi,protochlorophyllide (Pchlide) in the dark. These phenotypiccharacteristics of YFC1004 suggest that the cyanobacterium hastwo pathways for the reduction of Pchlide: a light-dependentand a lightindependent system. The FrxC protein appears to beinvolved in the light-independent reduction of Pchlide. (Received September 24, 1991; Accepted November 11, 1991)     

Plant Growth-promoting Rhizobacteria and Soybean [ Glycine max (L.) Merr.] Growth and Physiology at Suboptimal Root Zone Temperatures

Application of plant growth-promoting rhizobacteria (PGPR) hasbeen shown to increase legume growth and development under optimaltemperature conditions, and specifically to increase nodulationand nitrogen fixation of soybean [Glycine max (L.) Merr.] overa range of root zone temperatures (RZTs). Nine rhizobacteriaapplied into soybean rooting media were tested for their abilityto reduce the negative effects of low RZT on soybean growthand development by improving the physiological status of theplant. Three RZTs were tested: 25, 17.5, and 15 °C. At eachtemperature some PGPR strains increased plant growth and development,but the stimulatory strains varied with temperature. The strainsthat were most stimulatory at each temperatures were as follows:15 °C—Serratia proteamaculans 1–102; 17.5 °C—Aeromonashydrophila P73, and 25 °C—Serratia liquefaciens 2–68.Because enhancement of plant physiological activities were detectedbefore the onset of nitrogen fixation, these stimulatory effectscan be attributed to direct stimulation of the plant by thePGPR rather than stimulation of plant growth via improvementof the nitrogen fixation symbiosis. Legume; nitrogen fixation; nodulation; root zone temperature; PGPR     

Two Polypeptides Inducible by Low Levels of CO2 in Soluble Protein Fractions from Chlorella regularis Grown at Low or High pH

Previous studies suggested that certain protein(s) other thancarbonic anhydrase might play an important role in the facilitatedtransport of dissolved inorganic carbon (DIC) from the mediumto the site of CO₂ fixation by ribulose-1,5-bisphosphate carboxylase/oxygenasein the unicellular green alga Chlorella regularis adapted tolow-CO₂ (ordinary air) conditions [Shiraiwa et al. (1991) Jpn.J. Phycol. 39: 355; Satoh and Shiraiwa (1992) Research in Photosynthesis,Vol. III, p. 779]. The proteins that might be involved in thisfacilitated transport of DIC were investigated by pulse-labelingof induced proteins with ³⁵S-sulfate during adaptation of cellsgrown under high-CO₂ conditions to low CO₂. Analysis by SDS-PAGErevealed that synthesis of two polypeptides, with molecularmasses of 98 and 24 kDa, respectively, was induced under low-CO₂conditions. The 24-kDa polypeptide was induced at pH 5.5 butnot at pH 8.0, whereas the 98-kDa polypeptide was induced atboth pH 5.5 and pH 8.0. The possible role of these polypeptidesin the facilitated transport of DIC in Chlorella regularis isdiscussed. (Received October 30, 1995; Accepted February 26, 1996)     

Ammonia Assimilation in Canavalia ensiformis Plants under Water and Salt Stress

Nitrogen fixation and ammonia assimilation in nodules have beenthoroughly studied under stress conditions, but the behaviorof enzymes involved in ammonia assimilation to organic compoundsin plants of the Leguminosae family subjected to stress stillremains to be conclusively established. We found that understress conditions, C. ensiformis plants can switch from theirusual pathway of assimilation to an alternative one dependingon the nature of the stress and the tissue in which the processtakes place. In roots, it switches from the glutamate dehydrogenase(GDH) pathway to the glutamine synthetase (GS)/glutamate synthase(GOGAT) cycle under water stress but not under salt stress.However, in leaves under salt stress, GDH activity is maintainedbut GS activity markedly decreases (Received March 24, 1987; Accepted March 4, 1988)     

Validity of N2 fixation rate measurements in marine Oscillatoria (Trichodesmium)

No measurable differences in Trichodesmium nitrogenase activitywere observed between colonies collected by diving and incubatedunder ultra-clean conditions compared with those collected andincubated using standard techniques. Measurements were madein the northeastern Caribbean Sea, near the Bahama Islands andin the Sargasso Sea. Surprisingly, mean rates of ethylene productionwere high relative to most previous in situ measurements onTrichodesmium. The calculated cellular N doubling times (viaN₂ fixation) ranged from 1.13 days in the northeastern CaribbeanSea, 1.48 days in the Sargasso Sea to 1.8 days near the BahamaIslands. A comparison of these doubling times with those inthe literature illustrates the high variability in rate of N₂fixation by Trichodesmium. From this study, we conclude thatthe often observed slow rates of N₂ fixation are valid. Populationsof Trichodesmium can probably remain within the water columnat low growth rates via gas vesicles, which keep the colonysuspended, and low grazing rates by herbivores.     

The Cobalt Requirement of Non-legume Root Nodule Plants

As already shown for Alnus glutinosa, cobalt is found to beessential for the proper growth of nodulated plants of Casuarinacunninghamiana and Myrica gale in a nitrogen-free rooting medium.If cobalt is not supplied, the plants develop symptoms of nitrogendeficiency; under the conditions of the experiments such symptomsbecame pronounced during the second season of growth of theseperennial plants. No cobalt requirement could be detected innon-nodulated plants of Alnus and Myrica supplied with nitrateor ammonium-nitrogen, and this suggests that in nodulated plantsthe need for cobalt is confined to the nodules. Vitamin B₁₂analogues are shown to be present in the nodules in relativelylarge amounts when cobalt is supplied, their formation beingattributed to the endophytes, which may therefore require cobaltfor their growth. The great reduction in fixation of atmosphericnitrogen in cobalt-deficient nodules may be due to a retardedgrowth of the endophyte, though this is not the only possibility.The cobalt relation of these non-legumes appears to be basicallysimilar to that of legumes.     

The Effect of Hydroxymethanesulphonate on Photosynthesis in Chlorella pyrenoidosa

Photosynthesis under conditions known to favour glycollate excretionby algae did not result in glycollate excretion in a strainof Chlorella pyrenoidosa unless an inhibitor of glycollate oxidase,-hydroxypyridin-2yl-methane sulphonate (-HPMS), was present.This inhibitor increased the total amount of glycollate presentin the supernatant from the cells during photosynthetic carbondioxide fixation and gave accumulation of ¹⁴C in glycollateduring ¹⁴CO₂ fixation under conditions favouring glycollatesynthesis. At pH 8.3 -HPMS did not stimulate photosynthetic¹⁴CO₂ fixation in C. pyrenoidosa as occurs with some algae.Photoassimilation of acetate was inhibited by -HPMS, and thiswas shown to result from acetyl-CoA synthetase inhibition by-HPMS.     

Modulation of Sucrose Content by Fructose 2,6-bisphosphate during Photosynthesis in Rice Leaves Growing at Different Light Intensities

Reddy, A. R. and Das, V. S. R. 1987. Modulation of sucrose contentby fructose 2,6-bisphosphate during photosynthesis in rice leavesgrowing at different light intensities.—J. exp. Bot. 38:828–833. The relationship between the rate of CO₂ fixation and sucroseconcentration in the leaves of rice (Oryza sativa L.) grownat different light intensities was investigated. Maximum sucrosecontent coincided with maximum rates of CO₂ fixation, achievedat a photon flux density of 1600 µmol m^–2 s^–1.The levels of sucrose and fructose 2,6-bisphosphate were alsocompared in the leaves under different light intensities. Fructose2,6-Msphosphate accumulated during growth at low light. Theactivity of fructose-6-phosphate 2-kinase was high in the leavesgrown at low light while that of fructose-2,6-bisphosphatasewas low. The activities of phosphoglucose isomerase and phospho-glucomutasewere slightly increased by growth at low light The activitiesof UDP glucose pyrophosphorylase were adversely affected invitro with increased concentrations of fructose 2,6-bisphosphatewhile those of sucrose phosphate synthase were moderately affected.Phosphoglucose isomerase and phosphoglucomutase were activatedby fructose 2,6-bisphosphate (8-0 mmol m^–3) by 12-15%.The results suggested that low light intensities during growthresult in an accumulation of fructose 2,6-bisphosphate whichmodulates the key enzymes of sucrose biosynthesis thus regulatingcarbon flow under conditions of limited photosynthesis. Key words: Oryza sativa, photosynthesis, sucrose synthesis, fructose 2,6-bisphosphate, light     

The Effect of the Accumulation of Carbohydrate and Organic Nitrogen on Nitrogen Fixation (Acetylene Reduction) of Faba Bean cv. Fiord

Plants of faba bean cv. Fiord were grown under controlled conditions,without mineral N, in coarse river sand. Twenty-five days aftersowing when plants had at least eight fully opened leaves andwere nodulated and actively fixing N₂, half were topped andkept debudded for 21 d. Changes in dry weight, N₂ fixation (acetylenereduction activity), soluble carbohydrate, starch, soluble Nand total N in plants were monitored over the period. Both debudded and control plants grew and accumulated dry matter.Debudding resulted in a significant increase in the concentrationof soluble carbohydrate, starch and soluble N. but had onlya small effect on the total N concentration. A strong positivelinear relation between total plant weight and N content ofboth control and debudded plants showed that even under conditionsof excess supply of carbohydrate, faba beans have little capacityto store N. Soluble N accumulated in debudded plants presumablybecause less N was needed for the formation of new tissues thanin control plants. AR continued to increase throughout the experimentin control plants but declined in debudded plants from 6 to13 d after debudding and remained low until the end of the experiment.The decline was associated with an increase in available carbohydrateand in soluble N. The results of this experiment are consistentwith a feed back control of N₂ fixation by the soluble poolof N.Copyright 1994, 1999 Academic Press Vicia faba, faba bean, debudding, soluble N, inhibition of N₂ fixation     

Cloning, Nucleotide Sequences and Differential Expression of the nifH and nifH-Like (frxC) Genes from the Filamentous Nitrogen-Fixing Cyanobacterium Plectonema boryanum

The frxC gene, found in liverwort chloroplast DNA, encodes aprotein of unknown function. The deduced amino acid sequenceof the protein shows significant homology to that of ni-trogenaseFe-protein encoded by the nifH gene. We have cloned the frxCand nifH genes from the nitrogen-fixing cyanobacterium Plectonemaboryanum, using frxC- and nifH-specific probes, and have determinedtheir nucleotide sequences. The amino acid sequence deducedfrom the frxC gene of P. boryanum exhibits 83% homology to thatof the protein encoded by the/rxCgene from liverwort, whereasit exhibits only 34% homology to that encoded by the nifH genefrom the same organism, namely, P. boryanum. Northern blot analysisshowed that the frxC gene was transcribed more actively undernitrogenase-repressed conditions than under nitrogenase-inducedconditions, suggesting that the FrxC protein has a functiondistinct from nitrogen fixation. These results, together withthe phylogenetic relationship between the nifH and frxC genes,indicate that the frxC and nifH genes are derived from a commonancestral gene but have evolved independently to encode proteinswith different functions. (Received April 27, 1991; Accepted August 12, 1991)