木麻黄和桤木离体根瘤和立地土壤的固氮酶与N2O还原酶活性的研究

为了解非豆科固氮树种的固氮酶和N_2O还原酶(Nos)活性,采用乙炔还原法和乙炔抑制技术对细枝木麻黄(Casuarina cunninghamiana)和江南桤木(Alnus trabeculosa)离体根瘤及立地土壤的两种酶活性进行了研究。结果表明,离体根瘤只在厌氧条件下有固氮酶活性,在好氧条件下有Nos活性。根瘤区根际土和非根瘤区根际土的固氮酶活性在好氧条件大于厌氧条件,Nos活性只表现在厌氧条件下。在好氧条件下,根瘤区根际土和非根瘤区根际土的固氮酶活性无显著差异;根瘤区根际土的Nos活性显著大于非根瘤区根际土。除离体根瘤在好氧条件下不表现固氮酶活性外,细枝木麻黄和桤木的离体根瘤、根瘤区根际土和非根瘤区根际土的固氮酶活性均都大于Nos活性。好氧条件下根瘤区根际土的固氮酶活性与非根瘤区根际土的呈极显著正相关,而厌氧条件下根瘤的固氮酶活性与好氧条件下根瘤区根际土和非根瘤区根际土固氮酶活性、好氧条件下根瘤的Nos活性与厌氧条件下根瘤区根际土和非根瘤区根际土Nos活性均呈极显著负相关。这为研究弗兰克氏菌结瘤植物共生固氮体系对N2O汇强度的影响和调控奠定基础。     

Mutant Strain of Bradyrhizobium japonicum with Increased Symbiotic N2 Fixation Rates and Altered Mo Metabolism Properties

Mutant strains of Bradyrhizobium japonicum that required higher levels of molybdate than the wild-type strain for growth on NO₃-containing medium were obtained after transposon Tn5 mutagenesis of the wild-type strain. The mutant strains expressed more than fivefold-greater nitrate reductase activities in the range of 0.1 to 1.0 mM added molybdate compared with activities expressed upon incubation in non-Mo-supplemented medium, whereas the nitrate reductase activity of the wild-type strain (JH) was not markedly influenced by Mo supplementation. In free-living culture, mutant strains JH310 and JH359 expressed substantial nitrogenase activity, even in medium treated to remove molybdate, and nitrogenase activity was influenced little by Mo supplementation, whereas the wild-type strain required 100 nM added Mo for highest nitrogenase activity. Double-reciprocal plots of Mo uptake rates versus Mo concentration showed that both bacteroids and free-living cells of mutant strain JH359 had about the same affinity for Mo as did the parent strain. Bacteroids of both the mutants and the wild type also exhibited similar Mo accumulation rates over a 9-min period under very-low-Mo (4 nM) conditions. Nitrogenase activities for strain JH359 and for the wild-type strain in free-living culture were both strongly inhibited by tungsten; thus, the nitrogenase activities of both strains are probably the result of a “conventional” Mo-containing nitrogenase. Soybeans inoculated with strain JH359 and grown under either Mo-supplemented or Mo-deficient conditions had greater specific acetylene reduction rates and significantly greater plant fresh weight than those inoculated with the wild-type strain. Under Mo-deficient conditions, the acetylene reduction rates and plant fresh weights were up to 35 and 58% greater, respectively, for mutant-nodulated plants compared with wild-type-strain-nodulated plants.     

Isolation of an Asm− mutant of Rhizobium japonicum defective in symbiotic N2-fixation

Abstract A mutant strain of Rhizobium japonicum (CJ9) unable to assimilate ammonium (Asm⁻) was isolated following mutagenesis with N -methyl N -nitro-nitrosoguanidine (NTG). Glutamate synthase activity was not detectable in cell-free extracts of the mutant strain in contrast to the wild type and revertant strains. Although mutant CJ9 induced nitrogenase activity in an 'in vitro' assay system under microaerobic conditions, it failed to fix nitrogen (acetylene reduction) in soybean root nodules. These properties of mutant CJ9 constitute a new Asm⁻ mutant class in Rhizobium spp.     

Mutant Strain of Bradyrhizobium japonicum with Increased Symbiotic N(2) Fixation Rates and Altered Mo Metabolism Properties

Mutant strains of Bradyrhizobium japonicum that required higher levels of molybdate than the wild-type strain for growth on NO(3)-containing medium were obtained after transposon Tn5 mutagenesis of the wild-type strain. The mutant strains expressed more than fivefold-greater nitrate reductase activities in the range of 0.1 to 1.0 mM added molybdate compared with activities expressed upon incubation in non-Mo-supplemented medium, whereas the nitrate reductase activity of the wild-type strain (JH) was not markedly influenced by Mo supplementation. In free-living culture, mutant strains JH310 and JH359 expressed substantial nitrogenase activity, even in medium treated to remove molybdate, and nitrogenase activity was influenced little by Mo supplementation, whereas the wild-type strain required 100 nM added Mo for highest nitrogenase activity. Double-reciprocal plots of Mo uptake rates versus Mo concentration showed that both bacteroids and free-living cells of mutant strain JH359 had about the same affinity for Mo as did the parent strain. Bacteroids of both the mutants and the wild type also exhibited similar Mo accumulation rates over a 9-min period under very-low-Mo (4 nM) conditions. Nitrogenase activities for strain JH359 and for the wild-type strain in free-living culture were both strongly inhibited by tungsten; thus, the nitrogenase activities of both strains are probably the result of a "conventional" Mo-containing nitrogenase. Soybeans inoculated with strain JH359 and grown under either Mo-supplemented or Mo-deficient conditions had greater specific acetylene reduction rates and significantly greater plant fresh weight than those inoculated with the wild-type strain. Under Mo-deficient conditions, the acetylene reduction rates and plant fresh weights were up to 35 and 58% greater, respectively, for mutant-nodulated plants compared with wild-type-strain-nodulated plants.     

Rock phosphate solubilization and colonization of maize rhizosphere by wild and genetically modified strains of <Emphasis Type="Italic">Penicillium rugulosum</Emphasis>

Maize root colonization and phosphate solubilizing activity of the fungus Penicillium rugulosum were assessed in a greenhouse trial using soil-plant microcosms. The bacterial gene hph conferring resistance to hygromicin B was introduced by electroporation in the wild-type strain IR-94MF1 of P. rugulosum and one transformant, w-T3, was selected. Maize plants were grown for 5 weeks in a P-poor soil and fertilized with a Florida apatite mineral, with Navay, an apatite rock deposit from Venezuela, or with simple superphosphate. Inoculation treatments included strain IR-94MF1, transformant w-T3 and two IR-94MF1 UV-induced mutants with enhanced (Mps++) or reduced (Mps-) in vitro mineral phosphate solubilizing activity. In the absence of P fertilization, inoculation with any P. rugulosum isolate significantly reduced the size of the total and P-solubilizing bacterial community present in maize rhizosphere. The bacterial community significantly increased in maize inoculated with IR-94MF1 and w-T3 when P was added as apatites Navay or Florida. All P. rugulosum strains were able to stimulate the growth of maize plants as indicated by 3.6 to 28.6% increases in dry matter yields. In the presence of rock phosphate, P uptake by maize plants inoculated with the two mutants Mps++ and Mps- was not always in agreement with their P-solubilizing phenotypes. Strain IR-94MF1 and transformant w-T3 increased P assimilation by the plants fertilized with Navay rock phosphate by 26 and 38%, respectively. In this treatment, w-T3 showed its highest significant maize rhizosphere colonization. With the simple superphosphate treatment, w-T3 increased P uptake in plants by 8% over the uninoculated control and also decreased significantly the community size of total bacteria, total fungi, and P-solubilizing fungi in the rhizosphere.     

Establishment of phosphate-solubilizing strains of Azotobacter chroococcum in the rhizosphere and their effect on wheat cultivars under green house conditions

A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha^—1, 90 kg N + 60 kg P₂O₅ ha^—1, 120 kg N ha^—1 and 120 kg N + 60 kg P₂O₅ ha^—1. Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12—14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.     

Nitrogen Fixation and Carbon Dioxide Assimilation in Rhizobium japonicum

In free-living Rhizobium japonicum cultures, the stimulatory effect of CO₂ on nitrogenase (acetylene reduction) activity was mediated through ribulose bisphosphate carboxylase activity. Two mutant strains (CJ5 and CJ6) of R. japonicum defective in CO₂ fixation were isolated by mitomycin C treatment. No ribulose bisphosphate carboxylase activity could be detected in strain CJ6, but a low level of enzyme activity was present in strain CJ5. Mutant strain CJ5 also exhibited pleiotropic effects on carbon metabolism. The mutant strains possessed reduced levels of hydrogen uptake, formate dehydrogenase, and phosphoribulokinase activities, which indicated a regulatory relationship between these enzymes. The CO₂-dependent stimulation of nitrogenase activity was not observed in the mutant strains. Both mutant strains nodulated soybean plants and fixed nitrogen at rates comparable to that of the wild-type strain.     

Isolation and characterization of pigment mutants from a filamentous cyanobacterium

Five strains of a pigment mutant were isolated following UV irradiation and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) mutagenesis from a non-nitrogen fixing mutant of the cyanobacteriumGloeotrichia ghosei. Two of them (B-1 and V-1) were isolated by UV mutagenesis and other three (B-3, B-7 and Br-6) by MNNG mutagenesis. Among the five strains cultures of three strains (B-1, B-3 and B-7) were typically blue-green in colour. Culture of strain V-1 was found to be violet-pink and of Br-6 was brownish in colour. The parent strain of these mutants was dark-blue in colour. Blue-green mutants showed the predominance of phycocyanin (610 nm) whereas violet-pink and brown strains showed the predominance of phycoerythrin (550 nm) in the absorption spectra of water-soluble pigments. In contrast to these strains their parent strain showed both the absorption peaks (at 550 and 610 nm). Occurrence of stable pigment mutants of a filamentous cyanobacterium indicates that the synthesis of water-soluble pigments is genetically controlled in these mutant strains.     

Nitrate and Nitrite Reduction in Relation to Nitrogenase Activity in Soybean Nodules and Rhizobium japonicum Bacteroids

Soybean (Glycine max L. cv Williams) seeds were sown in pots containing a 1:1 perlite-vermiculite mixture and grown under greenhouse conditions. Nodules were initiated with a nitrate reductase expressing strain of Rhizobium japonicum, USDA 110, or with nitrate reductase nonexpressing mutants (NR⁻ 108, NR⁻ 303) derived from USDA 110. Nodules initiated with either type of strain were normal in appearance and demonstrated nitrogenase activity (acetylene reduction). The in vivo nitrate reductase activity of N₂-grown nodules initiated with nitrate reductase-negative mutant strains was less than 10% of the activity shown by nodules initiated with the wild-type strain. Regardless of the bacterial strain used for inoculation, the nodule cytosol and the cell-free extracts of the leaves contained both nitrate reductase and nitrite reductase activities. The wild-type bacteroids contained nitrate reductase but not nitrite reductase activity while the bacteroids of strains NR⁻ 108 and NR⁻ 303 contained neither nitrate reductase nor nitrite reductase activities.

Addition of 20 millimolar KNO₃ to bacteroids of the wild-type strain caused a decrease in nitrogenase activity by more than 50%, but the nitrate reductase-negative strains were insensitive to nitrate. The nitrogenase activity of detached nodules initiated with the nitrate reductase-negative mutant strains was less affected by the KNO₃ treatment as compared to the wild-type strain; however, the results were less conclusive than those obtained with the isolated bacteroids.

The addition of either KNO₃ or KNO₂ to detached nodules (wild type) suspended in a semisolid agar nutrient medium caused an inhibition of nitrogenase activity of 50% and 65% as compared to the minus N controls, and provided direct evidence for a localized effect of nitrate and nitrite at the nodule level. Addition of 0.1 millimolar sucrose stimulated nitrogenase activity in the presence or absence of nitrate or nitrite. The sucrose treatment also helped to decrease the level of nitrite accumulated within the nodules.

     

Root cell-wall properties are proposed to contribute to phosphorus (P) mobilization by groundnut and pigeonpea

Groundnuts showed a superior ability to take up phosphorus (P) from two soils of extremely low fertility, where sorghum and soybean died of P deficiency. This ability could not be attributed to differences in root development, to P uptake parameters such as C_min, or to the excretion of root exudates capable of solubilizing iron- (Fe-P) and aluminum-bound P (Al-P), the sparingly soluble P forms in soils. A new P solubilizing mechanism (called `contact reaction') which occurs at the interface between root surface and soil particles, is therefore proposed. Isolated cell walls from groundnut roots solubilized more P from P-fixing minerals than those from sorghum and soybean roots. The P-solubilizing activity of groundnut root cell-walls might therefore be related to the superior growth of this crop under P-deficient conditions. The P-solubilizing active sites in groundnut root cell walls were located at the root surface and could act as chelating agent with Fe(III). This P-solubilizing active component in the cell walls could be extracted by NaOH, but not by HCl, and was identified as a small molecule through column chromatography with Sephadex LH-20. The P-solubilizing ability of pigeonpea root cell-walls was examined and found to be as high as that of groundnut. As pigeonpea plants excrete significant amount of root exudates with Fe-P solubilizing ability only after they flower, the P-solubilizing ability of root cell-walls may partially explain the high P efficiency of this species before it flowers.     

Relationship of the Presence and Copy Number of Plasmids to Exopolysaccharide Production and Symbiotic Effectiveness in Rhizobium fredii USDA 206

Rhizobium fredii USDA 206 harbors four large plasmids, one of which carries nodulation and nitrogen fixation genes. Previously isolated groups of plasmid-cured derivatives of strain USDA 206 were compared with each other to determine possible plasmid functions. Mutant strain 206CANS was isolated as a nonmucoid (Muc⁻) derivative of strain 206CA, a mutant that was cured of two plasmids. The Muc⁻ phenotype of 206CANS was only expressed when the strain was grown on certain media, particularly those with polyols as carbon sources. Plasmid pRj206b of strain 206CANS was previously shown to have a higher copy number than the same plasmid in strains USDA 206 and 206CA. When this plasmid was transferred to Muc⁺ strains, it conferred a nonmucoid phenotype on recipient strains. The symbiotic effectiveness of the wild-type and cured strains was compared. Overall, few differences were shown, but strains 206CA and 206CANS were found to have higher nitrogenase activities than the other strains. Thus, there appeared to be a possible relationship among exopolysaccharide synthesis, plasmid copy number, and symbiotic effectiveness.     

一株高效解磷肠膜明串珠菌的分离鉴定及解磷能力研究

【背景】植物根际土壤含有多种溶磷微生物,但是具有溶磷能力的肠膜明串珠菌未见报道。【目的】从脐橙根际土壤分离高效解磷菌,研究其解磷应用。【方法】通过初筛和复筛从23株菌中筛选解磷能力较强的菌株,同时采用钼蓝比色法测定磷含量。通过测定发酵液中小分子有机酸含量、磷酸酯酶酶活及pH值的变化,探究菌株的解磷机理。【结果】经过筛选得到9株具有一定解磷能力的菌株。通过菌种16S rRNA基因序列分析和生理生化实验确定其中一株菌为肠膜明串珠菌,命名为肠膜明串珠菌G7。培养基初始pH6.0、碳源为葡萄糖、氮源为硫酸铵时G7的解磷能力较佳。G7发酵过程中产生大量有机酸,而其酸性磷酸酯酶活性高于碱性磷酸酯酶。【结论】碳源、氮源以及初始pH值都能影响G7的解磷能力,其解磷能力主要缘于在发酵过程中产生了大量小分子有机酸,关于G7的解磷机理还需要更深入的研究。     

Stress induced phosphate solubilization in bacteria isolated from alkaline soils

Phosphate solubilizing bacteria NBRI0603, NBRI2601, NBRI3246 and NBRI4003 were isolated from the rhizosphere of chickpea and alkaline soils. All four strains demonstrated diverse levels of phosphate solubilization activity under in vitro conditions in the presence of various carbon and nitrogen sources. Acid production may have contributed to phosphate solubilization, but was not the only reason for phosphate release into the medium. Among the four strains, NBRI2601 was the most efficient strain in terms of its capability to solubilize phosphorus in the presence of 10% salt, pH 12, or 45 degrees C. The strains showed varied levels of phosphate solubilization when the effects of different sources of nitrogen were examined during growth. The presence of low levels of Ca(2+) and EDTA in the medium enhanced phosphate solubilization.     

Effect of Sym Plasmid Curing on Symbiotic Effectiveness in Rhizobium fredii

A mutant, USDA 206C, of Rhizobium fredii USDA 206 was obtained by passage on acridine plates. This mutant was cured of its 197-megadalton Sym plasmid but retained its symbiotic effectiveness. Multiple plasmid and chromosomally borne nif gene copies have previously been shown in R. fredii USDA 206. HindIII and EcoRI restriction enzyme digests of plasmid and total DNA showed that at least two nif gene copies are probably missing in USDA 206C. To compare the symbiotic effectiveness of USDA 206 and USDA 206C, plant tests were carried out. Statistically significant differences were obtained for nodule number, nodule mass, nitrogenase activity per plant, nitrogenase specific activity, and total plant dry weight. There was an apparent correlation between loss of Sym plasmidborne nif gene copies and reduction of overall symbiotic effectiveness. Delayed nodulation by strain USDA 206C relative to strain USDA 206 also indicated an association with the loss of plasmidborne nodulation functions and the reduced symbiotic effectiveness of strain USDA 206C.     

Regulation of nitrogen fixation in Rhizobium spp. Isolation of mutants of Rhizobium trifulii which induce nitrogenase activity

This communication describes the isolation and characterization of mutants of Rhizobium trifolii which can induce nitrogenase activity in defined liquid medium. Two procedures were used for the isolation of these mutants from R. trifolii strain DT-6: (1) following chemical mutagenesis, slow growin mutants were selected which were unable to utilize NH₄⁺ as sole source of nitrogen; (2) as spontaneous mutants resistant to the glutamate analogue L-methionine-DL-sulfoximine.Mutants (DT-71, DT-125) isolated by these procedures induced nitrogenase activity in the free-living state, whereas the parent strain lacked this property. Induction of nitrogenase activity in these mutants occurred during the late exponential phase of growth when the rate of protein synthesis was decreasing. The addition of NH₄⁺ to a medium containing glutamate as the nitrogen-source resulted in a 50–70% reduction (repression?) of nitrogenase activity; in contrast, the rate of protein synthesis or the rate of respiration was not influenced by exogenous NH₄⁺.Biochemistry analysis showed that these mutants (strains DT-71 and DT-125) have defects in both nitrogen and carbon metabolism. The levels of glutamate synthase (both NADP⁺-and NAD⁺-dependent activities) and glutamate dehydrogenase (NAD⁺-dependent activity) were markedly lower. In addition, the mutants were found to have no detectable ribitol dehydrogenase or β-galactosidase activity. These findings are discussed in relation to a mechanism of regulation of symbiotic nitrogen fixation.     

Simultaneous phosphate solubilization potential and antifungal activity of new fluorescent pseudomonad strains, Pseudomonasaeruginosa, P. plecoglossicida and P. mosselii

Of 80 fluorescent pseudomonad strains screened for phosphate solubilization, three strains (BFPB9, FP12 and FP13) showed the ability to solubilize tri-calcium phosphate (Ca₃(PO₄)₂). During mineral phosphate solubilization, decrease of pH in the culture medium due to the production of organic acids by the strains was observed. These phosphate solubilizing strains produced indole-3-acetic acid (IAA) and protease as well as exhibited a broad-spectrum antifungal activity against phytopathogenic fungi. When tested in PCR using the gene-specific primers, strain BFPB9 showed the presence of hcnBC genes that encode hydrogen cyanide. On the basis of phenotypic traits, 16S rRNA sequence homology and subsequent phylogenetic analysis, strains BFPB9, FP12 and FP13 were designated as Pseudomonas aeruginosa, P. plecoglossicida and P. mosselii, respectively. Present investigation reports the phosphate solubilization potential and biocontrol ability of new strains that belong to P. plecoglossicida and P. mosselii. Because of the innate potential of phosphate solubilization, production of siderophore, IAA, protease, cellulase and HCN strains reported in this study can be used as biofertilizers as well as biocontrol agents.     

Establishment of phosphate-solubilizing strains of Azotobacter chroococcum in the rhizosphere and their effect on wheat cultivars under green house conditions

A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha(-1), 90 kg N + 60 kg P2O5 ha(-1), 120 kg N ha(-1) and 120 kg N + 60 kg P2O5 ha(-1). Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12-14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.     

Space mutagenesis of genetically engineered bacteria expressing recombinant human interferon α1b and screening of higher yielding strains

The aim of this study was to investigate the space mutagenesis of genetically engineered bacteria expressing recombinant human interferon α1b. The genetically engineered bacteria expressing the recombinant interferon α1b were sent into outer space on the Chinese Shenzhou VIII spacecraft. After the 17 day space flight, mutant strains that highly expressed the target gene were identified. After a series of screening of spaceflight-treated bacteria and the quantitative comparison of the mutant strains and original strain, we found five strains that showed a significantly higher production of target proteins, compared with the original strain. Our results support the notion that the outer space environment has unique effects on the mutation breeding of microorganisms, including genetically engineered strains. Mutant strains that highly express the target protein could be obtained through spaceflight-induced mutagenesis.     

Mutants of the Cyanobacterium Anabaena sp. PCC 7120 Altered in Nitrate Transport and Reduction

Nitrate assimilation-defective mutants SP7, SP9, and SP17 of the cyanobacterium Anabaena sp. PCC 7120 were isolated by use of transposon mutagenesis and screened on medium containing chlorate. SP7 and SP17 represented nitrate reductase-defective nature, while mutant SP9 appeared to be a regulatory mutant exhibiting pleiotropic behavior. Kinetics of nitrate uptake system exhibited K _s values of 31–38 μM for parent, SP7, and SP17 strains; however, mutant SP9 exhibited a high K _s value of 109.5 μM. Defective nitrate reductase was apparent in mutant SP7 and SP9, while mutant SP17 exhibited partial defective nature. Methyl viologen-dependent NR activity in parent strain presented a biphasic nature with K _m values of 0.13 and 2.47 mM, whereas a single K _m value (2.96 mM) was observed for mutant SP17. Mutant SP9 was also defective in nitrite uptake and reduction. Mutant strains exhibited derepressed nitrogenase activity in the presence of nitrate, while glutamine synthetase activity remained unaltered. Received: 20 April 1999 / Accepted: 22 May 1999     

Evidence for the lack of oxygen repression of Fe−Mo protein in mutants ofAnabaena variabilis

Mutants ofAnabaena variabilis, unable to fix nitrogen under aerobic conditions, were used to determine whether nitrogenase synthesis is subject to O₂ repression, as is the case in some heterotrophic bacteria. Nitrogenase activity in the mutants was induced as heterocysts matured under microaerophilic conditions. However, addition of 5% O₂ to the assay system inhibited activity by 95%. Under aerobic conditions, nitrogenase activity in the mutants could not be detected, but an activity-independent, immunological assay showed that the Fe-Mo protein was present at levels similar to those found in wild-type and mutant strains induced microaerophilically. Reducing the O₂ tension of an aerobically induced mutant resulted in a rate of nitrogenase activity induction twice the rate under continuous microaerophilic conditions. These results indicate that O₂ does not repress Fe-Mo protein synthesis in these mutants.