Glutamine synthetase of Chlamydomonas: its role in the control of nitrate assimilation

Work is described which suggests that glutamine synthetase (GS) could play an important and direct regulatory role in the control of NO₃ assimilation by the alga. In both steady-state cells and ones disturbed physiologically by changes in light or nitrogen supply the assimilation of NO₃ appears to be limited by the activity of GS. Moreover although in normal cells NH₃ can completely inhibit NO₃ uptake, promote the deactivation of nitrate reductase (NR) and repress the synthesis of NR and nitrite reductase (NIR), these controls are relaxed in cells in which GS is deactivated by treatment with L-methionine-DL-sulfoximine (MSO). It is proposed that the reversible deactivation of GS may play an important part in the regulation of NO₃ assimilation although it is still not clear whether the enzyme itself or products of its metabolism are responsible.Abbreviations GS glutamine synthetase - GS_s glutamine synthetase, synthetase activity - GS_t glutamine synthetase, transferase activity - NR nitrate reductase - NIR nitrite reductase - GDH glutamate dehydrogenase - CHX cycloheximide - MSO L-methionine-DL-sulfoximine - FAD flavine adenine dinucleotide     

Ammonium formation and assimilation in P(SARK)∷IPT tobacco transgenic plants under low N

Wild Type (WT) and transgenic tobacco plants expressing isopentenyltransferase (IPT), a gene encoding the enzyme regulating the rate-limiting step in cytokinins (CKs) synthesis, were grown under limited nitrogen (N) conditions. We analyzed nitrogen forms, nitrogen metabolism related-enzymes, amino acids and photorespiration related-enzymes in WT and P_SARK∷IPT tobacco plants. Our results indicate that the WT plants subjected to N deficiency displayed reduced nitrate (NO₃⁻) assimilation. However, an increase in the production of ammonium (NH₄⁺), by the degradation of proteins and photorespiration led to an increase in the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle in WT plants. In these plants, the amounts of amino acids decreased with N deficiency, although the relative amounts of glutamate and glutamine increased with N deficiency. Although the transgenic plants expressing P_SARK∷IPT and growing under suboptimal N conditions displayed a significant decline in the N forms in the leaf, they maintained the GS/GOGAT cycle at control levels. Our results suggest that, under N deficiency, CKs prevented the generation and assimilation of NH₄⁺ by increasing such processes as photorespiration, protein degradation, the GS/GOGAT cycle, and the formation of glutamine.     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.     

Pulse additions of soil carbon and nitrogen affect soil nitrogen dynamics in an arid Colorado Plateau shrubland

Biogeochemical cycles in arid and semi-arid ecosystems depend upon the ability of soil microbes to use pulses of resources. Brief periods of high activity generally occur after precipitation events that provide access to energy and nutrients (carbon and nitrogen) for soil organisms. To better understand pulse-driven dynamics of microbial soil nitrogen (N) cycling in an arid Colorado Plateau ecosystem, we simulated a pulsed addition of labile carbon (C) and N in the field under the canopies of the major plant species in plant interspaces. Soil microbial activity and N cycling responded positively to added C while NH₄⁺–N additions resulted in an accumulation of soil NO₃⁻. Increases in microbial activity were reflected in higher rates of respiration and N immobilization with C addition. When both C and N were added to soils, N losses via NH₃ volatilization decreased. There was no effect of soil C or N availability on microbial biomass N suggesting that the level of microbial activity (respiration) may be more important than population size (biomass) in controlling short-term dynamics of inorganic and labile organic N. The effects of C and N pulses on soil microbial function and pools of NH₄⁺–N and labile organic N were observed to last only for the duration of the moisture pulse created by treatment addition, while the effect on the NO₃⁻–N pool persisted after soils dried to pre-pulse moisture levels. We observed that increases in available C lead to greater ecosystem immobilization and retention of N in soil microbial biomass and also lowered rates of gaseous N loss. With the exception of trace gas N losses, the lack of interaction between available C and N on controlling N dynamics, and the subsequent reduction in plant available N with C addition has implications for the competitive relationships between plants species, plants and microbes, or both.     

氮沉降增加情景下植物-土壤-微生物交互对自然生态系统土壤有机碳的调控研究进展

大气氮沉降增加倾向于促进受氮限制陆地生态系统地上生物量,但是对地下碳过程和土壤碳截存的影响结果迥异,导致陆地生态系统“氮促碳汇”的评估存在很大的不确定性。大气氮沉降输入直接影响微生物活性或间接影响底物质量,改变凋落物和土壤有机质(SOM)的分解速率和分解程度,进而影响土壤有机碳(SOC)的积累与损耗过程。过去相关研究主要集中在土壤碳转化过程和碳储量动态方面,缺乏植物-微生物-SOM交互作用的理解,对土壤碳截存调控的生物化学和微生物学机理尚不清楚。本文以地下碳循环过程为主线,分别综述了氮沉降增加对植物地下碳分配、SOC激发效应、微生物群落碳代谢过程的影响,深入分析SOM化学稳定性与微生物群落动态的关系。该领域研究的薄弱环节体现在:(1)增氮倾向于降低根系的生长和周转,对根际沉积碳分配(数量和格局)的影响及驱动因素不明确;(2)虽然认识到氮素有效性影响土壤激发效应的方向和强度,但是氧化态NO-3和还原态NH+4输入对有机质激发效应的差异性影响及潜在机理知之甚少;(3)微生物碳利用效率(CUE)是微生物群落碳代谢的关键表征,能够很好地解释土壤碳的积累与损耗过程;由于缺乏适宜的测定方法,难以准确量化土壤微生物的CUE及微生物生物量的周转时间;(4)增氮会抑制土壤真菌群落及其胞外酶活性,对细菌群落组成的影响尚未定论,有关SOM化学质量与土壤微生物群落活性、组成之间的耦合关系尚不清楚。未来研究应基于长期的氮添加控制实验平台,结合碳氧稳定性同位素示踪、有机质化学、分子生物学和宏基因组学等方法,深入分析植物同化碳的地下分配规律、微生物碳代谢和周转、有机质化学结构与功能微生物群落的耦合关系等关键环节。上述研究将有助于揭示植物-土壤-微生物交互作用对SOC动态的调控机制,完善陆地生态系统碳-氮耦合循环模型,有效降低区域陆地碳汇评估的不确定性,并可为陆地生态系统应对全球变化提供科学依据。     

Short-Term Metabolite Changes during Transient Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum

In this study, we measured the total pool sizes of key cellular metabolites from nitrogen-limited cells of Selenastrum minutum before and during ammonium assimilation in the light. This was carried out to identify the sites at which N assimilation is acting to regulate carbon metabolism. Over 120 seconds following NH₄⁺ addition we found that: (a) N accumulated in glutamine while glutamate and α-ketoglutarate levels fell; (b) ATP levels declined within 5 seconds and recovered within 30 seconds of NH₄⁺ addition; (c) ratios of pyruvate/phosphoenolpyruvate, malate/phosphoenolpyruvate, Glc-1-P/Glc-6-P and Fru-1,6-bisphosphate/Fru-6-P increased; and (d) as previously seen, photosynthetic carbon fixation was inhibited. Further, we monitored starch degradation during N assimilation over a longer time course and found that starch breakdown occurred at a rate of about 110 micromoles glucose per milligram chlorophyll per hour. The results are consistent with N assimilation occurring through glutamine synthetase/glutamate synthase at the expense of carbon previously stored as starch. They also indicate that regulation of several enzymes is involved in the shift in metabolism from photosynthetic carbon assimilation to carbohydrate oxidation during N assimilation. It seems likely that pyruvate kinase, phosphoenolpyruvate carboxylase, and starch degradation are all activated, whereas key Calvin cycle enzyme(s) are inactivated within seconds of NH₄⁺ addition to N-limited S. minutum cells. The rapid changes in glutamate and triose phosphate, recently shown to be regulators of cytosolic pyruvate kinase, are consistent with them contributing to the short-term activation of this enzyme.     

Amino acid abundance and proteolytic potential in North American soils

Studies of nitrogen (N) cycling have traditionally focused on N mineralization as the primary process limiting plant assimilation of N. Recent evidence has shown that plants may assimilate amino acids (AAs) directly, circumventing the mineralization pathway. However, the general abundance of soil AAs and their relative importance in plant N uptake remains unclear in most ecosystems. We compared the concentrations and potential production rates of AAs and NH₄ ⁺, as well as the edaphic factors that influence AA dynamics, in 84 soils across the United States. Across all sites, NH₄ ⁺ and AA-N comprised similar proportions of the total bioavailable N pool (~20%), with NO₃ ⁻ being the dominant form of extractable N everywhere but in tundra and boreal forest soils. Potential rates of AA production were at least comparable to those of NH₄ ⁺ production in all ecosystems, particularly in semi-arid grasslands, where AA production rates were six times greater than for NH₄ ⁺ (P < 0.01). Potential rates of proteolytic enzyme activity were greatest in bacteria-dominated soils with low NH₄ ⁺ concentrations, including many grassland soils. Based on research performed under standardized laboratory conditions, our continental-scale analyses suggest that soil AA and NH₄ ⁺ concentrations are similar in most soils and that AAs may contribute to plant and microbial N demand in most ecosystems, particularly in ecosystems with N-poor soils.     

Pathways of Nitrogen Metabolism in Nodules of Alfalfa (Medicago sativa L.)

Exposure of intact alfalfa nodules to ¹⁵N₂ showed that in bacteroids the greatest flow of ¹⁵N was to NH₃. Label was also detected in glutamic acid, aspartic acid, and asparagine (Glu, Asp and Asn), but at far lower levels. In the host plant cytosols, more ¹⁵N was incorporated into Asn than into other compounds. Detached nodules were also used to study the metabolic pathway of N assimilation after exposure to ¹⁵N₂ or vacuum infiltration with (¹⁵NH₄)₂SO₄ in the presence or absence of different inhibitors of nitrogen assimilation: methionine sulfoximine (MSO), azaserine (AZA), or amino-oxyacetate (AOA). Treatment with MSO, an inhibitor of glutamine synthetase (GS), inhibited the flow of the label to glutamine (Gln)-amide, resulting in subsequently decreased label in Asnamide. Aza, which inhibits the formation of Glu from Gln by glutamate synthase (GOGAT), enhanced the labeling of the amide groups of both Gln and Asn, while that of Asn-amino decreased. When AOA was used to block the transamination reaction very little label was found in Asp and Asn-amino. The results are consistent with the role of GS/GOGAT in the cytosol for the assimilation of NH₃ produced by N₂ fixation in the bacteroids of alfalfa nodules. Asn, a major nitrogen transport compound in alfalfa, is mainly synthesized by a Gln-dependent amidation of Asp, according to feeding experiments using the ¹⁵N-labeled amide group of glutamine. Data from ¹⁵NH₄⁺ feeding support some direct amidation of Asp to form Asn.     

Nitrogen Metabolism of the Marine Microalga Chlorella autotrophica

The levels of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in Chlorella autotrophica (clone 580) are strongly regulated by the nitrogen source and salt concentration of the medium. GS is present at high levels in NO₃⁻-grown cells, and at maximum levels in nitrogen-starved cells. However, the levels of GS in these cells are somewhat decreased by increasing salinity. Cells growing on NH₄⁺ have high NADPH-GDH activity, the levels of which increase with increasing NH₄⁺ supply, while GS decreases to a very low level under these conditions. Salinity intensifies the induction of NADPH-GDH activity in NH₄⁺-grown cells. The levels of NADH-GDH are low in this alga, but present under all growth conditions. Methionine sulfoximine (MSX) has little effect on growth and nitrogen assimilation of the alga in the presence of NH₄⁺.     

Prolonged root hypoxia effects on enzymes involved in nitrogen assimilation pathway in tomato plants

In order to investigate the effects of root hypoxia (1–2% oxygen) on the nitrogen (N) metabolism of tomato plants (Solanum lycopersicum L. cv. Micro-Tom), a range of N compounds and N-assimilating enzymes were performed on roots and leaves of plants submitted to root hypoxia at the second leaf stage for three weeks. Obtained results showed that root hypoxia led to a significant decrease in dry weight (DW) production and nitrate content in roots and leaves. Conversely, shoot to root DW ratio and nitrite content were significantly increased. Contrary to that in leaves, glutamine synthetase activity was significantly enhanced in roots. The activities of nitrate and nitrite reductase were enhanced in roots as well as leaves. The higher increase in the NH₄⁺ content and in the protease activities in roots and leaves of hypoxically treated plants coincide with a greater decrease in soluble protein contents. Taken together, these results suggest that root hypoxia leaded to higher protein degradation. The hypoxia-induced increase in the aminating glutamate dehydrogenase activity may be considered as an alternative N assimilation pathway involved in detoxifying the NH₄⁺, accumulated under hypoxic conditions. With respect to hypoxic stress, the distinct sensitivity of the enzymes involved in N assimilation is discussed.Key words: tomato, hypoxia, nitrogen, glutamine synthetase, protease, glutamate dehydrogenase     

NUTRIENT CONTROLS ON NITROGEN UPTAKE AND METABOLISM BY NATURAL POPULATIONS AND CULTURES OF TRICHODESMIUM (CYANOBACTERIA)

The effects of inorganic nutrient (ammonium [NH₄ ⁺ ] and nitrate [NO₃ ⁻ ]) and amino acid (glutamate [glu] and glutamine [gln]) additions on rates of N₂ fixation, N uptake, glutamine synthetase (GS) activity, and concentrations of intracellular pools of gln and glu were examined in natural and cultured populations of Trichodesmium. Additions of 1 μM glu, gln, NO₃ ⁻ , or NH₄ ⁺ did not affect short-term rates of N₂ fixation. This may be an important factor that allows for continued N₂ fixation in oligotrophic areas where recycling processes are active. N₂ fixation rates decreased when nutrients were supplied at higher concentrations (e.g. 10 μM). Uptake of combined N (NH₄ ⁺ , NO₃ ⁻ , and amino acids) by Trichodesmium was stimulated by increased concentrations. For NO₃ ⁻ , proportional increases in NO₃ ⁻ uptake and decreases in N₂ fixation were observed when additions were made to cultures before the onset of the light period. GS activity did not change much in response to the addition of NH₄ ⁺ , NO₃ ⁻ , glu, or gln. GS is necessary for N metabolism, and the bulk of this enzyme pool may be conserved. Intracellular pools of glu and gln varied in response to 10 μM additions of NH₄ ⁺ , glu, or gln. Cells incubated with NH₄ ⁺ became depleted in intracellular glu and enriched with intracellular gln. The increase in the gln/glu ratio corresponded to a decrease in the rate of N₂ fixation. Although the gln/glu ratio decreased in cells exposed to the amino acids, there was only a corresponding decrease in N₂ fixation after the gln addition. The results presented here suggest that combined N concentrations on the order of 1 μM do not affect rates of N₂ fixation and metabolism, although higher concentrations (e.g. 10 μM) can. Moreover, these effects are exerted through products of NH₄ ⁺ assimilation rather than exogenous N, as has been suggested for other species. These results may help explain how cultures of Trichodesmium are able to simultaneously fix N₂ and take up NH₄ ⁺ and how natural populations continue to fix N₂ once combined N concentrations increase within a bloom.     

Impacts of urea N addition on soil microbial community in a semi-arid temperate steppe in northern China

Nitrogen (N) addition has been well documented to decrease plant biodiversity across various terrestrial ecosystems. However, such generalizations about the impacts of N addition on soil microbial communities are lacking. This study was conducted to examine the impacts of N addition (urea-N fertilizer) on soil microbial communities in a semi-arid temperate steppe in northern China. Soil microbial biomass carbon (C), biomass N (MBN), net N mineralization and nitrification, and bacterial and fungal community level physiological profiles (CLPP) along an N addition gradient (0–64 g N m^?2 year^?1) were measured. Three years of N addition caused gradual or step increases in soil NH₄-N, NO₃-N, net N mineralization and nitrification in the early growing season. The reductions in microbial biomass under high N addition levels (32 and 64 g N m^?2 year^?1) are partly attributed to the deleterious effects of soil pH. An N optimum between 16 and 32 g N m^?2 year^?1 in microbial biomass and functional diversity exists in the temperate steppe in northern China. Similar N loading thresholds may also occur in other ecosystems, which help to interpret the contrasting observations of microbial responses to N addition.     

Root and Nodule Enzymes of Ammonia Assimilation in Two Plant-Conditioned Symbiotically Ineffective Genotypes of Alfalfa (Medicago sativa L.)

Biochemical and physiological parameters associated with nitrogen metabolism were measured in nodules and roots of glasshouse-grown clones of two symbiotically ineffective alfalfa (Medicago sativa L.) genotypes supplied with either NO₃⁻ or NH₄⁺. Significant differences were observed between genotypes for nodule soluble protein concentrations and glutamine synthetase (GS) and glutamate synthase (GOGAT) specific activities, both in untreated controls and in response to applied N. Nodule soluble protein of both genotypes declined in response to applied N, while nodule GS, GOGAT, and glutamate dehydrogenase (GDH) specific activities either decreased or remained relatively constant. In contrast, no genotype differences were observed in roots for soluble protein concentrations and GS, GOGAT, and GDH specific activities, either in untreated controls or in response to applied N. Root soluble protein levels and GS and GOGAT specific activities of N-treated plants increased 2- to 4-fold within 4 days and then decreased between days 13 and 24. Root GDH specific activity of NH₄⁺-treated plants increased steadily throughout the experiment and was 50 times greater than root GS or GOGAT specific activities by day 24.     

Tightly-Coupled Plant-Soil Nitrogen Cycling: Comparison of Organic Farms across an Agricultural Landscape

How farming systems supply sufficient nitrogen (N) for high yields but with reduced N losses is a central challenge for reducing the tradeoffs often associated with N cycling in agriculture. Variability in soil organic matter and management of organic farms across an agricultural landscape may yield insights for improving N cycling and for evaluating novel indicators of N availability. We assessed yields, plant-soil N cycling, and root expression of N metabolism genes across a representative set of organic fields growing Roma-type tomatoes (Solanum lycopersicum L.) in an intensively-managed agricultural landscape in California, USA. The fields spanned a three-fold range of soil carbon (C) and N but had similar soil types, texture, and pH. Organic tomato yields ranged from 22.9 to 120.1 Mg ha^-1 with a mean similar to the county average (86.1 Mg ha^-1), which included mostly conventionally-grown tomatoes. Substantial variability in soil inorganic N concentrations, tomato N, and root gene expression indicated a range of possible tradeoffs between yields and potential for N losses across the fields. Fields showing evidence of tightly-coupled plant-soil N cycling, a desirable scenario in which high crop yields are supported by adequate N availability but low potential for N loss, had the highest total and labile soil C and N and received organic matter inputs with a range of N availability. In these fields, elevated expression of a key gene involved in root N assimilation, cytosolic glutamine synthetase GS1, confirmed that plant N assimilation was high even when inorganic N pools were low. Thus tightly-coupled N cycling occurred on several working organic farms. Novel combinations of N cycling indicators (i.e. inorganic N along with soil microbial activity and root gene expression for N assimilation) would support adaptive management for improved N cycling on organic as well as conventional farms, especially when plant-soil N cycling is rapid.     

γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings

Excessive use of nitrogen (N) fertilizer has increased ammonium (NH₄⁺) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH₄⁺ toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH₄⁺ concentrations above 0.75 mM inhibited the growth of rice and caused NH₄⁺ accumulation in both shoots and roots. Use of excessive NH₄⁺ also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH₄⁺ conditions, exogenous γ‐aminobutyric acid (GABA) treatment limited NH₄⁺ accumulation in rice seedlings, reduced NH₄⁺ toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH₄⁺. Furthermore, we found that the activity of glutamine synthetase/NADH‐glutamate synthase (GS; EC 6.3.1.2/NADH‐GOGAT; EC1.4.1.14) in root increased gradually as the NH₄⁺ concentration increased. However, when the concentration of NH₄⁺ is more than 3 mM, GABA treatment inhibited NH₄⁺‐induced increases in GS/NADH‐GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH₄⁺. These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA‐dependent alleviation of ammonium toxicity in rice seedlings.     

The Role of Ammonium Assimilating Enzymes in Lentil Roots and Nodules

Activities of ammonium assimilating enzymes glutamate dehydrogenase (GDH), glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) as well as the amino acid content were higher in nodules compared to roots. Their activities increased at 40 and 60 d after sowing, with a peak at 90 d, a time of maximum nitrogenase activity. The GS/GOGAT ratio had a positive correlation with the amino acid content in nodules. Higher activities of AST than ALT may be due to lower glutamine and higher asparagine content in xylem. The data indicated that glutamine synthetase and glutamate synthase function as the main route for the assimilation of fixed N, while NADH-dependent glutamate dehydrogenase may function at higher NH₄ ⁺ concentration in young and senescing nodules. Enzyme activities in lentil roots reflected a capacity to assimilate N for making the amino acids they may need for both growth and export to upper parts of the plant. This revised version was published online in July 2006 with corrections to the Cover Date.     

Responses of terrestrial nitrogen pools and dynamics to different patterns of freeze‐thaw cycle: A meta‐analysis

Altered freeze‐thaw cycle (FTC) patterns due to global climate change may affect nitrogen (N) cycling in terrestrial ecosystems. However, the general responses of soil N pools and fluxes to different FTC patterns are still poorly understood. Here, we compiled data of 1519 observations from 63 studies and conducted a meta‐analysis of the responses of 17 variables involved in terrestrial N pools and fluxes to FTC. Results showed that under FTC treatment, soil NH₄⁺, NO₃^?, NO₃^? leaching, and N₂O emission significantly increased by 18.5%, 18.3%, 66.9%, and 144.9%, respectively; and soil total N (TN) and microbial biomass N (MBN) significantly decreased by 26.2% and 4.7%, respectively; while net N mineralization or nitrification rates did not change. Temperate and cropland ecosystems with relatively high soil nutrient contents were more responsive to FTC than alpine and arctic tundra ecosystems with rapid microbial acclimation. Therefore, altered FTC patterns (such as increased duration of FTC, temperature of freeze, amplitude of freeze, and frequency of FTC) due to global climate warming would enhance the release of inorganic N and the losses of N via leaching and N₂O emissions. Results of this meta‐analysis help better understand the responses of N cycling to FTC and the relationships between FTC patterns and N pools and N fluxes.     

Genetic transformation of glutamine auxotrophy to prototrophy in the cyanobacterium Nostoc muscorum

Glutamine auxotrophic (Gln ^-) and l-methionine d,l-sulfoximine (MSX) resistant (MSX ^r) mutants of N. muscorum were isolated and characterized for nitrogen nutrition, nitrogenase activity, glutamine synthetase (GS) activity and glutamine amide, -keto-glutarate amido transferase (GOGAT) activity. The glutamine auxotroph was found to the GOGAT-containing GS-defective, incapable of growth with N₂ or NH ₄ ⁺ but capable of growth with glutamine as nitrogen source, thus, suggesting GS to be the primary enzyme of both ammonia assimilation and glutamine formation in the cyanobacterium. The results of transformation and reversion studies suggests that glutamine auxotrophy is the result of a mutation in the gln A gene and that gln A gene can be transferred from one strain to another by transformation.