Nitrite assimilation

Bird, C., and Wyman, M. (2003). Nitrate/nitrite assimilation system of the marine picoplanktonic cyanobacterium Synechococcus sp strain WH 8103 Effect of nitrogen source and avaflabflity on gene expression. Appl. Environ. Microbiol. 69, 7009-7018. 

Pahnska, K. A., Laloui, W., Bedu, S., Loiseaux-de Goer, S., Castets, A. M., Rippka, R., and Tandeau de Marsac, N. (2002). The signal transducer Pn and bicarbonate acquisition in Prochlorocoaus marinus PCC 9511, a marine cyanobacterium naturally deficient in nitrate and nitrite assimilation. Microbiology. 148, 2405-2412. 

Ammonium is normally exchanged with protons to balance charge in passing through each cell membrane, so that each ammonium absorbed and assimilated by chloroplasts has half the alkaline effect of each nitrite assimilation. When taken together, the processes of nitrite and ammonium assimilation might therefore be of a similar order in terms of alkaline effect as the acidic effect of oxygenase activity of Rubisco,... 

Regardless of whether the process of nitrate reduction is located in photosynthetic or nonphotosynthetic tissues, it still involves a cytoplasmically located nitrate reductase [reaction (6)] and a nitrite reductase complex [reaction (7)], which is located in plastids. Possible sources of reductant for these reactions have been discussed in several reviews (e.g., Lee, 1980 Abrol et al., 1983 Smirnoff and Stewart, 1985) and the conclusion reached that in heterotrophic (nonphotosynthetic) nitrate assimilation the NADH required by nitrate reductase might be derived from glycolysis, from the oxidative pentose phosphate pathway, or even from mitochondrial dehydrogenases (see I e, 1980), whereas the pentose phosphate pathway may be of singular significance in supplying NADPH for nitrite assimilation. As indicated for root tissue by Ernes et al. 

In recent years greater attention has been given to nitrogen containing pesticides and the possibility of their nitrosation in soil. The N-nitrosamines that form may arise from the parent pesticide or from a pesticide metabolite. The reaction calls for favourable pH conditions (pH 3-4) and excess nitrite. Under field conditions, the nitrosable residues are usually present in traces and only small quantities of these will actually be nitrosated in soils. However, the possibility exists that the small amounts of N-nitrosamines could be assimilated by plants. 

A distinctive feature of the genus Saccharomyces as compared to other yeasts is its inability to use nitrate or nitrite as the sole nitrogen souree. Assimilation of... 

Klebsiella pneumoniae uses Bromoxynil as a nitrogen source, but it does so only after converting the nitrite to NH3, which is then assimilated. 

Nitrogen uptake that results in the formation of new biomolecules is termed an assimilation process, such as assimilatory nitrogen reduction. The processes that result in the release of DIN into seawater are referred to as dissimilations, such as dissimi-latory nitrogen reduction. An example of the latter is denitrification, in which nitrate and nitrite obtained from seawater serve as electron acceptors to enable the oxidation of organic matter. This causes the nitrate and nitrite to be transformed into reduced species, such as N2O and N2, which are released back into seawater. 

The phototrophs that assimilate fixed nitrogen preferentially utilize ammonium over nitrate or nitrite. (Some phototrophs assimilate DON, such as urea, to meet their nitrogen needs, and, hence, are not strict autotrophs.) Ammonia is fevored over the more oxidized forms of DIN as less redox energy is required to incorporate its nitrogen into biomolecules, primarily amino acids and the nucleotide bases. 

If ammonium concentrations in seawater are low, phytoplankton will assimilate nitrate and nitrite using chemical-specific permeases. Once inside the cell, these DIN species are transformed into ammonium via redox reactions in which nitrogen is reduced to the -III oxidation state ... 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

Green plants, algae, fungi, cyanobacteria and bacteria that assimilate nitrate also produce assimilatory nitrite reductases, which catalyze the six-electron reduction of nitrite to ammonia (equation 89). The formation of heme-nitrosyl intermediates has been detected in several cases,1515 while hydroxylamine is commonly thought to be an intermediate. Added hydroxylamine is rapidly reduced to ammonia. However, no intermediates are released, and ammonia is the only product... 

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). 

Nitrification seems limited to a number of autotrophic bacteria. The dominant genus that is capable of oxidizing ammonia to nitrite in soils is Nitmsomonas, and the dominant genus capable of oxidizing nitrite to nitrate is Nitrobacter. Normally, the two processes are closely connected and nitrite accumulation does not occur. Nitrifying bacteria are chemolithotrophs that utilize the energy derived from nitrification to assimilate C02. 

GS in plants is the key enzyme of the GS/GOGAT (glutamine synthetase/g1utamine 2-oxyg1utarate aminotransferase) pathway and thus plays a crucial role in ammonia assimilation/reassimilation (117. 118). GS inhibition by phosphinothricin causes accumulation of toxic levels of ammonia. Since ammonia production is increased by photorespiration and conversion of nitrite to ammonia (also light-dependent), nitrogen fertilizers and light act to promote phosphinothricin efficacy (119). 

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