Light and Dark Reduction of Nitrite in a Reconstituted Enzymic System

Photosynthetic nitrate assimilation in cyanobacteria

Nitrate uptake and reduction to nitrite and ammonium are driven in cyanobacteria by photosynthetically generated assimilatory power, i.e., ATP and reduced ferredoxin. High-affinity nitrate and nitrite uptake takes place in different cyanobacteria through either an ABC-type transporter or a permease from the major facilitator superfamily (MFS). Nitrate reductase and nitrite reductase are ferredoxin-dependent metalloenzymes that carry as prosthetic groups a [4Fe-4S] center and Mo-bis-molybdopterin guanine dinucleotide (nitrate reductase) and [4Fe-4S] and siroheme centers (nitrite reductase). Nitrate assimilation genes are commonly found forming an operon with the structure: nir (nitrite reductase)-permease gene(s)-narB (nitrate reductase). When the cells perceive a high C to N ratio, this operon is transcribed from a complex promoter that includes binding sites for NtcA, a global nitrogen-control regulator that belongs to the CAP family of bacterial transcription factors, and NtcB, a pathway-specific regulator that belongs to the LysR family of bacterial transcription factors. Transcription is also affected by other factors such as CnaT, a putative glycosyl transferase, and the signal transduction protein P(II). The latter is also a key factor for regulation of the activity of the ABC-type nitrate/nitrite transporter, which is inhibited when the cells are incubated in the presence of ammonium or in the absence of CO(2). Notwithstanding significant advance in understanding the regulation of nitrate assimilation in cyanobacteria, further post-transcriptional regulatory mechanisms are likely to be discovered.

Differential regulation of glucose-6-phosphate dehydrogenase isoenzyme activities in potato

In plants, Glc-6-phosphate dehydrogenase (G6PDH) isoenzymes are present in the cytosol and in plastids. The plastidic enzymes (P1 and P2) are subject to redox regulation, but mechanisms that adjust cytosolic G6PDH activity are largely unknown. We adopted a leaf disc system for monitoring the effects of various conditions on G6PD isoform expression and enzyme activities in potato (Solanum tuberosum). Cytosolic G6PDH activity remained constant during water incubation in the dark. In continuous light or in the presence of metabolizable sugars in the dark, cytosolic G6PDH activity increased 6-fold within 24 h. Cycloheximide incubation demonstrated that enhanced cytosolic G6PDH activity depends on de novo protein synthesis. Osmotic change, phosphate sequestration, or oxidative stress did not affect cytosolic G6PDH activity. Furthermore, enzyme activity and protein contents closely followed the corresponding mRNA levels. Together with the fact that multiple SURE elements are present in the promoter region of the gene, these results suggest that cytosolic G6PDH activity is regulated by sugar availability at the transcriptional level. Plastidic G6PDH activity stayed constant during water incubation in the light and dropped to minimal levels within 6 h in the dark. Conversely, plastidic G6PDH activity of leaf discs incubated on Paraquat rose to 10-fold higher levels, which was not prevented by cycloheximide. Similar increases were found with nitrite, nitrate, or sulfate. No major changes in protein or mRNA contents of the plastidic P1 and P2 isoforms were registered. K(m) (Glc-6-phosphate) values of plastidic G6PDH activity differed between samples incubated on water or Paraquat, suggesting posttranslational modification of the plastidic enzyme(s). Immunoprecipitation of (32)P-labeled samples with P1 isoform-specific antibodies showed that the chloroplast enzyme is subject to protein phosphorylation. Obviously, in extended dark periods, G6PDH activity in the stroma is restricted but can be stimulated in response to high demands for NADPH.

Nitrite reduction in barley-root plastids: Dependence on NADPH coupled with glucose-6-phosphate and 6-phosphogluconate dehydrogenases, and possible involvement of an electron carrier and a diaphorase

Plastids from roots of barley (Hordeum vulgare L.) seedlings were isolated by discontinuous Percoll-gradient centrifugation. Coinciding with the peak of nitrite reductase (NiR; EC 1.7.7.1, a marker enzyme for plastids) in the gradients was a peak of a glucose-6-phosphate (Glc6P) and NADP(+)-linked nitrite-reductase system. High activities of phosphohexose isomerase (EC 5.3.1.9) and phosphoglucomutase (EC 2.7.5.1) as well as glucose-6-phosphate dehydrogenase (Glc6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) were also present in the isolated plastids. Thus, the plastids contained an overall electron-transport system from NADPH coupled with Glc6PDH and 6PGDH to nitrite, from which ammonium is formed stoichiometrically. However, NADPH alone did not serve as an electron donor for nitrite reduction, although NADPH with Glc6P added was effective. Benzyl and methyl viologens were enzymatically reduced by plastid extract in the presence of Glc6P+ NADP(+). When the plastids were incubated with dithionite, nitrite reduction took place, and ammonium was formed stoichiometrically. The results indicate that both an electron carrier and a diaphorase having ferredoxin-NADP(+) reductase activity are involved in the electron-transport system of root plastids from NADPH, coupled with Glc6PDH and 6PGDH, to nitrite.

Nitrate and nitrite as 'in vivo' quenchers of chlorophyll fluorescence in blue-green algae

The effect of nitrate and nitrite on long-term chlorophyll fluorescence has been studied in filamentous blue-green algae. Cells grown autotrophically with nitrate as nitrogen source show, under argon atmosphere, a high level of fluorescence. The addition of either nitrete or nitrite induces a significant fluorescence quenching, but, whereas in the case of nitrite no previous treatment is required, in the case of nitrate the cells have to be sonicated or treated with Triton X-100 in advance without destroying their cellular integrity. DCMU Abbreviations: BQ, p-benzoquinone; DCMU, 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea; FCCP, carbonylcyanide 4-trifluoromethoxyphenylhydrazone; FeCy, potassium ferricyanide. strongly inhibits the quenching of fluorescence caused by nitrate or nitrite. Using cells grown with ammonia, a nutritional repressor of the two enzymes of the nitrate-reducing system, the fluorescence quenching observed in either case becomes negligible. These results clearly indicate that both nitrate and nitrite can physiologically act as primary Hill reagents in photosynthesis in blue-green algae.

Nitrite reduction in leaves; Studies on isolated chloroplasts

Chloroplast preparations from spinach leaves containing a high percentage of intact chloroplasts were capable of light dependent nitrite reduction at rates around 9 μmol/mg chlorophyll/h for at least 50 min. This reduction was inhibited by DCMU but unaffected by uncouplers of photosynthetic phosphorylation. Nitrite reduction was not accompanied by a stoichiometric evolution of oxygen evolution. The disappearance of nitrite was accompanied by an approximately stoichiometric formation of reduced nitrogen.

Studies on nitrite reductase in barley

Nitrite reductase from barley seedlings was purified 50-60 fold by ammonium sulphate precipitation and gel filtration. No differences were established in the characteristics of nitrite reductases isolated in this way from either leaf or root tissues. The root enzyme accepted electrons from reduced methyl viologen, ferredoxin, or an unidentified endogenous cofactor. Enzyme activity in both tissues was markedly increased by growth on nitrate. This activity was not associated with sulphite reductase activity. Microbial contamination could not account for the presence of nitrite reductase activity in roots. Nitrite reductase assayed in vitro with reduced methyl viologen as the electron donor was inhibited by 2,4-dinitrophenol but not by arsenate.

Effect of nitrite and nitrate on chlorophyll fluorescence in green algae

The influence of nitrite and nitrate on chlorophyll fluorescence, a very sensitive indicator for the redox state of the primary acceptor of photosystem II of photosynthesis, was studied in green algae (several species of Chlorella, and Ankistrodesmus braunii). In phosphate solution under an atmosphere of nitrogen (i.e., in the absence of O2 and CO2, and without nitrite or nitrate), fluorescence shows a pronounced induction and then rises to a high steady-state level. In the presence of nitrite, however, fluorescence decreases after a rather short induction peak to a much lower steady-state. Nitrate, on the other hand, does not have any influence on either induction or steady-state of fluorescence. These results clearly demonstrate that nitrite reduction in the light is very closely coupled to the photosynthetic electron transport system, whereas nitrate is not reduced photosynthetically in vivo.

The role of light in nitrite reduction; Studies with leaf discs

The reduction of nitrite by leaf discs has been studied. In short term experiments the reduction is markedly stimulated by light, but is not affected by the absence of oxygen or carbon dioxide from the gas phase. Carbon dioxide assimilation is more sensitive than nitrite reduction to 3-(3',4'-dichloro-)-1,1-dimethyl urea (DCMU) inhibition. Uncouplers such as carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) do not inhibit nitrite reduction although dinitrophenol (DNP) has a small effect.Although nitrite stimulates oxygen evolution in the light in the absence of CO2 the stoichiometry of nitrite reduction to oxygen evolution is much less than would be predicted if nitrite is simply acting as a classical Hill reagent.

[Dependence of nitrite assimilation in green algae on energy supplied by respiration and photosynthesis]

Inhibitors and uncouplers of phosphorylation, i.e., arsenate, 2.4-dinitrophenol (DNP), pentachlorophenol (PCP), and carbonyl cyanide m-chlorophenylhydrazone (CCCP), inhibit the assimilation of nitrite by the green alga Ankistrodesmus braunii in the dark and in the light. In a medium containing nitrate, these inhibitors interrupt nitrate reduction at the level of nitrite. In phosphatedeficient algae, the assimilation of nitrite can be decreased by a concomitant, energy-dependent uptake of chloride and phosphate ions. These results support the assumption that high-energy phosphate is required for the assimilation of nitrite.CO2 and glucose (after pre-illumination) increase nitrite assimilation in the light. Photosynthetic nitrite reduction is inhibited by 3-(3',4'-dichlorophenyl)-1,1-dimethyl urea (DCMU), an inhibitor of oxygen evolution, and by disalicylidene-propanediamine-(1,3) (DSPD), an inhibitor of the photosynthetic reduction of ferredoxin.

The effect of nitrate and nitrite on oxygen evolution and carbon-dioxide assimilation and the reduction of nitrate and nitrite by intact chloroplasts

Intact chloroplasts isolated from spinach reduced NO3 (-) and NO2 (-) in the light without the addition of either co-factors or added enzymes. The maximum rate observed, however, for the reduction of NO3 (-) was approximately 3μMoles hr(-1) mg(-1) (chlorophyll) and for NO2 (-) 6 μMoles hr(-1) mg(-1) (chlorophyll). These rates were consistent with the enzyme content of whole chloroplasts, but much lower than those found in whole leaf extracts.The addition of both NO3 (-) and NO2 (-) in low concentrations resulted in transient increases in both O2 evolution and CO2 fixation. The increases in oxygen evolution were not consistent in amount and bore no relation to the amount of substrate reduced. Similar transients were observed in a number of experiments when NaCl or NH4Cl were added.The addition of NO2 (-) at concentrations of 10(-4) M and above resulted in marked inhibition of both O2 evolution and CO2 fixation. NO2 (-) appears to inhibit by blocking the reduction of NADP. NO3 (-) at similar concentrations had no such effect.An increase in the soluble amino nitrogen content of the chloroplasts was observed when NO3 (-) or NO2 (-) was reduced. There was, however, no increase in the incorporation of (14)C from (14)CO2 into amino acids under these conditions. Even with the addition of ammonia the amount of (14)C incorporated into the amino acids was not changed from less than 5% of the total (14)C fixed. We conclude that while intact chloroplasts do have the ability to reduce both NO3 (-) and NO2 (-) at low rates, they do not synthesize appreciable amounts of amino acid directly, and this fact must be considered when formulating any pathways for nitrogen metabolism during photosynthesis.

[Polarographic measurement of nitrite reduction by chlorella in monochromatic light]

In green plant cells nitrite is reduced by two systems, one dependent on photosynthesis and the other upon respiration. Using a polarographic method for continuous measurement of nitrite uptake, the relationship between light driven and respiration linked nitrite reduction of Chlorella cells was studied.Photosynthetic nitrite reduction is characterized by a pronounced increase in the velocity of nitrite uptake upon illumination. After the light is turned off the velocity immediately returns to the preillumination value. Photosynthetic nitrite reduction of Chlorella is separated from respiration linked nitrite reduction by illumination with red light under anaerobic conditions; it is stimulated by CO2 and is inhibited by DCMU, findings which confirm earlier observations.In white light a special blue light stimulation of nitrite uptake is overlapped by photosynthetic nitrite reduction. In contrast to photosynthetic nitrite reduction this type of light stimulation is characterized by a lag period of about I min from the onset of illumination; it continues about 10 min when the light is turned off. It is separated from photosynthetic nitrite reduction by irradiation of the algae with low intensities of short wavelength light (<500 nm). Blue light stimulation of nitrite uptake of Chlorella is strongly dependent on the developmental stage of the cells. It is observed with young cells (autospores) of synchronized algae only.There is no evidence for any connection between blue light stimulation of nitrite uptake and photosynthesis. From the sensitivity of this process towards anaerobic conditions and antimycin A it is concluded to be a stimulation of respiration linked nitrite reduction.Under conditions of low exogenous nitrite concentration a temporary inhibition of steady state dark nitrite reduction appears immediately after the light is turned off. From several observations it is concluded that the inhibition already exists during the preceding illumination and decreases the rate of total nitrite uptake in the light. This process is suppressed by inhibition of respiration as well as by the inhibitor of photosynthesis, DCMU.If nitrate is the source of nitrogen an excretion of nitrite is found following illumination. The kinetics of this process agree with those observed for the light induced inhibition of steady state dark nitrite reduction immediately after illumination.

[Studies on light-dependent nitrite reduction in Chlorella]

1. The dependence of nitrite reduction activity in Chlorella fusca SHIHIRA et KRAUSS, as measured by uptake of nitrite, on the developmental stage, light intensity and CO2 concentration was investigated. 2. There is a quantitative relation between uptake of nitrite-nitrogen and increase of organic cell-nitrogen. 3. The kinetics of nitrite reduction in light are linear with time at high CO2 concentrations (3% in air) but biphasic at low CO2 concentrations (0 to 0.03%). In the steady state phase nitrite uptake is stimulated by an increase in the CO2 concentration from 0 to 0.03%. The initial phase and the steady state phase exhibit a different temperature dependence. 4. Nitrite uptake in the steady state phase is completely inhibited by iodoacetamide at concentrations which inhibit the photosynthetic CO2 reduction (5×10(-5) M). At lower iodoacetamide concentrations (1 to 2×10(-5) M) the linear time course of nitrite uptake at high CO2 concentrations is changed in the same manner as it is when the CO2 concentration is lowered. It is concluded that under special conditions (e.g. low CO2 concentration) the rate of steady state nitrite reduction in light is limited by the supply of carbon skeleton (i.e. N acceptors in the synthesis of organic nitrogen compounds) from the photosynthetic CO2 reduction cycle. 5. Pre-illumination in the presence of nitrite followed by a dark period depresses the initial phase of nitrite uptake in light at low CO2 concentrations. Pretreatment with nitrite in the dark lowers the nitrite uptake of the cells in the initial phase by exactly the same amount of nitrite which has been taken up during the pre-darkening period. It is concluded that there is a correlation between the initial phase of nitrite reduction in light and the respiration-linked nitrite reduction in the dark.