Nitrate as a hill reagent in a reconstituted chloroplast system

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.

Ferredoxin-dependent photosynthetic reduction of nitrate and nitrite by particles of Anacystis nidulans

The dark and light reduction of nitrate and nitrite by cell-free preparations of the blue-green alga Anacystis nidulans has been investigated. The three following methods have been successfully applied to the preparation of active particulate fractions from the alga cells: (a) shaking with glass beads, (b) lysozyme treatment and lysis of the resulting protoplasts, and (c) sonication. The two enzymes of the nitrate-reducing system-namely, nitrate reductase and nitrite reductase-are firmly bound to the isolated pigment-containing particles, and can be easily solubilized by prolonging the vibration or sonication time. Both enzymes-whether solubilized or bound to the particles-depend on reduced ferredoxin as the immediate electron donor. In its presence, the alga particles catalyze the gradual photoreduction of nitrate to nitrite and ammonia, a process that can thus be considered as one of the most simple and relevant examples of Photosynthesis. Some of the properties of nitrate reductase have been studied. Nitrate reductase as well as nitrite reductase are adaptive enzymes repressed by ammonia.

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.

[Nitrate-dependent noncyclic photophosphorylation in Ankistrodesmus braunii in the absence of CO2 and O 2]

Manometric measurements show that oxygen evolution proceeds in synchronised cells of Ankistrodesmus braunii even in an atmosphere of pure nitrogen. In this case the slow oxygen evolution is dependent on the presence of nitrate (Table 1). Light saturation is found at a low light intensity at pH 5.6, at a higher light intensity at pH 8.0 (Fig. 1). The light saturation curves are in good agreement with those of (32)P-labelling in Ankistrodesmus under the same conditions (Fig. 2).DCMU inhibition in N2 of both O2-evolution and (32)P-labelling begins only at a DCMU concentration of 5×10(-7)M or more. Complete inhibition of O2-evolution is reached only at 10(-5)M (Fig.3). In (32)P-labelling a variable percentage is still left uninhibited at 10(-5) M DCMU (Fig. 4, Table 2), which is at least partly due to cyclic photophsphorylation. Nitrate starvation for several hours causes a considerable decrease in O2-evolution and also in the sensitivity to those high concentrations of DCMU (Fig. 5), but it leads to a sensitivity to antimycin A not observed under normal conditions (Table 3). The effects of nitrate starvation thus become comparable to those of far-red light, under which noncyclic electron transport is slow or completely prevented.The inhibition by DCMU of electron transport in photosystem II is also estimated by measuring the increase in fluorescence at 684 nm in air containing additional CO2. This fluorescence is saturated only at 10(-5)M DCMU and shows that a certain percentage of photosystem II remains uninhibited at 5×10(-7)M (Fig. 6), a concentration found to be almost ineffective in inhibiting O2-evolution and (32)P-labelling in an N2-atmosphere.The results indicate that in synchronised cells of Ankistrodesmus noncyclic electron flow and noncyclic photophosphorylation can proceed in an atmosphere of pure nitrogen if nitrate is available as the electron acceptor. In this case noncyclic photophosphorylation, inspite of its low rates, still dominates over cyclic photphosphorylation. At low pH, when nitrate reduction is slow, cyclic photophosphorylation accounts for a greater part of the total phosphorylation than at high pH. Thus in the absence of CO2 and O2 cyclic photophosphorylation can be regarded as the main process of ATP formation only after nitrate starvation, in far-red light or in the presence of high concentrations of DCMU.Inhibition by DCMU, though very efficient under conditions of high photosynthetic activity, becomes rate-limiting only if the electron transport is so far reduced by DCMU that the remaining rate is of the same order as the low rate of the control or less. Therefore high concentrations of DCMU are required for the inhibition of low rates of noncyclic photophosphorylation.

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.

Intracellular location of nitrate reductase and nitrite reductase in spinach and sunflower leaves

Chloroplasts have been isolated from spinach and from sunflower which retain their outer membrane and their stroma protein as determined both by ability to fix CO2 and evolve O2 at high rates, and by appearance under the phase contrast microscope. Such chloroplasts contain both nitrate and nitrite reductase activity. However, calculations on the distribution of these enzymes, when compared with the distribution of pyruvate kinase and cytochrome c oxidase activity, demonstrate that the larger part of both nitrate and nitrite reductase is located outside of the chloroplast.