Nitrite uptake and its regulation in the cyanobacterium Anacystis nidulans

Abiotic variables affect STX concentration in a meso-oligotrophic subtropical coastal lake dominated by Cylindrospermopsis raciborskii (Cyanophyceae)

The cyanobacterium Cylindrospermopsis raciborskii is capable of producing toxins including saxitoxin (STX). Few studies have verified the influence of environmental variables on the production of STX and most have only been studied in the laboratory. The goal of this work was to identify the abiotic variables related to STX concentration in situ. The relationship among STX concentration and the physical variables, nutrients and chlorophyll-a (chl-a) concentration was examined in a meso-oligotrophic subtropical coastal lake dominated by C. raciborskii. A generalized linear model was developed, incorporating all variables measured monthly over a 45-month monitoring period. Conductivity and dissolved inorganic nitrogen (DIN) concentration provided the greatest explanatory power for STX concentration in situ. Previous studies suggested that C. raciborskii cells exposed to stress associated with higher ionic concentrations appear to activate the biosynthesis of STX suggesting that STX can elicit changes cell permeability and may contribute to the homeostasis of this organism. An increase of DIN concentration results in a higher concentration of STX which may be related to a reduced metabolic demand, since the uptake of inorganic nitrogen requires less energy than N₂-fixation. Thus, increased DIN can favor the growth of C. raciborskii population or improve cellular homeostasis, both potentially increasing STX concentration in the aquatic system, which was observed through a delayed response pattern. The developed model, while providing only a moderate predictive power, can assist in the understanding of the environmental variables associated with increases in STX concentration, and in monitoring and minimizing the risks of toxic blooms.

Microalgal and cyanobacterial cultivation: the supply of nutrients

Microalgae and cyanobacteria are a promising new source of biomass that may complement agricultural crops to meet the increasing global demand for food, feed, biofuels and chemical production. Microalgae and cyanobacteria cultivation does not interfere directly with food production, but care should be taken to avoid indirect competition for nutrient (fertilizer) supply. Microalgae and cyanobacteria production requires high concentrations of essential nutrients (C,N,P,S,K,Fe, etc.). In the present paper the application of nutrients and their uptake by microalgae and cyanobacteria is reviewed. The main focus is on the three most significant nutrients, i.e. carbon, nitrogen and phosphorus; however other nutrients are also reviewed. Nutrients are generally taken up in the inorganic form, but several organic forms of them are also assimilable. Some nutrients do not display any inhibition effect on microalgal or cyanobacterial growth, while others, such as NO2 or NH3 have detrimental effects when present in high concentrations. Nutrients in the gaseous form, such as CO2 and NO face a major limitation which is related mainly to their mass transfer from the gaseous to the liquid state. Since the cultivation of microalgae and cyanobacteria consumes considerable quantities of nutrients, strategies to improve the nutrient application efficiency are needed. Additionally, a promising strategy to improve microalgal and cyanobacterial production sustainability is the utilization of waste streams by recycling of waste nutrients. However, major constraints of using waste streams are the reduction of the range of the biomass applications due to production of contaminated biomass and the possible low bio-availability of some nutrients.

Nitrite transport activity of a novel HPP family protein conserved in cyanobacteria and chloroplasts

Some cyanobacterial genomes encode an integral membrane protein of the HPP family, which exhibited nitrite transport activity when expressed in the nitrite transport-less NA4 mutant of the cyanobacterium Synechococcus elongatus strain PCC 7942. AT5G62720 and AT3G47980 were found to encode Arabidopsis homologs of the cyanobacterial protein. The product of AT5G62720 was localized to the chloroplast envelope membrane and was shown to confer nitrite uptake activity on the NA4 mutant when expressed with an N-terminally truncated transit peptide or as a fusion with the N-terminal region of the cyanobacterial HPP family protein. Kinetic analyses showed that the Arabidopsis protein has much higher affinity for nitrite (K(m) = 13 µM) than the cyanobacterial protein (K(m) = 150 µM). Illuminated chloroplasts isolated from the mutant lines of AT5G62720 showed much lower activity of nitrite uptake than the chloroplasts isolated from the wild-type Col-0 plants, while the chloroplasts of the mutants of AT1G68570 (AtNPF3.1), the gene previously reported to encode a plastid nitrite transporter AtNitr1, showed wild-type levels of nitrite uptake activity. AT3G47980 was expressed in roots but not in shoots. It has a putative transit peptide similar to that of AT5G62720 and its fusion with the N-terminal region of the cyanobacterial HPP protein showed low but significant activity of nitrite transport in the cyanobacterial cell. Transcription of AT5G62720 (AtNITR2;1) and AT3G47980 (AtNITR2;2) was stimulated by nitrate under the control of the NIN-like proteins, suggesting that the HPP proteins represent nitrate-inducible components of the nitrite transport system of plastids.

Negative regulation of expression of the nitrate assimilation nirA operon in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120

In the filamentous, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120, expression of the nitrate assimilation nirA operon takes place in the absence of ammonium and the presence of nitrate or nitrite. Several positive-action proteins that are required for expression of the nirA operon have been identified. Whereas NtcA and NtcB exert their action by direct binding to the nirA operon promoter, CnaT acts by an as yet unknown mechanism. In the genome of this cyanobacterium, open reading frame (ORF) all0605 (the nirB gene) is found between the nirA (encoding nitrite reductase) and ntcB genes. A nirB mutant was able to grow at the expense of nitrate as a nitrogen source and showed abnormally high levels of nirA operon mRNA both in the presence and in the absence of nitrate. This mutant showed increased nitrate reductase activity but decreased nitrite reductase activity, an imbalance that resulted in excretion of nitrite, which accumulated in the extracellular medium, when the nirB mutant was grown in the presence of nitrate. A nirA in-frame deletion mutant also showed a phenotype of increased expression of the nirA operon in the absence of ammonium, independent of the presence of nitrate in the medium. Both NirB and NirA are therefore needed to keep low levels of expression of the nirA operon in the absence of an inducer. Because NirB is also needed to attain high levels of nitrite reductase activity, NirA appears to be a negative element in the nitrate regulation of expression of the nirA operon in Anabaena sp. strain PCC 7120.

Nitrite transport activity of the ABC-type cyanate transporter of the cyanobacterium Synechococcus elongatus

In addition to the ATP-binding cassette (ABC)-type nitrate/nitrite-bispecific transporter, which has a high affinity for both substrates (K(m), approximately 1 microM), Synechococcus elongatus has an active nitrite transport system with an apparent K(m) (NO(2)(-)) value of 20 microM. We found that this activity depends on the cynABD genes, which encode a putative cyanate (NCO(-)) ABC-type transporter. Accordingly, nitrite transport by CynABD was competitively inhibited by NCO(-) with a K(i) value of 0.025 microM. The transporter was induced under conditions of nitrogen deficiency, and the induced cells showed a V(max) value of 11 to 13 micromol/mg of chlorophyll per h for cyanate or nitrite, which could supply approximately 30% of the amount of nitrogen required for optimum growth. Its relative specificity for the substrates and regulation at transcriptional and posttranslational levels suggested that the physiological role of the bispecific cyanate/nitrite transporter in S. elongatus is to allow nitrogen-deficient cells to assimilate low concentrations of cyanate in the medium. Its contribution to nitrite assimilation was significant in a mutant lacking the ABC-type nitrate/nitrite transporter, suggesting a possible role for CynABD in nitrite assimilation by cyanobacterial species that lack another high-affinity mechanism(s) for nitrite transport.

Lack of control of nitrite assimilation by ammonium in an oceanic picocyanobacterium, Synechococcus sp. strain WH 8103

In cyanobacteria, the transcriptional activator NtcA is involved in global nitrogen control and, in the absence of ammonium, regulates the expression of genes involved in the assimilation of alternative nitrogen sources. The oceanic picocyanobacterium Synechococcus sp. strain WH 8103 harbors a copy of ntcA, but in the present study, we show that unlike other marine cyanobacteria that have been investigated, this strain is capable of coassimilating nitrite when grown in the presence of ammonium. Transcript levels for the genes encoding the nitrate/nitrite-bispecific permease NrtP and nitrate reductase (NarB) were substantially down-regulated by ammonium, whereas the abundances of nitrite reductase (NirA) transcripts were similar in nitrite- and ammonium-grown cells. The growth of Synechococcus sp. strain WH 8103 in medium containing both ammonium and nitrite resulted in only minor changes in the expression profile in comparison to that of nitrite-grown cells with the exception that the gene encoding the high-affinity ammonium transporter Amt1 was down-regulated to the levels seen in ammonium-grown cells. Whereas the expression of nrtP, narB, and amt1 appears to be NtcA dependent in this marine cyanobacterium, the transcription and expression of nirA appear not to be. The ability to coassimilate nitrite and reduced-nitrogen sources like ammonium may be an adaptive trait that enables oceanic strains like Synechococcus sp. strain WH 8103 to exploit the low nitrite concentrations found in oceanic surface waters that are not available to their principal and more numerous competitor, Prochlorococcus.

The nitrate/nitrite ABC transporter of Phormidium laminosum: Phosphorylation state of NrtA is not involved in its substrate binding activity

Most cyanobacteria take up nitrate or nitrite through a multisubunit ABC transporter (ATP-binding cassette) located in the cytoplasmic membrane. Nitrate and nitrite transport activity is instantaneously blocked by the presence of ammonium in the medium. Previous biochemical studies reported the existence of phosphorylation/dephosphorylation events of the nitrate transporter (NRT) related to the presence of ammonium-sensitive kinase/phosphatase activities in plasma membranes of the cyanobacterium Synechococcus elongatus PCC 6301. In this work, we have analyzed the biochemical properties of the periplasmic nitrate/nitrite-binding subunit (NrtA) of NRT from the thermophilic nondiazotrophic cyanobacterium Phormidium laminosum. Our results show that cyanobacterial NrtA is phosphorylated in vivo. However, substrate binding activity in vitro is not affected by the phosphorylation state of the protein, ruling out the possibility that phosphorylation/dephosphorylation of NrtA is involved in the regulation of the nitrate/nitrite uptake by NRT transporter. Moreover, NrtA is present as multiple isoforms showing the same molecular mass but different isoelectric points ranging from pI 5 to 6. Mass spectrometric characterization of NrtA isoforms shows that the protein is phosphorylated at residue Tyr203, and contains several methionine sulphoxide residues which account for the observed isoforms. Both phosphorylated and non-phosphorylated forms of NrtA are active in vitro, showing comparable binding affinity for nitrate and nitrite. Both substrates behave as pure competitive inhibitors with a binding stoichiometry of one molecule of anion per NrtA monomer.

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.

Ecological aspects of ntcA gene expression and its use as an indicator of the nitrogen status of marine Synechococcus spp

Nitrogen nutrition in cyanobacteria is regulated by NtcA, a transcriptional activator that is subject to negative control by ammonium. Using Synechococcus sp. strain WH7803 as a model organism, we show that ntcA expression was induced when cells were exposed to nitrogen stress but not when they were subjected to phosphorus or iron deprivation. Transcript levels accumulated in cells grown on a variety of inorganic and organic nitrogen sources, with the sole exception of ammonium. ntcA transcription was induced when ammonium levels dropped below 1 microM and reached maximal levels within 2 h. Furthermore, the addition of more than 1 microM ammonium led to a rapid decline in ntcA mRNA. The negative effect of ammonium was prevented by the addition of L-methionine-D,L-sulfoximine (MSX) and azaserine, inhibitors of ammonium assimilation. Thus, basal ntcA transcript levels are indicative of ammonium utilization. Conversely, the highest ntcA transcript levels were found in cells lacking a nitrogen source capable of supporting growth. Therefore, maximal ntcA expression would indicate nitrogen deprivation. This state of nitrogen deprivation was induced by a 1-h incubation with MSX. The rapid response of ntcA gene expression to the addition of ammonium and MSX was used to design a protocol for assessing relative ntcA transcript levels in field populations of cyanobacteria, from which their nitrogen status can be inferred. ntcA was basally expressed in Synechococcus at a nutrient-enriched site at the northern tip of the Gulf of Aqaba, Red Sea. Therefore, these cyanobacteria were not nitrogen stressed, and their nitrogen requirements were met by regenerated nitrogen in the form of ammonium.

Phosphorylation of the signal transducer PII protein and an additional effector are required for the PII-mediated regulation of nitrate and nitrite uptake in the Cyanobacterium synechococcus sp. PCC 7942

In the cyanobacterium Synechococcus sp. strain PCC 7942, the phosphorylation states of the signal transducer PII protein (GlnB) can change rapidly depending on the nitrogen and carbon supply. A PII-null mutant (MP2) shows no ammonium-dependent inhibition of the nitrate and nitrite uptake, in contrast to the wild-type. New mutants with different types of PII, which may mimic either the phosphorylated (GlnBS49E or GlnBS49D) or unphosphorylated (GlnBS49A) form of the protein, were constructed using site-directed in vitro mutagenesis. Mutant MP2-A (GlnBS49A) grew poorly using nitrate as a nitrogen source and was unable to take up nitrate supplied at 100 microM, even in the absence of externally added ammonium. Mutants MP2-D and MP2-E (GlnBS49D and GlnBS49E, respectively), however, showed nitrate-dependent growth and regulation of nitrate uptake by ammonium, as in the wild-type. Characterization of the mutants also included an analysis of nitrite uptake and of the levels of the nir (nitrate/nitrite assimilation) operon transcripts, the presence of NrtA (nitrate/nitrite transport binding protein), and nitrate and nitrite reductase activities. In vitro, no significant difference was observed in the cooperative binding of ATP and 2-oxoglutarate between the wild-type and the unphosphorylated or phosphorylated-like forms of the mutant PII proteins. The results obtained indicate that both unphosphorylated and phosphorylated-like forms of PII are able to inhibit nitrate uptake in the presence of ammonium, but the unphosphorylated form also has a negative effect in the absence of this nitrogen source. Therefore, an additional effector, possibly 2-oxoglutarate, is required for the PII protein to relieve inhibition of nitrate uptake in the absence of ammonium.

Nitrate transport and not photoinhibition limits growth of the freshwater Cyanobacterium synechococcus species PCC 6301 at low temperature

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20 degreesC and 38 degreesC, the growth rate decreased with decreasing temperature, and growth ceased at 15 degreesC. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38 degreesC remained at 15 degreesC. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m-2 s-1) at 15 degreesC. Little or no loss of oxygen evolution was observed under the normal light intensity (250 microE m-2 s-1) for growth at 15 degreesC. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15 degreesC, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.

Nitrite-specific active transport system of the cyanobacterium Synechococcus sp. strain PCC 7942

Studies on the nitrite uptake capability of a mutant of Synechococcus sp. strain PCC 7942 lacking the ATP-binding cassette-type nitrate-nitrite-bispecific transporter revealed the occurrence of a nitrite-specific active transport system with an apparent Km (NO2-) of about 20 microM. Similar to the nitrate-nitrite-bispecific transporter, the nitrite-specific transporter was reversibly inhibited by ammonium in the medium.

A role for the signal transduction protein PII in the control of nitrate/nitrite uptake in a cyanobacterium

In the cyanobacterium Synechococcus sp. strain PCC 7942, ammonium exerts a rapid and reversible inhibition of the nitrate and nitrite uptake, and the PII protein (GlnB) is differentially phosphorylated depending on the intracellular N/C balance. RNA/DNA hybridizations, as well as nitrate and nitrite uptake experiments, were carried out with the wild-type strain and a PII-null mutant. The transcriptional control by ammonium of the expression of the nir-nrt ABCD-narB operon remained operative in the mutant but, in contrast to the wild-type strain, the mutant took up nitrate and nitrite even in the presence of ammonium. Moreover, the wild-type phenotype was restored by insertion of a copy of the wild-type glnB gene in the genome of the PII-null mutant. These results indicate that the unphosphorylated form of PII is involved in the short-term inhibition by ammonium of the nitrate and nitrite uptake in Synechococcus sp. strain PCC 7942.

Regulation of ntcA expression and nitrite uptake in the marine Synechococcus sp. strain WH 7803

NtcA is a transcriptional activator involved in global nitrogen control in cyanobacteria. In the absence of ammonium it regulates the transcription of a series of genes encoding proteins required for the uptake and assimilation of alternative nitrogen sources (I. Luque, E. Flores, and A. Herrero, EMBO J. 13:2862-2869, 1994). ntcA, present in a single copy in the marine Synechococcus sp. strain WH 7803, was cloned and sequenced. The putative amino acid sequence shows a high degree of identity to NtcA from freshwater cyanobacteria in two functional domains. The expression of ntcA was negatively regulated by ammonium from a putative transcription start point located downstream of an NtcA consensus recognition sequence. Addition of either rifampin or ammonium led to a rapid decline in ntcA transcript levels with half-lives of less than 2 min in both cases. Nitrate-grown cells showed high ntcA transcript levels, as well as the capacity for active nitrite uptake. However, ammonium-grown cells showed low levels of the ntcA transcript and did not utilize nitrite. The addition of ammonium to nitrite uptake-active cells resulted in a gradual decline in the rate of uptake over a 24-h period. Active nitrite uptake was not induced in cells transferred to medium lacking a nitrogen source despite evidence of elevated expression of ntcA, indicating that ntcA expression is not sufficient for uptake capacity to develop. Nitrate and nitrite addition led to the development of nitrite uptake, whereas the addition of leucine did not. Furthermore, nitrite addition triggered the de novo protein synthesis required for uptake capacity to develop. These data suggest that nitrite and nitrate act as specific inducers for the synthesis of proteins required for nitrite uptake.

Nitrate assimilation by bacteria

Nitrate is a significant nitrogen source for plants and microorganisms. Recent molecular genetic analyses of representative bacterial species have revealed structural and regulatory genes responsible for the nitrate-assimilation phenotype. Together with results from physiological and biochemical studies, this information has unveiled fundamental aspects of bacterial nitrate assimilation and provides the foundation for further investigations. Well-studied genera are: the cyanobacteria, including the unicellular Synechococcus and the filamentous Anabaena; the gamma-proteobacteria Klebsiella and Azotobacter; and a Gram-positive bacterium, Bacillus. Nitrate uptake in most of these groups seems to involve a periplasmic binding protein-dependent system that presumably is energized by ATP hydrolysis (ATP-binding cassette transporters). However, Bacillus may, like fungi and plants, utilize electrogenic uptake through a representative of the major facilitator superfamily of transport proteins. Nitrate reductase contains both molybdenum cofactor and an iron-sulfur cluster. Electron donors for the enzymes from cyanobacteria and Azotobacter are ferredoxin and flavodoxin, respectively, whereas the Klebsiella and Bacillus enzymes apparently accept electrons from a specific NAD(P)H-reducing subunit. These subunits share sequence similarity with the reductase components of bacterial aromatic ring-hydroxylating dehydrogenases such as toluene dioxygenase. Nitrite reductase contains sirohaem and an iron-sulfur cluster. The enzymes from cyanobacteria and plants use ferredoxin as the electron donor, whereas the larger enzymes from other bacteria and fungi contain FAD and NAD(P)H binding sites. Nevertheless, the two forms of nitrite reductase share recognizable sequence and structural similarity. Synthesis of nitrate assimilation enzymes and uptake systems is controlled by nitrogen limitation in all bacteria examined, but the relevant regulatory proteins exhibit considerable structural and mechanistic diversity in different bacterial groups. A second level of control, pathway-specific induction by nitrate and nitrite in Klebsiella, involves transcription antitermination. Several issues await further experimentation, including the mechanism and energetics of nitrate uptake, the pathway(s) for nitrite uptake, the nature of electron flow during nitrate reduction, and the action of transcriptional regulatory circuits. Fundamental knowledge of nitrate assimilation physiology should also enhance the study of nitrate metabolism in soil, water and other natural environments, a challenging topic of considerable interest and importance.

Involvement of the C-terminal domain of an ATP-binding subunit in the regulation of the ABC-type nitrate/nitrite transporter of the Cyanobacterium synechococcus sp. strain PCC 7942

In Synechococcus sp. strain PCC 7942, an ATP-binding cassette transporter encoded by the genes nrtA, nrtB, nrtC, and nrtD mediates active transport of nitrate and nitrite, which is inhibited by ammonium, a preferred source of nitrogen for the cyanobacterium. One of the ATP-binding subunits of the transporter, NrtC, has a distinct C-terminal domain of 380 amino acid residues. A mutant NC2, constructed by removal of this domain using genetic engineering techniques, assimilated low concentrations of nitrate and nitrite and accumulated nitrate intracellularly, showing that the domain is not essential for the transporter activities. Assimilation of low concentrations of nitrite was only partially inhibited by ammonium in NC2 but was completely inhibited in the wild-type cells. Cells of NC2 and its derivative (nitrate reductase-less strain NC4) carrying the truncated NrtC but not the cells with the wild-type NrtC accumulated nitrate intracellularly in the presence of ammonium in medium. These findings indicated that the C-terminal domain of NrtC is involved in the ammonium-promoted inhibition of the nitrate/nitrite transporter. In the presence of ammonium, NC2 could not assimilate nitrate despite its ability to accumulate nitrate intracellularly, which suggested that reduction of intracellular nitrate by nitrate reductase is also subject to inhibition by ammonium.

Nitrogen source-dependent expression of a 126 kDa protein in the plasma membrane of the cyanobacterium Synechococcus PCC 7942

The expression of a 126 kDa protein in the cytoplasmic membrane of Synechococcus PCC 7942 is shown to be dependent on the nitrogen source. It is absent in ammonium-grown cells and its quantity is inversely related to the concentration of nitrate or nitrite in the growth medium. Addition of ammonium-grown cells to a medium containing nitrate or L-methionine-DL-sulfoximine results in the expression of this protein. It is present in the plasmalemma of the Synechococcus NC3 mutant (nrtC gene deleted) and absent in the NA3 mutant (nrtABCD genes deleted). These results may suggest involvement of the 126 kDa protein in nitrate transport through Synechococcus cytoplasmic membrane.

Substrate-binding lipoprotein of the cyanobacterium Synechococcus sp. strain PCC 7942 involved in the transport of nitrate and nitrite

Of the four genes (nrtABCD) required for active transport of nitrate in the cyanobacterium Synechococcus sp. strain PCC 7942, nrtBCD encode membrane components of an ATP-binding cassette transporter involved in the transport of nitrite as well as of nitrate, whereas nrtA encodes a 45-kDa cytoplasmic membrane protein, the biochemical function of which remains unclear. Characterization of the nrtA deletional mutants showed that the 45-kDa protein is essential for the functioning of the nitrate/nitrite transporter. A truncated NrtA protein lacking the N-terminal 81 amino acids, expressed in Escherichia coli cells as a histidine-tagged soluble protein, was shown to bind nitrate and nitrite with high affinity (Kd = 0.3 microM). Immunoblotting analysis using the antibody against the 45-kDa protein revealed a 48-kDa precursor of the protein, which accumulated in the cyanobacterial cells treated with globomycin, an antibiotic that specifically inhibits cleavage of the signal peptide of lipoprotein precursors. These findings indicated that the nrtA gene product is a nitrate- and nitrite-binding lipoprotein. The N-terminal sequences of putative cyanobacterial substrate-binding proteins suggested that lipoprotein modification of substrate-binding proteins of ATP-binding cassette transporters is common in cyanobacteria.

Evidence for the nitrate assimilation-dependent nitrite excretion in cyanobacterium Nostoc MAC

Nitrate uptake and nitrite efflux patterns in Nostoc MAC showed a rapid phase followed by their saturation. Nitrite efflux was maximum in nitrate medium whereas the cells incubated in N2 and NH 4 (+) media exhibited a decreased nitrite efflux activity. The simultaneous presence of NH 4 (+) and nitrate significantly decreased nitrite efflux. L-Methionine-DL-sulphoximine (MSX) prevented inhibition of nitrite efflux by NH 4 (+) . In the dark there was negligible nitrite efflux, whereas illumination increased the rate of nitrite efflux significantly. The nitrite efflux system was maximally operative at pH 8.0, 30°C and a photon fluence rate of 50 μmol m(-2). s(-1). These results confirm that (i) the nitrite efflux system in Nostoc MAC is dependent upon nitrate uptake and assimilation and is repressible by NH 4 (+) ; (ii) NH 4 (+) itself is not the actual repressor of nitrite efflux; a product of NH 4 (+) assimilation via glutamine synthetase (GS) is required for repression to occur; (iii) the catalytic function of GS does not appear to be involved in nitrate assimilation-dependent nitrite efflux, and (iv) the optimum pH, temperature and illumination for maximum nitrite efflux were found to be 8.0, 30°C and 50μmol m(-2). s respectively.

Nitrate and nitrate are transported by different specific transport systems and by a bispecific transporter in Chlamydomonas reinhardtii

Nitrate transport mutants from Chlamydomonas reinhardtii and strains derived from them upon transformation with plasmids containing the C. reinhardtii nar2/Nrt2;1 or nar2/Nrt2;2 genes have been used to study nitrate and nitrite transport systems. Mutants lacking nitrate assimilation clustered genes showed a high affinity nitrite transporter activity (system 3), which was subject to ammonium inhibition and appeared to be independent of a functional nar2 gene. Transformants carrying nar2/Nrt2;2 recovered a high affinity nitrate transporter activity (system 2) and showed nitrite transport activities with properties similar to those in nonrecovered high affinity nitrate transporter activity (system 1) together with a considerably enhanced nitrite transport activity. Nitrite transport mediated by system 1 was very sensitive to inhibition by nitrate at microM concentrations. Results strongly suggest that three nitrate assimilation related high affinity transport systems operate in C. reinhardtii: one specific for nitrite, a second one encoded by nar2/Nrt2;2 specific for nitrate, and another one encoded by nar2/Nrt2;1, which is bispecific for these two anions.

Uptake and assimilation of nitrite in the acidophilic red alga Cyanidium caldarium Geitler

The acidophilic red alga, Cyanidium caldarium Geitler, could use nitrite as a nitrogen source for growth, although this compound was very toxic in acidic media. Growth could be sustained when nitrite was added continuously at a rate lower than the maximum rate of nitrite assimilation for the culture. Nitrite assimilation was derepressed in cells growing on nitrate or nitrite, under nitrogen limitation and by nitrogen starvation. Ammonium-grown cells showed a limited capacity for nitrite reduction in the light, even if nitrite reductase was not detectable in cell extracts. In acidic media nitrite was taken up mainly through HNO, influx. Nitrite assimilation, in viva, was restricted by a saturable step under conditions in which nitrite uptake was not limiting. The pH of the cell suspension affected nitrite assimilation by changing the K_½ over the pH range tested but not the V_max . The K_½ was inversely proportional to the concentration of H⁺ in the medium. The apparent K_m value for nitrite of nitrite reductase, in vitro, and its expected apparent K_m value, in vivo, estimated from the K_½ of nitrite assimilation, suggested that the activity of this enzyme may not be the limiting step of nitrite assimilation.