Nitrate reductase activity and heterocyst suppression on nitrate in Anabaena sp. strain PCC 7120 require moeA

Hfq is required for optimal nitrate assimilation in the Cyanobacterium Anabaena sp. strain PCC 7120

Hfq is an RNA binding protein involved in posttranscriptional regulation of gene expression in bacteria. It acts by binding to regulatory small RNAs (sRNAs), which confer specificity for the regulation. Recently, orthologues of the Hfq protein were annotated in cyanobacterial genomes, although its capacity to regulate gene expression by interacting with sRNAs has not been yet demonstrated. Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that, in the absence of combined nitrogen, is able to fix atmospheric nitrogen by differentiating specialized cells called heterocysts. We have generated an hfq knockout mutant of Anabaena sp. PCC 7120. Deletion of this gene results in differentiation of heterocysts in the presence of nitrate, suggesting a defect in nitrate assimilation. We show that hfq mutant cells are affected in transport and use of nitrate and nitrite. An analysis of the expression of several genes in the nir operon, encoding different elements of the nitrate assimilation pathway, demonstrates a downregulation of their transcription in mutant cells. We also observed that genes ntcB and cnaT, involved in the regulation of the nir operon, show a lower expression in cells lacking Hfq. Finally, when hfq was reintroduced in the mutant, heterocyst differentiation was no longer observed in the presence of nitrate. Therefore, our results indicate that the RNA chaperone Hfq is involved in the regulation of the nir operon, although the mechanism for this regulation is still unknown.

The Anabaena sp. strain PCC 7120 asr1734 gene encodes a negative regulator of heterocyst development

The novel asr1734 gene of Anabaena (Nostoc) sp. strain PCC 7120 inhibited heterocyst development when present in extra copies. Overexpression of asr1734 inhibited heterocyst development in several strains including the wild type and two strains that form multiple contiguous heterocysts (Mch phenotype): a PatS null mutant and a hetR(R223W) mutant. Overexpression of asr1734 also caused increased nblA messenger RNA levels, and increased loss of autofluorescence in vegetative cells throughout filaments after nitrogen or sulphur depletion. Unlike the wild type, an asr1734 knockout mutant formed 5% heterocysts after a nitrogen shift from ammonium to nitrate, and formed 15% heterocysts and a weak Mch phenotype after step-down to medium lacking combined nitrogen. After nitrogen step-down, the asr1734 mutant had elevated levels of ntcA messenger RNA. A green fluorescent protein reporter driven by the asr1734 promoter, P(asr1734)-gfp, was expressed specifically in differentiating proheterocysts and heterocysts after nitrogen step-down. Strains overexpressing asr1734 and containing P(hetR)-gfp or P(patS)-gfp reporters failed to show normal patterned upregulation 24 h after nitrogen step-down even though hetR expression was upregulated at 6 h. Apparent orthologues of asr1734 are found only in two other filamentous nitrogen-fixing cyanobacteria, Anabaena variabilis and Nostoc punctiforme.

Regulation of CO on heterocyst differentiation and nitrate uptake in the cyanobacterium Anabaena sp. PCC 7120

AIMS

The aim of the present investigation was to study the effects of different inorganic carbon and nitrogen sources on nitrate uptake and heterocyst differentiation in the culture of cyanobacterium Anabaena sp. PCC 7120.

METHODS AND RESULTS

Anabaena was cultivated in media BG11 containing combined nitrogen and supplementary NaHCO3 or CO2. Cell growth, heterocyst differentiation, nitrate reductase (NR, EC 1.7.7.2), glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) and NO uptake were analysed. The cells cultivated in BG11(0) medium with aeration were taken as reference. Experimental results showed that the differentiation frequency of heterocysts when the cells were cultivated with elevated CO2 was higher than that of the cells grown with air or bicarbonate. Heterocysts appeared unexpectedly when CO2 was introduced into the medium containing nitrate. However, no heterocysts emerged when CO2 was added to medium containing NH or urea, or when NaHCO3 was supplied to the medium with nitrate. Both nitrate uptake rate and nitrate reduction enzyme activity were depressed by the supplement of CO2 to the culture. The activity of G6PDH was enhanced with the increase in heterocyst differentiation frequency.

CONCLUSION

CO2 might compete with NO for energy and electrons in the uptake process and CO2 appears favoured. This led to a high intracellular C/N ratio and a relative N limitation. So the process of heterocyst differentiation was activated to supplement nitrogen uptake.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided an attractive possibility to form more heterocysts by rapid growth of Anabaena cells cultivated in the medium containing nitrate in order to increase nitrogen fixation and hydrogen production.

Molybdate transport and its effect on nitrogen utilization in the cyanobacterium Anabaena variabilis ATCC 29413

Molybdenum is an essential component of the cofactors of many metalloenzymes including nitrate reductase and Mo-nitrogenase. The cyanobacterium Anabaena variabilis ATCC 29413 uses nitrate and atmospheric N2 as sources of nitrogen for growth. Two of the three nitrogenases in this strain are Mo-dependent enzymes, as is nitrate reductase; thus, transport of molybdate is important for growth of this strain. High-affinity transport of molybdate in A. variabilis was mediated by an ABC-type transport system encoded by the products of modA and modBC. The modBC gene comprised a fused orf including components corresponding to modB and modC of Escherichia coli. The deduced ModC part of the fused gene lacked a recognizable molybdate-binding domain. Expression of modA and modBC was induced by starvation for molybdate. Mutants in modA or modBC were unable to grow using nitrate or Mo-nitrogenase. Growth using the alternative V-nitrogenase was not impaired in the mutants. A high concentration of molybdate (10 microM) supported normal growth of the modBC mutant using the Nif1 Mo-nitrogenase, indicating that there was a low-affinity molybdate transport system in this strain. The modBC mutant did not detectably transport low concentrations of 99Mo (molybdate), but did transport high concentrations. However, such transport was observed only after cells were starved for sulphate, suggesting that an inducible sulphate transport system might also serve as a low-affinity molybdate transport system in this strain.

hetL overexpression stimulates heterocyst formation in Anabaena sp. strain PCC 7120

The cyanobacterium Anabaena sp. strain PCC 7120 forms single heterocysts about every 10 to 15 vegetative cells along filaments. PatS is thought to be a peptide intercellular signal made by developing heterocysts that prevents neighboring cells from differentiating. Overexpression of the patS gene suppresses heterocyst formation. The hetL gene (all3740) was isolated in a genetic screen to identify genes involved in PatS signaling. Extracopy hetL allowed heterocyst formation in a patS overexpression strain. hetL overexpression from a heterologous promoter in wild-type Anabaena PCC 7120 induced multiple-contiguous heterocysts (Mch) in nitrate-containing medium. The predicted HetL protein is composed almost entirely of pentapeptide repeats with a consensus of A(D/N)L*X, where * is a polar amino acid. Thirty Anabaena PCC 7120 genes contain this repeat motif. A synthetic pentapeptide corresponding to the last 5 amino acids of PatS, which suppresses heterocyst formation in the wild type, did not suppress heterocyst formation in a hetL overexpression strain, indicating that HetL overexpression is affecting heterocyst regulation downstream of PatS production. The transcription regulator NtcA is required for the initiation of heterocyst formation. hetL overexpression allowed the initiation of heterocyst development in an ntcA-null mutant, but differentiation was incomplete. hetR and hetC mutations that block heterocyst development are epistatic to hetL overexpression. A hetL-null mutant showed normal heterocyst development and diazotrophic growth, which could indicate that it is not normally involved in regulating development, that it normally plays a nonessential accessory role, or perhaps that its loss is compensated by cross talk or redundancy with other pentapeptide repeat proteins.

Transport of molybdate in the cyanobacterium Anabaena variabilis ATCC 29413

Heterocyst-forming filamentous cyanobacteria, such as Anabaena variabilis ATCC 29413, require molybdenum as a component of two essential cofactors for the enzymes nitrate reductase and nitrogenase. A. variabilis efficiently transported (99)Mo (molybdate) at concentrations less than 10(-9) M. Competition experiments with other oxyanions suggested that the molybdate-transport system of A. variabilis also transported tungstate but not vanadate or sulfate. Although tungstate was probably transported, tungsten did not function in place of molybdenum in the Mo-nitrogenase. Transport of (99)Mo required prior starvation of the cells for molybdate, suggesting that the Mo-transport system was repressed by molybdate. Starvation, which required several generations of growth for depletion of molybdate, was enhanced by growth under conditions that required synthesis of nitrate reductase or nitrogenase. These data provide evidence for a molybdate storage system in A. variabilis. NtcA, a regulatory protein that is essential for synthesis of nitrate reductase and nitrogenase, was not required for transport of molybdate. The closely related strain Anabaena sp. PCC 7120 transported (99)Mo in a very similar way to A. variabilis.

Functional analysis of the Escherichia coli molybdopterin cofactor biosynthesis protein MoeA by site-directed mutagenesis

Five moeA mutants were generated by replacing some conserved amino acids of MoeA by site-directed mutagenesis. The mutants were assayed for the ability to restore in vivo nitrate reductase activity of the moeA mutant Escherichia coli JRG97 and in vitro Neurospora crassa nit-1 nitrate reductase activity. The replacements Asp59AlaGly60Ala, Asp259Ala, Pro298AlaPro301Ala abolished the function of MoeA in Mo-molybdopterin formation and stabilization, reflected in the inability to restore nitrate reductase activity. The replacements Gly251AlaGly252Ala reduced, and that of Pro283Ala had no effect, on nitrate reductase activity. E. coli JRG97 cells transformed with mutants that failed to restore nitrate reductase activity showed by HPLC analysis a decreased level of molybdopterin-derived dephospho FormA as compared to bacteria transformed with wild-type moeA. The effects of the amino acid replacements on MoeA function may be explained in correlation with the MoeA crystal structure.

The crystal structure of Escherichia coli MoeA, a protein from the molybdopterin synthesis pathway

MoeA is involved in synthesis of the molybdopterin cofactor, although its function is not yet clearly defined. The three-dimensional structure of the Escherichia coli protein was solved at 2.2 A resolution. The locations of highly conserved residues among the prokaryotic and eukaryotic MoeA homologs identifies a cleft in the dimer interface as the likely functional site. Of the four domains of MoeA, domain 2 displays a novel fold and domains 1 and 4 each have only one known structural homolog. Domain 3, in contrast, is structurally similar to many other proteins. The protein that resembles domain 3 most closely is MogA, another protein required for molybdopterin cofactor synthesis. The overall similarity between MoeA and MogA, and the similarities in a constellation of residues that are strongly conserved in MoeA, suggests that these proteins bind similar ligands or substrates and may have similar functions.

Heterocyst formation in Anabaena

Heterocystous cyanobacteria grow as multicellular organisms with a distinct one-dimensional developmental pattern of single nitrogen-fixing heterocysts separated by approximately ten vegetative cells. Several genes have been identified that are required for heterocyst development and pattern formation. A key regulator, HetR, has been recently shown to be aserine-type protease.

Heterocyst pattern formation controlled by a diffusible peptide

Many filamentous cyanobacteria grow as multicellular organisms that show a developmental pattern of single nitrogen-fixing heterocysts separated by approximately 10 vegetative cells. Overexpression of a 54-base-pair gene, patS, blocked heterocyst differentiation in Anabaena sp. strain PCC 7120. A patS null mutant showed an increased frequency of heterocysts and an abnormal pattern. Expression of a patS-gfp reporter was localized in developing proheterocysts. The addition of a synthetic peptide corresponding to the last five amino acids of PatS inhibited heterocyst development. PatS appears to control heterocyst pattern formation through intercellular signaling mechanisms.

The narA locus of Synechococcus sp. strain PCC 7942 consists of a cluster of molybdopterin biosynthesis genes

The narA locus required for nitrate reduction in Synechococcus sp. strain PCC 7942 is shown to consist of a cluster of genes, namely, moeA, moaC, moaD, moaE, and moaA, involved in molybdenum cofactor biosynthesis. The product of the moaC gene of strain PCC 7942 shows homology in its N-terminal half to MoaC from Escherichia coli and in its C-terminal half to MoaB or Mog. Overexpression of the Synechococcus moaC gene in E. coli resulted in the synthesis of a polypeptide of 36 kDa, a size that would conform to a protein resembling a fusion of the MoaC and MoaB or Mog polypeptides of E. coli. Insertional inactivation of the moeA, moaC, moaE, and moaA genes showed that the moeA-moa gene cluster is required for growth on nitrate and expression of nitrate reductase activity in strain PCC 7942. The moaCDEA genes constitute an operon which is transcribed divergently from the moeA gene. Expression of the moeA gene and the moa operon was little affected by the nitrogen source present in the culture medium.