High recovery of nitrogenase activity and of Fe-labeled nitrogenase in heterocysts isolated from Anabaena variabilis

Single-Cell Measurements of Fixation and Intercellular Exchange of C and N in the Filaments of the Heterocyst-Forming Cyanobacterium Anabaena sp. Strain PCC 7120

Under diazotrophic conditions, the model filamentous, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 develops a metabolic strategy based on the physical separation of the processes of oxygenic photosynthesis, in vegetative cells, and N₂ fixation, in heterocysts. This strategy requires the exchange of carbon and nitrogen metabolites and their distribution along the filaments, which takes place through molecular diffusion via septal junctions involving FraCD proteins. Here, Anabaena was incubated in a time course (up to 20 h) with [¹³C]bicarbonate and ¹⁵N₂ and analyzed by secondary ion mass spectrometry imaging (SIMS) (large-geometry SIMS [LG-SIMS] and NanoSIMS) to quantify C and N assimilation and distribution in the filaments. The ¹³C/¹²C and ¹⁵N/¹⁴N ratios measured in wild-type filaments showed a general increase with time. The enrichment was relatively homogeneous in vegetative cells along individual filaments, while it was reduced in heterocysts. Heterocysts, however, accumulated recently fixed N at their poles, in which the cyanophycin plug [multi-l-arginyl-poly(l-aspartic acid)] is located. In contrast to the rather homogeneous label found along stretches of vegetative cells, ¹³C/¹²C and ¹⁵N/¹⁴N ratios were significantly different between filaments both at the same and different time points, showing high variability in metabolic states. A fraC fraD mutant did not fix N₂, and the ¹³C/¹²C ratio was homogeneous along the filament, including the heterocyst in contrast to the wild type. Our results show the consumption of reduced C in the heterocysts associated with the fixation and export of fixed N and present an unpredicted heterogeneity of cellular metabolic activity in different filaments of an Anabaena culture under controlled conditions.

Comparative genomic insights into culturable symbiotic cyanobacteria from the water fern Azolla

Species of the floating, freshwater fern Azolla form a well-characterized symbiotic association with the non-culturable cyanobacterium Nostoc azollae, which fixes nitrogen for the plant. However, several cyanobacterial strains have over the years been isolated and cultured from Azolla from all over the world. The genomes of 10 of these strains were sequenced and compared with each other, with other symbiotic cyanobacterial strains, and with similar strains that were not isolated from a symbiotic association. The 10 strains fell into three distinct groups: six strains were nearly identical to the non-symbiotic strain, Nostoc (Anabaena) variabilis ATCC 29413; three were similar to the symbiotic strain, Nostoc punctiforme, and one, Nostoc sp. 2RC, was most similar to non-symbiotic strains of Nostoc linckia. However, Nostoc sp. 2RC was unusual because it has three sets of nitrogenase genes; it has complete gene clusters for two distinct Mo-nitrogenases and an alternative V-nitrogenase. Genes for Mo-nitrogenase, sugar transport, chemotaxis and pili characterized all the symbiotic strains. Several of the strains infected the liverwort Blasia, including N. variabilis ATCC 29413, which did not originate from Azolla but rather from a sewage pond. However, only Nostoc sp. 2RC, which produced highly motile hormogonia, was capable of high-frequency infection of Blasia. Thus, some of these strains, which grow readily in the laboratory, may be useful in establishing novel symbiotic associations with other plants.

Vegetative cells may perform nitrogen fixation function under nitrogen deprivation in Anabaena sp. strain PCC 7120 based on genome-wide differential expression analysis

Nitrogen assimilation is strictly regulated in cyanobacteria. In an inorganic nitrogen-deficient environment, some vegetative cells of the cyanobacterium Anabaena differentiate into heterocysts. We assessed the photosynthesis and nitrogen-fixing capacities of heterocysts and vegetative cells, respectively, at the transcriptome level. RNA extracted from nitrogen-replete vegetative cells (NVs), nitrogen-deprived vegetative cells (NDVs), and nitrogen-deprived heterocysts (NDHs) in Anabaena sp. strain PCC 7120 was evaluated by transcriptome sequencing. Paired comparisons of NVs vs. NDHs, NVs vs. NDVs, and NDVs vs. NDHs revealed 2,044 differentially expressed genes (DEGs). Kyoto Encyclopedia of Genes and Genomes enrichment analysis of the DEGs showed that carbon fixation in photosynthetic organisms and several nitrogen metabolism-related pathways were significantly enriched. Synthesis of Gvp (Gas vesicle synthesis protein gene) in NVs was blocked by nitrogen deprivation, which may cause Anabaena cells to sink and promote nitrogen fixation under anaerobic conditions; in contrast, heterocysts may perform photosynthesis under nitrogen deprivation conditions, whereas the nitrogen fixation capability of vegetative cells was promoted by nitrogen deprivation. Immunofluorescence analysis of nitrogenase iron protein suggested that the nitrogen fixation capability of vegetative cells was promoted by nitrogen deprivation. Our findings provide insight into the molecular mechanisms underlying nitrogen fixation and photosynthesis in vegetative cells and heterocysts at the transcriptome level. This study provides a foundation for further functional verification of heterocyst growth, differentiation, and water bloom control.

FtsZ of Filamentous, Heterocyst-Forming Cyanobacteria Has a Conserved N-Terminal Peptide Required for Normal FtsZ Polymerization and Cell Division

Filamentous cyanobacteria grow by intercalary cell division, which should involve distinct steps compared to those producing separate daughter cells. The N-terminal region of FtsZ is highly conserved in the clade of filamentous cyanobacteria capable of cell differentiation. A derivative of the model strain Anabaena sp. PCC 7120 expressing only an FtsZ lacking the amino acids 2–51 of the N-terminal peptide (ΔN-FtsZ) could not be segregated. Strain CSL110 expresses both ΔN-FtsZ, from the endogenous ftsZ gene promoter, and the native FtsZ from a synthetic regulated promoter. Under conditions of ΔN-FtsZ predominance, cells of strain CSL110 progressively enlarge, reflecting reduced cell division, and show instances of asymmetric cell division and aberrant Z-structures notably differing from the Z-ring formed by FtsZ in the wild type. In bacterial 2-hybrid assays FtsZ interacted with ΔN-FtsZ. However, ΔN-FtsZ-GFP appeared impaired for incorporation into Z-rings when expressed together with FtsZ. FtsZ, but not ΔN-FtsZ, interacted with the essential protein SepF. Both FtsZ and ΔN-FtsZ polymerize in vitro exhibiting comparable GTPase activities. However, filaments of FtsZ show a distinct curling forming toroids, whereas ΔN-FtsZ form thick bundles of straight filaments. Thus, the N-terminal FtsZ sequence appears to contribute to a distinct FtsZ polymerization mode that is essential for cell division and division plane location in Anabaena.

Increased heterocyst frequency by patN disruption in Anabaena leads to enhanced photobiological hydrogen production at high light intensity and high cell density

The effects of increasing the heterocyst-to-vegetative cell ratio on the nitrogenase-based photobiological hydrogen production by the filamentous heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 were studied. Using the uptake hydrogenase-disrupted mutant (ΔHup) as the parent, a deletion-insertion mutant (PN1) was created in patN, known to be involved in heterocyst pattern formation and leading to multiple singular heterocysts (MSH) in Nostoc punctiforme strain ATCC 29133. The PN1 strain showed heterocyst differentiation but failed to grow in medium free of combined-nitrogen; however, a spontaneous mutant (PN22) was obtained on prolonged incubation of PN1 liquid cultures and was able to grow robustly on N₂. The disruption of patN was confirmed in both PN1 and PN22 by PCR and whole genome resequencing. Under combined-nitrogen limitation, the percentage of heterocysts to total cells in the PN22 filaments was 13-15 and 16-18% under air and 1% CO₂-enriched air, respectively, in contrast to the parent ΔHup which formed 6.5-11 and 9.7-13% heterocysts in these conditions. The PN22 strain exhibited a MSH phenotype, normal diazotrophic growth, and higher H₂ productivity at high cell concentrations, and was less susceptible to photoinhibition by strong light than the parent ΔHup strain, resulting in greater light energy utilization efficiency in H₂ production on a per unit area basis under high light conditions. The increase in MSH frequency shown here appears to be a viable strategy for enhancing H₂ productivity by outdoor cultures of cyanobacteria in high-light environments.

Compartmentalized cyanophycin metabolism in the diazotrophic filaments of a heterocyst-forming cyanobacterium

Heterocyst-forming cyanobacteria are multicellular organisms in which growth requires the activity of two metabolically interdependent cell types, the vegetative cells that perform oxygenic photosynthesis and the dinitrogen-fixing heterocysts. Vegetative cells provide the heterocysts with reduced carbon, and heterocysts provide the vegetative cells with fixed nitrogen. Heterocysts conspicuously accumulate polar granules made of cyanophycin [multi-L-arginyl-poly (L-aspartic acid)], which is synthesized by cyanophycin synthetase and degraded by the concerted action of cyanophycinase (that releases β-aspartyl-arginine) and isoaspartyl dipeptidase (that produces aspartate and arginine). Cyanophycin synthetase and cyanophycinase are present at high levels in the heterocysts. Here we created a deletion mutant of gene all3922 encoding isoaspartyl dipeptidase in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. The mutant accumulated cyanophycin and β-aspartyl-arginine, and was impaired specifically in diazotrophic growth. Analysis of an Anabaena strain bearing an All3922-GFP (green fluorescent protein) fusion and determination of the enzyme activity in specific cell types showed that isoaspartyl dipeptidase is present at significantly lower levels in heterocysts than in vegetative cells. Consistently, isolated heterocysts released substantial amounts of β-aspartyl-arginine. These observations imply that β-aspartyl-arginine produced from cyanophycin in the heterocysts is transferred intercellularly to be hydrolyzed, producing aspartate and arginine in the vegetative cells. Our results showing compartmentalized metabolism of cyanophycin identify the nitrogen-rich molecule β-aspartyl-arginine as a nitrogen vehicle in the unique multicellular system represented by the heterocyst-forming cyanobacteria.

Complete genome sequence of Anabaena variabilis ATCC 29413

Anabaena variabilis ATCC 29413 is a filamentous, heterocyst-forming cyanobacterium that has served as a model organism, with an extensive literature extending over 40 years. The strain has three distinct nitrogenases that function under different environmental conditions and is capable of photoautotrophic growth in the light and true heterotrophic growth in the dark using fructose as both carbon and energy source. While this strain was first isolated in 1964 in Mississippi and named Anabaena flos-aquae MSU A-37, it clusters phylogenetically with cyanobacteria of the genus Nostoc. The strain is a moderate thermophile, growing well at approximately 40(°) C. Here we provide some additional characteristics of the strain, and an analysis of the complete genome sequence.

Cell-specific gene expression in Anabaena variabilis grown phototrophically, mixotrophically, and heterotrophically

BACKGROUND

When the filamentous cyanobacterium Anabaena variabilis grows aerobically without combined nitrogen, some vegetative cells differentiate into N2-fixing heterocysts, while the other vegetative cells perform photosynthesis. Microarrays of sequences within protein-encoding genes were probed with RNA purified from extracts of vegetative cells, from isolated heterocysts, and from whole filaments to investigate transcript levels, and carbon and energy metabolism, in vegetative cells and heterocysts in phototrophic, mixotrophic, and heterotrophic cultures.

RESULTS

Heterocysts represent only 5% to 10% of cells in the filaments. Accordingly, levels of specific transcripts in vegetative cells were with few exceptions very close to those in whole filaments and, also with few exceptions (e.g., nif1 transcripts), levels of specific transcripts in heterocysts had little effect on the overall level of those transcripts in filaments. In phototrophic, mixotrophic, and heterotrophic growth conditions, respectively, 845, 649, and 846 genes showed more than 2-fold difference (p < 0.01) in transcript levels between vegetative cells and heterocysts. Principal component analysis showed that the culture conditions tested affected transcript patterns strongly in vegetative cells but much less in heterocysts. Transcript levels of the genes involved in phycobilisome assembly, photosynthesis, and CO2 assimilation were high in vegetative cells in phototrophic conditions, and decreased when fructose was provided. Our results suggest that Gln, Glu, Ser, Gly, Cys, Thr, and Pro can be actively produced in heterocysts. Whether other protein amino acids are synthesized in heterocysts is unclear. Two possible components of a sucrose transporter were identified that were upregulated in heterocysts in two growth conditions. We consider it likely that genes with unknown function represent a larger fraction of total transcripts in heterocysts than in vegetative cells across growth conditions.

CONCLUSIONS

This study provides the first comparison of transcript levels in heterocysts and vegetative cells from heterocyst-bearing filaments of Anabaena. Although the data presented do not give a complete picture of metabolism in either type of cell, they provide a metabolic scaffold on which to build future analyses of cell-specific processes and of the interactions of the two types of cells.

Expression of Shewanella oneidensis MR-1 [FeFe]-hydrogenase genes in Anabaena sp. strain PCC 7120

H(2) generated from renewable resources holds promise as an environmentally innocuous fuel that releases only energy and water when consumed. In biotechnology, photoautotrophic oxygenic diazotrophs could produce H(2) from water and sunlight using the cells' endogenous nitrogenases. However, nitrogenases have low turnover numbers and require large amounts of ATP. [FeFe]-hydrogenases found in other organisms can have 1,000-fold higher turnover numbers and no specific requirement for ATP but are very O(2) sensitive. Certain filamentous cyanobacteria protect nitrogenase from O(2) by sequestering the enzyme within internally micro-oxic, differentiated cells called heterocysts. We heterologously expressed the [FeFe]-hydrogenase operon from Shewanella oneidensis MR-1 in Anabaena sp. strain PCC 7120 using the heterocyst-specific promoter P(hetN). Active [FeFe]-hydrogenase was detected in and could be purified from aerobically grown Anabaena sp. strain PCC 7120, but only when the organism was grown under nitrate-depleted conditions that elicited heterocyst formation. These results suggest that the heterocysts protected the [FeFe]-hydrogenase against inactivation by O(2).

Localized Tetrazolium Reduction in Relation to N(2) Fixation, CO(2) Fixation, and H(2) Uptake in Aquatic Filamentous Cyanobacteria

The aquatic filamentous cyanobacteria Anabaena oscillarioides and Trichodesmium sp. reveal specific cellular regions of tetrazolium salt reduction. The effects of localized reduction of five tetrazolium salts on N(2) fixation (acetylene reduction), CO(2) fixation, and H(2) utilization were examined. During short-term (within 30 min) exposures in A. oscillarioides, salt reduction in heterocysts occurred simultaneously with inhibition of acetylene reduction. Conversely, when salts failed to either penetrate or be reduced in heterocysts, no inhibition of acetylene reduction occurred. When salts were rapidly reduced in vegetative cells, CO(2) fixation and H(2) utilization rates decreased, whereas salts exclusively reduced in heterocysts were not linked to blockage of these processes. In the nonheterocystous genus Trichodesmium, the deposition of reduced 2,3,5-triphenyl-2-tetrazolium chloride (TTC) in the internal cores of trichomes occurs simultaneously with a lowering of acetylene reduction rates. Since TTC deposition in heterocysts of A. oscillarioides occurs contemporaneously with inhibition of acetylene reduction, we conclude that the cellular reduction of this salt is of use in locating potential N(2)-fixing sites in cyanobacteria. The possible applications and problems associated with interpreting localized reduction of tetrazolium salts in cyanobacteria are presented.

NblA is essential for phycobilisome degradation in Anabaena sp. strain PCC 7120 but not for development of functional heterocysts

Phycobilisomes (PBS) are the major light-harvesting complexes of cyanobacteria. These usually blue-coloured multiprotein assemblies are rapidly degraded when the organisms are starved for combined nitrogen. This proteolytic process causes a colour change of the cyanobacterial cells from blue-green to yellow-green ('bleaching'). As is well documented for the unicellular, non-diazotrophic cyanobacteria Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803, a gene termed nblA plays a key role in PBS degradation. Filamentous, diazotrophic cyanobacteria like Anabaena adapt to nitrogen deprivation by differentiation of N(2)-fixing heterocysts. However, during the first hours after nitrogen deprivation all cells degrade their PBS. When heterocysts mature and nitrogenase becomes active, vegetative cells resynthesize their light-harvesting complexes while in heterocysts the phycobiliprotein content remains very low. Expression and function of nblA in Anabaena sp. PCC 7120 was investigated. This strain has two nblA homologous genes, one on the chromosome (nblA) and one on plasmid delta (nblA-p). Northern blot analysis indicated that only the chromosomal nblA gene is up-regulated upon nitrogen starvation. Mutants with interrupted nblA and nblA-p genes, respectively, grew on N(2) and developed functional heterocysts. Mutant DeltanblA-p behaved like the wild-type. However, mutant DeltanblA was unable to degrade its PBS, which was most obvious in non-bleaching heterocysts. The results show that NblA, encoded by the chromosomal nblA gene, is required for PBS degradation in Anabaena but is not essential for heterocyst differentiation.

Cost-effective and uniform (13)C- and (15)N-labeling of the 24-kDa N-terminal domain of the Escherichia coli gyrase B by overexpression in the photoautotrophic cyanobacterium Anabaena sp. PCC 7120

Structural studies of biomolecules using nuclear magnetic resonance (NMR) rely on the availability of samples enriched in (13)C and (15)N isotopes. While (13)C/(15)N-labeled proteins are generally obtained by overexpression in transformed Escherichia coli cells cultured in the presence of an expensive mixture of labeled precursors, those of the photoautotrophic cyanobacterium Anabaena sp. PCC 7120 can be uniformly labeled by growing them in medium containing Na(15)NO(3) and NaH(13)CO(3) as the sole nitrogen and carbon sources. We report here a novel vector-host system suitable for the efficient preparation of uniformly (13)C/(15)N-labeled proteins in Anabaena sp. PCC 7120. The 24-kDa N-terminal domain of the E. coli gyrase B subunit, used as a test protein, was cloned into the pRL25C shuttle vector under the control of the tac promoter. The transformed Anabaena cells were grown in the presence of the labeled mineral salts and culture conditions were optimized to obtain over 90% of (13)C and (15)N enrichment in the constitutively expressed 24-kDa polypeptide. The yield of purified 24-kDa protein after dual isotope labeling under anaerobic conditions was similar to that obtained with E. coli cells bearing a comparable expression vector and cultured in parallel in a commercially available labeling medium. Furthermore, as probed by NMR spectroscopy and mass spectrometry, the 24-kDa N-terminal domain expressed in Anabaena was identical to the E. coli sample, demonstrating that it was of sufficient quality for 3D-structure determination. Because the Anabaena system was far more advantageous taking into consideration the expense for the labels that were necessary, these results indicate that Anabaena sp. PCC 7120 is an economic alternative for the (13)C/(15)N-labeling of soluble recombinant proteins destined for structural studies.

The permeability of heterocysts to the gases nitrogen and oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retina gas vacuoles for several days after differentiation. It is demonstrated that the rate of gas diffusion into a heterocyst that is near an overlying gas phase can be determined approximately from observations on the rate of gas pressure rise required to collapse 50% of its gas vacuoles. The mean permeability coefficient ( α ) of heterocysts O 2 and N 2 was found to be 0.3 s -1 . From this it was calculated that the average permeability ( k ) of the heterocyst surface layer is about 0.4 μm s -1 (within a factor of 2). This is probably within the range that could be provided by a few layers of the 26-C glycolipids in the heterocyst envelope. It is likely, but not proven, that the main route for gas diffusion is through the envelope rather than through the terminal pores of the heterocyst. From measurements of cell nitrogen content (2.7 pg). doubling time (3 days) and heterocyst: vegetative cell ratio (1:24) it was calculated that the average heterocyst fixed 5.9 x 10 -18 mol N 2 s -1 ; this must equal the diffusion rate of N 2 inside the average heterocyst that was 22% below the outside air-saturated concentration. the maximum N 2 fixation rate allowed by the estimated permeability coefficeint would be 2.7 x 10 -17 mol s -1 per heterocyst, slightly greater than the maximum calcualted N 2 fixation rate. The observed permeability coefficient is low enough for the oxygen concentration in the heterocyst to be maintained close to zero by the probable rate of respiration, providing an anaerobic environment for nitrogenase. The rate of O 2 diffusion will limit the N 2 -fixation rate in the dark by limiting the rate at which ATP is supplied by oxidative phosphorylation.

Heterocyst formation

Heterocysts are microaerobic, N2-fixing cells that form in a patterned array within O2-producing filamentous cyanobacteria. Structural features of heterocysts can be predicted from consideration of their physiology. This review focuses on the spacing mechanism that determines which cells will differentiate, and on the regulation of the progression of the differentiation process. Applicable genetic tools, developed primarily using Anabaena PCC 7120, but employed also with Nostoc spp., are reviewed. These tools include localization of transcription using fusions to lux, lac, and gfp, and mutagenesis with oriV-containing derivatives of transposon Tn5. Mature and developing heterocysts inhibit nearby vegetative cells from differentiating; genes patA, devA, hetC, and the hetMNI locus may hold keys to understanding intercellular interactions that influence heterocyst formation. Regulatory and other genes that are transcriptionally activated at different times after nitrogen stepdown have been identified, and should permit analysis of mechanisms that underlie the progression of heterocyst differentiation.

Isolation and characterization of the VnfEN genes of the cyanobacterium Anabaena variabilis

The filamentous cyanobacterium Anabaena variabilis fixes nitrogen in the presence of vanadium (V) and in the absence of molybdenum (Mo), using a V-dependent nitrogenase (V-nitrogenase) encoded by the vnfDGK genes. Downstream from these genes are two genes that are similar to the vnfEN genes of Azotobacter vinelandii. Like the vnfDGK genes, the vnfEN genes were transcribed in the absence of Mo, whether or not V was present. A mutant with an insertion in the vnfN gene lacked V-nitrogenase activity; thus, the vnfEN genes were essential for the V-nitrogenase system in A. variabilis. Growth and acetylene reduction assays with wild-type and mutant strains suggested that the V-nitrogenase reduced dinitrogen better than acetylene. The similarity of the vnfEN genes of A. variabilis and A. vinelandii was not strong. The vnfEN genes of A. variabilis showed greater similarity to the vnfDK genes just upstream than to the A. vinelandii vnfEN genes. Sequence comparisons provide support for the idea that if the vnf genes were transferred laterally among bacterial strains, the vnf cluster was not transferred intact. It appears likely that the structural genes were transferred before a duplication event led to the evolution of the vnfEN genes independently in the two strains. The divergence of the vnfEN genes from the vnfDK genes suggests that this duplication, and hence the transfer of vnf genes, was an ancient event.

Characterization of nifB, nifS, and nifU genes in the cyanobacterium Anabaena variabilis: NifB is required for the vanadium-dependent nitrogenase

Anabaena variabilis ATCC 29413 is a heterotrophic, nitrogen-fixing cyanobacterium containing both a Mo-dependent nitrogenase encoded by the nif genes and V-dependent nitrogenase encoded by the vnf genes. The nifB, nifS, and nifU genes of A. variabilis were cloned, mapped, and partially sequenced. The fdxN gene was between nifB and nifS. Growth and acetylene reduction assays using wild-type and mutant strains indicated that the nifB product (NifB) was required for nitrogen fixation not only by the enzyme encoded by the nif genes but also by the enzyme encoded by the vnf genes. Neither NifS nor NifU was essential for nitrogen fixation in A. variabilis.

Requirement of the regulatory protein NtcA for the expression of nitrogen assimilation and heterocyst development genes in the cyanobacterium Anabaena sp. PCC 7120

The cyanobacterial ntcA gene encodes a DNA-binding protein that belongs to the Crp family of bacterial transcriptional regulators. In this work, we describe the isolation of an ntcA insertional mutant of the dinitrogen-fixing, heterocyst-forming cyanobacterium Anabaena sp. PCC 7120. The Anabaena ntcA mutant was able to use ammonium as a source of nitrogen for growth, but was unable to assimilate atmospheric nitrogen (dinitrogen) or nitrate. Nitrogenase and enzymes of the nitrate reduction system were not synthesized in the ntcA mutant under derepressing conditions, and glutamine synthetase levels were lower in the mutant than in the wild-type strain. In the ntcA mutant, in response to removal of ammonium, accumulation of mRNA of the genes encoding nitrogenase (nifHDK), nitrite reductase (nir, the first gene of the nitrate assimilation operon), and glutamine synthetase (glnA) was not observed. A transcription start point of the Anabaena glnA gene (corresponding to RNAl), that has been shown to be used preferentially after nitrogen step-down, was not used in the ntcA insertional mutant. Heterocyst development (which is necessary for the aerobic fixation of dinitrogen) and induction of hetR (a regulatory gene that is required for heterocyst development) were also impaired in the ntcA mutant. These results showed that the ntcA gene product, NtcA, is required in Anabaena sp. PCC 7120 for the expression of genes encoding proteins involved in the assimilation of nitrogen sources alternative to ammonium including dinitrogen and nitrate, and that the process of heterocyst development is also controlled by NtcA.

Posttranslational modification of nitrogenase. Differences between the purple bacterium Rhodospirillum rubrum and the cyanobacterium Anabaena variabilis

In the photosynthetic bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulatus post-translational regulation of nitrogenase is due to ADP-ribosylation of the Fe-protein, the dinitrogenase reductase [Burris, R. H. (1991) J. Biol. Chem. 266, 9339-9342]. This mechanism has been assumed to be responsible for nitrogenase modification in a variety of organisms. In the present study, we examined whether ADP-ribosylation holds true for the filamentous cyanobacterium Anabaena variabilis. Genes coding for the nitrogenase-modifying enzymes dinitrogenase reductase-activating glycohydrolase (DRAG) and dinitrogenase reductase ADP-ribosyl transferase (DRAT) from R. rubrum have been subcloned and overexpressed in Escherichia coli. After isolation under anaerobic conditions, both proteins were functional as determined by in-vitro assays using nitrogenase from R. rubrum as substrate. In contrast to the R. rubrum enzyme, nitrogenase from A. variabilis was not affected by DRAG or DRAT. Neither could inactive nitrogenase be restored by DRAG, nor nitrogenase activity suppressed by DRAT. Using specific antibodies against arginine-bound ADP-ribose [Meyer, T. & Hilz, H. (1986) Eur. J. Biochem. 155, 157-165], immunoblotting of the inactive, modified form of the Fe-protein from R. rubrum but not that from A. variabilis showed a strong cross reaction. Furthermore, differently to R. rubrum no ADP-ribosylated proteins could be detected at all, indicating the absence of this posttranslational modification in A. variabilis.

Characterization of genes for an alternative nitrogenase in the cyanobacterium Anabaena variabilis

Anabaena variabilis ATCC 29413 is a heterotrophic, nitrogen-fixing cyanobacterium that has been reported to fix nitrogen and reduce acetylene to ethane in the absence of molybdenum. DNA from this strain hybridized well at low stringency to the nitrogenase 2 (vnfDGK) genes of Azotobacter vinelandii. The hybridizing region was cloned from a lambda EMBL3 genomic library of A. variabilis, mapped, and sequenced. The deduced amino acid sequences of the vnfD and vnfK genes of A. variabilis showed only about 56% similarity to the nifDK genes of Anabaena sp. strain PCC 7120 but were 76 to 86% similar to the anfDK or vnfDK genes of A. vinelandii. The organization of the vnf gene cluster in A. variabilis was similar to that of A. vinelandii. However, in A. variabilis, the vnfG gene was fused to vnfD; hence, this gene is designated vnfDG. A vnfH gene was not contiguous with the vnfDG gene and has not yet been identified. A mutant strain, in which a neomycin resistance cassette was inserted into the vnf cluster, grew well in a medium lacking a source of fixed nitrogen in the presence of molybdenum but grew poorly when vanadium replaced molybdenum. In contrast, the parent strain grew equally well in media containing either molybdenum or vanadium. The vnf genes were transcribed in the absence of molybdenum, with or without vanadium. The vnf gene cluster did not hybridize to chromosomal DNA from Anabaena sp. strain PCC 7120 or from the heterotrophic strains, Nostoc sp. strain Mac and Nostoc sp. strain ATCC 29150. A hybridizing ClaI fragment very similar in size to the A. variabilis ClaI fragment was present in DNA isolated from several independent, cultured isolates of Anabaena sp. from the Azolla symbiosis.

In vitro activation of dinitrogenase reductase from the cyanobacterium Anabaena variabilis (ATCC 29413)

Nitrogenase of the heterocystous cyanobacterium Anabaena variabilis was inactivated in vivo (S. Reich, H. Almon, and P. Böger, FEMS Microbiol. Lett. 34:53-56, 1986). Partially purified and modified (inactivated) dinitrogenase reductase (Fe-protein) of such cells was reactivated by isolated membrane fractions of A. variabilis or of Rhodospirillum rubrum, and acetylene reduction was measured. Reactivation requires ATP, Mg2+, and Mn2+. The activating principle is localized in the heterocyst and was found effective only when prepared from cells exhibiting active nitrogenase. It also restores the activity of modified Fe-protein from R. rubrum.

Identification and characterization of hetA, a gene that acts early in the process of morphological differentiation of heterocysts

Envelope polysaccharide is a major early diagnostic of differentiating heterocysts. The mutation in mutant EF116 of Anabaena sp. strain PCC 7120 reduces the cohesiveness of this polysaccharide. A 3.5-kilobase fragment of DNA cloned from the wild type of this Anabaena sp. was previously shown to complement this mutation. We present the nucleotide sequence of a 2,555-base-pair portion of this fragment containing an open reading frame (ORF) of 601 amino acids. Complementation analysis using deletion derivatives of the 3.5-kilobase fragment showed that the gene mutated in EF116, which we designate hetA, lies within this ORF. Transcription of hetA was induced as a result of deprivation for nitrate and yielded a monocistronic mRNA that was present at greatest abundance 7 h after nitrogen stepdown. At that time, proheterocysts could not be distinguished by light microscopy; transcription of nifHD, structural genes of nitrogenase, peaked much later. Situated 3' to hetA are 4 identical repeats of the sequence 5'-TTCAAAA-3' and 12 repeats (10 identical) of the sequence 5'-CCCCAAT-3'. The 12 repeats, present within and near the 5' end of a second ORF, are almost identical to repeats that have been reported to be present in the region between the petC and petA genes of a related cyanobacterium.

Excision of an 11-kilobase-pair DNA element from within the nifD gene in anabaena variabilis heterocysts

The 3' region of the Anabaena variabilis nifD gene contains an 11-kilobase-pair element which is excised from the chromosome during heterocyst differentiation. We have sequenced the recombination sites which border the element in vegetative cells and the rearranged heterocyst sequences. In vegetative cells, the element was flanked by 11-base-pair direct repeats which were identical to the repeats present at the ends of the nifD element in Anabaena sp. strain PCC 7120 (Anabaena strain 7120). Although Anabaena strain 7120 and A. variabilis are quite distinct in many ways, the overall sequence similarity between the two strains for the regions sequenced was 96%. Like the Anabaena strain 7120 element, the A. variabilis element was excised in heterocysts to produce a functional nifD gene and a free circularized element which was neither amplified nor degraded. The Anabaena strain 7120 xisA gene is located at the nifK-proximal end of the nifD element and is required for excision of the element in heterocysts. The A. variabilis element also contained an xisA gene which could complement a defective Anabaena strain 7120 xisA gene. A. variabilis did not contain the equivalent of the Anabaena strain 7120 fdxN 55-kilobase-pair element.

Structure and regulation of genes encoding phycocyanin and allophycocyanin from Anabaena variabilis ATCC 29413

Gene clones encoding phycocyanin and allophycocyanin were isolated from an Anabaena variabilis ATCC 29413-Charon 30 library by using the phycocyanin (cpc) genes of Agmenellum quadruplicatum and the allophycocyanin (apc) genes of Cyanophora paradoxa as heterologous probes. The A. variabilis cpcA and cpcB genes occur together in the genome, as do the apcA and apcB genes; the two sets of genes are not closely linked, however. The cpc and apc genes appear to be present in only one copy per genome. DNA-RNA hybridization analysis showed that expression of the cpc and apc genes is greatly decreased during nitrogen starvation; within 1 h no cpc or apc mRNA could be detected. The source of nitrogen for growth did not influence expression of the genes; vegetative cells from nitrogen-fixing and ammonia-grown cultures had approximately the same levels of cpc and apc mRNAs. Heterocysts had less than 5% as much cpc mRNA as vegetative cells from nitrogen-fixing cultures. Northern hybridization (RNA blot) analysis showed that the cpc genes are transcribed to give an abundant 1.4-kilobase (kb) RNA as well as two less prominent 3.8- and 2.6-kb species. The apc genes gave rise to two transcripts, a 1.4-kb predominant RNA and a minor 1.75-kb form.

Isolation and complementation of mutants of Anabaena sp. strain PCC 7120 unable to grow aerobically on dinitrogen

Mutants of Anabaena sp. strain PCC 7120 unable to grow aerobically on dinitrogen were isolated by mutagenesis with UV irradiation, followed by a period of incubation in yellow light and then by penicillin enrichment. A cosmid vector, pRL25C, containing replicons functional in Escherichia coli and in Anabaena species was constructed. DNA from wild-type Anabaena sp. strain PCC 7120 was partially digested with Sau3AI, and size-fractionated fragments about 40 kilobases (kb) in length were ligated into the phosphatase-treated unique BamHI site of pRL25C. A library of 1,054 cosmid clones was generated in E. coli DH1 bearing helper plasmid pDS4101. A derivative of conjugative plasmid RP-4 was transferred to this library by conjugation, and the library was replicated to lawns of mutant Anabaena strains with defects in the polysaccharide layer of the envelopes of the heterocysts. Mutant EF116 was complemented by five cosmids, three of which were subjected to detailed restriction mapping; a 2.8-kb fragment of DNA derived from one of the cosmids was found to complement EF116. Mutant EF113 was complemented by a single cosmid, which was also restriction mapped, and was shown to be complemented by a 4.8-kb fragment of DNA derived from this cosmid.

Effect of nitrogenous compounds on nitrogenase gene expression in anaerobic cultures of Anabaena variabilis

The effects of several organic and inorganic nitrogen compounds on nitrogenase mRNA and enzyme activity levels were examined in anaerobic cultures of Anabaena variabilis 29413. Even low concentrations of exogenous ammonia (20 microM) prevented nitrogenase gene expression. Nitrate, in contrast, had little effect, even at very high concentrations. Neither compound had a significant direct effect on existing enzyme activity. The amino acids glutamine and glutamate did not repress nif gene expression. Methionine sulfoximine, but not 7-azatryptophan, was shown to eliminate the repressive effect of ammonia, and this action occurred at the mRNA level. Low concentrations of carbamyl phosphate caused a rapid decrease in nitrogenase mRNA levels. These results are consistent with the ideas that nif gene regulation in Anabaena spp. occurs primarily at the mRNA level and that ammonia, and possibly also glutamine and glutamate, is not the immediate effector of regulation.

Regulation of nitrogenase gene expression in anaerobic cultures of Anabaena variabilis

Derepression of nitrogenase gene expression was studied at the mRNA and enzyme activity levels in anaerobic cultures of Anabaena variabilis 29413. Cells, previously grown with ammonium chloride, were incubated in the absence of fixed nitrogen compounds under an Ar atmosphere with dichlorophenyldimethyl-urea present to inhibit oxygen evolution. The appearance of nitrogenase mRNA (measured by dot blot hybridization analysis) and nitrogenase activity (measured as acetylene-reducing activity) was followed, and the cells were also observed by phase-contrast microscopy. Nitrogenase mRNA could be detected after 1.5 to 2.0 h of nitrogen starvation; enzyme activity appeared about 1 h later. Although enzyme activity increased for many hours, mRNA levels reached a steady state rapidly. Neither heterocysts nor proheterocysts formed under these conditions; however, the cells were observed to shrink and become chlorotic. When anaerobic, derepressed cultures were exposed to oxygen, nitrogenase mRNA levels decreased very rapidly.

Molecular aspects of nitrogen fixation by photosynthetic prokaryotes

The photosynthetic prokaryotes possess diverse metabolic capabilities, both in carrying out different types of photosynthesis and in their other growth modes. The nature of the coupling of these energy-generating processes with the basic metabolic demands of the cell, such as nitrogen fixation, has stimulated research for many years. In addition, nitrogen fixation by photosynthetic prokaryotes exhibits several unique features; the oxygen-evolving cyanobacteria have developed various strategies for protection of the oxygen-labile nitrogenase proteins, and some photosynthetic bacteria have been found to regulate their nitrogenase (N2ase) activity in a rapid response to fixed nitrogen, thus saving substantial amounts of energy. Recent advances in the biochemistry, physiology, and genetics of nitrogen fixation by cyanobacteria and photosynthetic bacteria are reviewed, with special emphasis on the unique features found in these organisms. Several major topics in cyanobacterial nitrogen fixation are reviewed. The isolation and characterization of N2ase and the isolation and sequence of N2ase structural genes have shown a great deal of similarity with other organisms. The possible pathways of electron flow to N2ase, the mechanisms of oxygen protection, and the control of nif expression and heterocyst differentiation will be discussed. Several recent advances in the physiology and biochemistry of nitrogen fixation by the photosynthetic bacteria are reviewed. Photosynthetic bacteria have been found to fix nitrogen microaerobically in darkness. The regulation of nif expression and possible pathways of electron flow to N2ase are discussed. The isolation of N2ase proteins, particularly the covalent modification of the Fe protein, the nature of the modifying group, properties of the activating enzyme, and regulating factors of the inactivation/activation process are reviewed.

Effect of glutamine on growth and heterocyst differentiation in the cyanobacterium Anabaena variabilis

Mutants of the cyanobacterium Anabaena variabilis that were capable of increased uptake of glutamine, as compared with that in the parental strains, were isolated. Growth of these mutants and their parental strains was measured in media containing N2, ammonia, or glutamine as a source of nitrogen. All strains grew well with any one of these sources of fixed nitrogen. Much of the glutamine taken up by the cells was converted to glutamate. The concentrations of glutamine, glutamate, arginine, ornithine, and citrulline in free amino acid pools in glutamine-grown cells were high compared with the concentrations of these amino acids in ammonia-grown or N2-grown cells. All strains capable of heterocyst differentiation, including a strain which produced nonfunctional heterocysts, grew and formed heterocysts in the presence of glutamine. However, nitrogenase activity was repressed in glutamine-grown cells. Glutamine may not be the molecule directly responsible for repression of the differentiation of heterocysts.

Estimation of gene expression in heterocysts of Anabaena variabilis by using DNA-RNA hybridization

In the filamentous cyanobacterium Anabaena variabilis, specialized cells called heterocysts occur in a regular pattern along the filament and are the sites of nitrogen fixation. We used two different types of DNA-excess RNA hybridization techniques to estimate the number of genes expressed in recently differentiated, mature heterocysts. In the first, RNA and DNA were incubated in a phosphate buffer at 60 degrees C, and the hybrids were separated from the unhybridized material by hydroxylapatite chromatography. In the second, the nucleic acids were incubated at 50 degrees C in a buffer containing 50% formamide, and the fraction of DNA in duplexes was assayed by S1 nuclease digestion. Both techniques revealed that approximately 65% of the A. variabilis genome was expressed in vegetative cells and 45% of the genome was expressed in heterocysts. Two experiments were conducted to estimate the number of heterocyst-specific mRNA transcripts. In one, hybridization of heterocyst RNA to a null DNA probe (DNA not transcribed in vegetative cells) revealed that heterocyst-specific transcripts were encoded by 25% of the DNA sense strand, representing approximately 1,000 genes (assuming each to be 1,500 nucleotides in length). The second approach, in which total cell DNA was hybridized to a mixture of heterocyst and vegetative cell RNA, indicated that 14.7% of the DNA sense strand, or about 600 genes, was transcribed exclusively in the heterocyst. The remaining 900 to 1,300 transcripts present in the heterocyst appeared to be constitutively produced in both vegetative cells and heterocysts. The heterocyst-specific transcripts were present in abundant copies in the cell, while transcripts that occurred in both cell types were present at much lower frequency.

Isolation of cyanobacterial heterocysts with high and sustained dinitrogen-fixation capacity supported by endogenous reductants

A method is described for the preparation of cyanobacterial heterocysts with high nitrogen-fixation (acetylene-reduction) activity supported by endogenous reductants. The starting material was Anabaena variabilis ATCC 29413 grown in the light in the presence of fructose. Heterocysts produced from such cyanobacteria were more active than those from photoautotrophically-grown A. variabilis, presumably because higher reserves of carbohydrate were stored within the heterocysts. It proved important to avoid subjecting the cyanobacteria to low temperatures under aerobic conditions, as inhibition of respiration appeared to lead to inactivation of nitrogenase. Low temperatures were not harmful in the absence of O2. A number of potential osmoregulators at various concentrations were tested for use in heterocyst isolation. The optimal concentration (0.2 M sucrose) proved to be a compromise between adequate osmotic protection for isolated heterocysts and avoidance of inhibition of nitrogenase by high osmotic strength. Isolated heterocysts without added reductants such as H2 had about half the nitrogen-fixation activity expected on the basis of intact filaments. H2 did not increase the rate of acetylene reduction, suggesting that the supply of reductant from heterocyst metabolism did not limit nitrogen fixation under these conditions. Such heterocysts had linear rates of acetylene reduction for at least 2 h, and retained their full potential for at least 12 h when stored at 0 degree C under N2.

The Azolla-Anabaena azollae Relationship : XII. Nitrogenase Activity and Phycobiliproteins of the Endophyte as a Function of Leaf Age and Cell Type

Nitrogenase activity was measured in leaves along the main stem axes of Azolla pinnata R. Br. The activity was negligible in leaves of the apical region, rapidly increased to a maximum as leaves matured, and declined in aging leaves. In situ absorption and fluorescence emission spectra were obtained for individual vegetative cells and heterocysts in filaments of the A. pinnata and Azolla caroliniana endophytes removed from the cavities of progressively older leaves. These spectra unequivocally demonstrate the occurrence of phycobiliproteins in the two cell types of both endophytes at the onset of heterocyst differentiation in filaments from young leaves, during the period of maximal nitrogenase activity in filaments from mature leaves, and in filaments from leaves entering senescence. Phycobiliproteins of the A. caroliniana endophyte were purified and extinction coefficients determined for the phycoerythrocyanin, phycocyanin, and allophycocyanin. The phycobiliprotein content and complement of sequential leaf segments from main stem axes and of vegetative cell and heterocyst preparations were measured in crude extracts. There was no obvious alteration of the phycobiliprotein complement associated with increasing heterocyst frequency of the endophyte in sequential leaf segments and the phycobiliprotein complement of heterocysts was not appreciably different from that of vegetative cells. These findings indicate that the phycobiliprotein complement of the vegetative cell precursor is retained in the heterocysts of the endophyte.

Identification of facultatively heterotrophic, N2-fixing cyanobacteria able to receive plasmid vectors from Escherichia coli by conjugation

Plasmid vectors transferable by conjugation from Escherichia coli to obligately photoautotrophic strains of Anabaena spp. are also transferred to and maintained in heterotrophic, filamentous cyanobacteria of the genus Nostoc. These organisms can be used for the genetic analysis of oxygenic photosynthesis, chromatic adaptation, nitrogen fixation, and heterocyst development.

Role of Chemotaxis in Establishing a Specific Nitrogen-Fixing Cyanobacterial-Bacterial Association

A specific association with the cyanobacterium Anabaena oscillarioides was established by positive bacterial (pseudomonad) chemotaxis to Anabaena oscillarioides heterocysts. This association enhanced nitrogen fixation in A. oscillarioides, and positive chemotaxis was particularly strong during periods of active nitrogen fixation. Addition of compounds known to elicit positive chemotaxis in pseudomonads interfered with the establishment of the association, while removal of these compounds led to reestablishment of the association. Anabaena oscillarioides excretion products, some of which are exuded from heterocyst-vegetative cell junctions, are likely to be responsible for positive chemotactic responses. Chemotaxis-controlled associations such as this one explain in part why aquatic bacterial-algal and bacterial-particle associations occur sporadically and are heterogeneously distributed in time and space.

Permeabilization of isolated heterocysts of Anabaena sp. strain 7120 with detergent

Heterocysts isolated from Anabaena sp. strain 7120 with lysozyme plus sonication were permeabilized with the cationic detergent cetyltrimethylammonium bromide, and they then exhibited comparable acetylene reduction activity in the light and dark with an ATP-regenerating system plus dithionite. The detergent diminished the effect of H2 in enhancing acetylene reduction.

High endogenous nitrogenase activity in isolated heterocysts of Anabaena sp. strain CA after nitrogen starvation

Metabolically active heterocysts were isolated from a mutant of Anabaena sp. strain CA with fragile vegetative cells. Heterocysts isolated from cultures grown in 1% CO2 in air reduced C2H2 at 57 and 10 nmol of C2H2 per mg (dry weight) per min under H2 and Ar, respectively. However, if whole filaments were sparged with 1% CO2 in 99% Ar for 12 h before heterocyst isolation, these heterocysts showed C2H2 reduction rates of 83 nmol of C2H4 per mg (dry weight) per min under either H2 or Ar, or 40% the activity of whole filaments grown in 1% CO2 in air. Heterocysts isolated from cultures sparged with 100% Ar or 1% CO2 in 99% N2 had the same C2H2 reduction pattern as heterocysts from cultures grown in 1% CO2 in air, i.e., low activity under Ar and high activity under H2. Labeling of whole filaments incubated with NaH14CO3 for 12 h under 1% CO2 in air or 1% CO2 in 99% Ar resulted in a twofold higher accumulation of 14C-labeled compounds in vegetative cells and heterocysts of Ar-incubated cells. Our results suggest that during incubation under 1% CO2 in 99% Ar, presumably a nitrogen starvation condition, continuing photosynthetic fixation of CO2 leads to accumulation of material(s) in the heterocysts that supports a high, persistent endogenous rate of C2H2 reduction. This material appears to be, in part, glycogen.