Expression of furA is modulated by NtcA and strongly enhanced in heterocysts of Anabaena sp. PCC 7120

Adaptive Response of Thermophiles to Redox Stress and Their Role in the Process of dye Degradation From Textile Industry Wastewater

Release of dye-containing textile wastewater into the environment causes severe pollution with serious consequences on aquatic life. Bioremediation of dyes using thermophilic microorganisms has recently attracted attention over conventional treatment techniques. Thermophiles have the natural ability to survive under extreme environmental conditions, including high dye concentration, because they possess stress response adaptation and regulation mechanisms. Therefore, dye detoxification by thermophiles could offer enormous opportunities for bioremediation at elevated temperatures. In addition, the processes of degradation generate reactive oxygen species (ROS) and subject cells to oxidative stress. However, thermophiles exhibit better adaptation to resist the effects of oxidative stress. Some of the major adaptation mechanisms of thermophiles include macromolecule repair system; enzymes such as superoxide dismutase, catalase, and glutathione peroxidase; and non-enzymatic antioxidants like extracellular polymeric substance (EPSs), polyhydroxyalkanoates (PHAs), etc. In addition, different bacteria also possess enzymes that are directly involved in dye degradation such as azoreductase, laccase, and peroxidase. Therefore, through these processes, dyes are first degraded into smaller intermediate products finally releasing products that are non-toxic or of low toxicity. In this review, we discuss the sources of oxidative stress in thermophiles, the adaptive response of thermophiles to redox stress and their roles in dye removal, and the regulation and crosstalk between responses to oxidative stress.

FurC (PerR) from Anabaena sp. PCC7120: a versatile transcriptional regulator engaged in the regulatory network of heterocyst development and nitrogen fixation

FurC (PerR) from Anabaena sp. PCC7120 was previously described as a key transcriptional regulator involved in setting off the oxidative stress response. In the last years, the cross-talk between oxidative stress, iron homeostasis and nitrogen metabolism is becoming more and more evident. In this work, the transcriptome of a furC-overexpressing strain was compared with that of a wild-type strain under both standard and nitrogen-deficiency conditions. The results showed that the overexpression of furC deregulates genes involved in several categories standing out photosynthesis, iron transport and nitrogen metabolism. The novel FurC-direct targets included some regulatory elements that control heterocyst development (hetZ and asr1734), genes directly involved in the heterocyst envelope formation (devBCA and hepC) and genes which participate in the nitrogen fixation process (nifHDK and nifH2, rbrA rubrerythrin and xisHI excisionase). Likewise, furC overexpression notably impacts the mRNA levels of patA encoding a key protein in the heterocyst pattern formation. The relevance of FurC in these processes is bringing out by the fact that the overexpression of furC impairs heterocyst development and cell growth under nitrogen step-down conditions. In summary, this work reveals a new player in the complex regulatory network of heterocyst formation and nitrogen fixation.

Differential Effects of Varying Concentrations of Phosphorus, Iron, and Nitrogen in N2-Fixing Cyanobacteria

Diazotrophs or N₂-fixers are one of the most ecologically significant groups in marine ecosystems (pelagic and benthic). Inorganic phosphorus (PO₄^3–) and iron (Fe) can limit the growth and N₂-fixing capacities of cyanobacteria. However, studies investigating co-limitation of these factors are lacking. Here, we added different concentrations of PO₄^3– and Fe in two cyanobacterial species whose relatives can be found in seagrass habitats: the unicellular Halothece sp. (PCC 7418) and the filamentous Fischerella muscicola (PCC 73103), grown under different nitrate (NO₃^–) concentrations and under N₂ as sole N source, respectively. Their growth, pigment content, N₂-fixation rates, oxidative stress responses, and morphological and cellular changes were investigated. Our results show a serial limitation of NO₃^– and PO₄^3– (with NO₃^– as the primary limiting nutrient) for Halothece sp. Simultaneous co-limitation of PO₄^3– and Fe was found for both species tested, and high levels of Fe (especially when added with high PO₄^3– levels) inhibited the growth of Halothece sp. Nutrient limitation (PO₄^3–, Fe, and/or NO₃^–) enhanced oxidative stress responses, morphological changes, and apoptosis. Furthermore, an extensive bio-informatic analysis describing the predicted Pho, Fur, and NtcA regulons (involved in the survival of cells to P, Fe, and N limitation) was made using the complete genome of Halothece sp. as a model, showing the potential of this strain to adapt to different nutrient regimes (P, Fe, or N).

2-oxoglutarate modulates the affinity of FurA for the ntcA promoter in Anabaena sp. PCC 7120

2-oxoglutarate (2-OG) is a central metabolite that acts as a signaling molecule informing about the status of the carbon/nitrogen balance of the cell. In recent years, some transcriptional regulators and even two-component systems have been described as 2-OG sensors. In the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120, two master regulators, NtcA and FurA, are deeply involved in the regulation of nitrogen metabolism. Both of them show a complex intertwined regulatory circuit to achieve a suitable regulation of nitrogen fixation. In this work, 2-OG is found to bind FurA, modulating the specific binding of FurA to the ntcA promoter. This study provides evidence of a new additional control point in the complex network controlled by the NtcA and FurA proteins.

Iron deficiency influences NtcA-dependent regulation of fatty acid desaturation and heterocyte envelop formation in Anabaena sp. PCC 7120

In Anabaena sp. PCC 7120, iron is an essential trace element and its availability determines proper functioning of several kinds of metabolisms. Iron deficiency leads to several unavoidable consequences including membrane damage. In the present study, we dealt with the impact of iron deficiency on NtcA (global nitrogen regulator)-dependent regulation of two important processes, i.e. fatty acid desaturation and heterocyte envelop formation in cyanobacterium Anabaena sp. PCC 7120. In Anabaena sp. PCC 7120, NtcA regulates fatty acid desaturation by regulating enzyme fatty acid desaturases. The NtcA-based regulation of fatty acid desaturation may be direct or indirect. Furthermore, the expression of genes involved in the heterocyte envelope polysaccharide (HEP) layer formation (hepABCK) and heterocyte-specific glycolipids (HGLs) synthesis (devH, hglE_A , prpJ and devB) were also under the control of NtcA and reduced under iron deficiency background. The enhanced expression of furA and early downregulation of ntcA under iron deficiency is responsible for reduction in fatty acid desaturation as well as decrease in the expression of genes involved in HEP layer formation and HGL synthesis. Overall results confirmed that iron deficiency influences the NtcA-based regulation of fatty acid desaturation and heterocyte envelop formation in Anabaena sp. PCC 7120.

Differential regulation of ssb genes in the nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC71201

Anabaena sp. PCC7120 possesses three genes coding for single-stranded DNA-binding (SSB) protein, of which ssb1 was a single gene, and ssb2 and ssb3 are the first genes of their corresponding operons. Regulation of the truncated ssb genes, ssb1 (alr0088) and ssb2 (alr7559), was unaffected by N-status of growth. They were negatively regulated by the SOS-response regulatory protein LexA, as indicated by the (i) binding of Anabaena LexA to the LexA box of regulatory regions of ssb1 and ssb2, and (ii) decreased expression of the downstream gfp reporter gene in Escherichia coli upon co-expression of LexA. However, the full-length ssb gene, ssb3 (all4779), was regulated by the availability of Fe²⁺ and combined nitrogen, as indicated by (i) increase in the levels of SSB3 protein on Fe²⁺ -depletion and decrease under Fe²⁺ -excess conditions, and (ii) 1.5- to 1.6-fold decrease in activity under nitrogen-fixing conditions compared to nitrogen-supplemented conditions. The requirement of Fe²⁺ as a co-factor for repression by FurA and the increase in levels of FurA under nitrogen-deficient conditions in Anabaena (Lopez-Gomollon et al. 2007) indicated a possible regulation of ssb3 by FurA. This was substantiated by (i) the binding of FurA to the regulatory region of ssb3, (ii) repression of the expression of the downstream gfp reporter gene in E. coli upon co-expression of FurA, and (iii) negative regulation of ssb3 promoter activity by the upstream AT-rich region in Anabaena. This is the first report on possible role of FurA, an important protein for iron homeostasis, in DNA repair of cyanobacteria.

Multiplicity and specificity of siderophore uptake in the cyanobacterium Anabaena sp. PCC 7120

Many cyanobacteria secrete siderophores to sequester iron. Alternatively, mechanisms to utilize xenosiderophores have evolved. The overall uptake systems are comparable to that of other bacteria involving outer membrane transporters energized by TonB as well as plasma membrane-localized transporters. However, the function of the bioinformatically-inferred components is largely not established and recent studies showed a high diversity of the complexity of the uptake systems in different cyanobacteria. Thus, we approached the systems of the filamentous Anabaena sp. PCC 7120 as a model of a siderophore-secreting cyanobacterium. Anabaena sp. produces schizokinen and uptake of Fe-schizokinen involves the TonB-dependent transporter, schizokinen transporter (SchT), and the ABC-type transport system FhuBCD. We confirm that this system is also relevant for the uptake of structurally similar Fe-siderophore complexes like Fe-aerobactin. Moreover, we demonstrate a function of the TonB-dependent transporter IutA2 in Fe-schizokinen uptake in addition to SchT. The iutA2 mutant shows growth defects upon iron limitation, alterations in Fe-schizokinen uptake and in the transcription profile of the Fe-schizokinen uptake system. The physiological properties of the mutant confirm the importance of iron uptake for cellular function, e.g. for the Krebs cycle. Based on the relative relation of expression of schT and iutA2 as well as of the iron uptake rate to the degree of starvation, a model for the need of the co-existence of two different outer membrane transporters for the same substrate is discussed.

Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport

Iron-containing metalloproteins are the main cornerstones for efficient electron transport in biological systems. The abundance and diversity of iron-dependent proteins in cyanobacteria makes those organisms highly dependent of this micronutrient. To cope with iron imbalance, cyanobacteria have developed a survey of adaptation strategies that are strongly related to the regulation of photosynthesis, nitrogen metabolism and other central electron transfer pathways. Furthermore, either in its ferrous form or as a component of the haem group, iron plays a crucial role as regulatory signalling molecule that directly or indirectly modulates the composition and efficiency of cyanobacterial redox reactions. We present here the major mechanism used by cyanobacteria to couple iron homeostasis to the regulation of electron transport, making special emphasis in processes specific in those organisms.

Expanding the Role of FurA as Essential Global Regulator in Cyanobacteria

In the nitrogen-fixing heterocyst-forming cyanobacterium Anabaena sp. PCC 7120, the ferric uptake regulator FurA plays a global regulatory role. Failures to eliminate wild-type copies of furA gene from the polyploid genome suggest essential functions. In the present study, we developed a selectively regulated furA expression system by the replacement of furA promoter in the Anabaena sp. chromosomes with the Co²⁺/Zn²⁺ inducible coaT promoter from Synechocystis sp. PCC 6803. By removing Co²⁺ and Zn²⁺ from the medium and shutting off furA expression, we showed that FurA was absolutely required for cyanobacterial growth. RNA-seq based comparative transcriptome analyses of the furA-turning off strain and its parental wild-type in conjunction with subsequent electrophoretic mobility shift assays and semi-quantitative RT-PCR were carried out in order to identify direct transcriptional targets and unravel new biological roles of FurA. The results of such approaches led us to identify 15 novel direct iron-dependent transcriptional targets belonging to different functional categories including detoxification and defences against oxidative stress, phycobilisome degradation, chlorophyll catabolism and programmed cell death, light sensing and response, heterocyst differentiation, exopolysaccharide biosynthesis, among others. Our analyses evidence novel interactions in the complex regulatory network orchestrated by FurA in cyanobacteria.

Unraveling the redox properties of the global regulator FurA from Anabaena sp. PCC 7120: disulfide reductase activity based on its CXXC motifs

Cyanobacterial FurA works as a global regulator linking iron homeostasis to photosynthetic metabolism and the responses to different environmental stresses. Additionally, FurA modulates several genes involved in redox homeostasis and fulfills the characteristics of a heme-sensor protein whose interaction with this cofactor negatively affects its DNA binding ability. FurA from Anabaena PCC 7120 contains five cysteine residues, four of them arranged in two redox CXXC motifs.

AIMS

Our goals were to analyze in depth the putative contribution of these CXXC motifs in the redox properties of FurA and to identify potential interacting partners of this regulator.

RESULTS

Insulin reduction assays unravel that FurA exhibits disulfide reductase activity. Simultaneous presence of both CXXC signatures greatly enhances the reduction rate, although the redox motif containing Cys(101) and Cys(104) seems a major contributor to this activity. Disulfide reductase activity was not detected in other ferric uptake regulator (Fur) proteins isolated from heterotrophic bacteria. In vivo, FurA presents different redox states involving intramolecular disulfide bonds when is partially oxidized. Redox potential values for CXXC motifs, -235 and -238 mV, are consistent with those reported for other proteins displaying disulfide reductase activity. Pull-down and two-hybrid assays unveil potential FurA interacting partners, namely phosphoribulokinase Alr4123, the hypothetical amidase-containing domain All1140 and the DNA-binding protein HU.

INNOVATION

A novel biochemical activity of cyanobacterial FurA based on its cysteine arrangements and the identification of novel interacting partners are reported.

CONCLUSION

The present study discloses a putative connection of FurA with the cyanobacterial redox-signaling pathway.

α-fur, an antisense RNA gene to fur in the extreme acidophile Acidithiobacillus ferrooxidans

A large non-coding RNA, termed α-Fur, of ~1000 nt has been detected in the extreme acidophile Acidithiobacillus ferrooxidans encoded on the antisense strand to the iron-responsive master regulator fur (ferric uptake regulator) gene. A promoter for α-fur was predicted bioinformatically and validated using gene fusion experiments. The promoter is situated within the coding region and in the same sense as proB, potentially encoding a glutamate 5-kinase. The 3′ termination site of the α-fur transcript was determined by 3′ rapid amplification of cDNA ends to lie 7 nt downstream of the start of transcription of fur. Thus, α-fur is antisense to the complete coding region of fur, including its predicted ribosome-binding site. The genetic context of α-fur is conserved in several members of the genus Acidithiobacillus but not in all acidophiles, indicating that it is monophyletic but not niche specific. It is hypothesized that α-Fur regulates the cellular level of Fur. This is the fourth example of an antisense RNA to fur, although it is the first in an extreme acidophile, and underscores the growing importance of cis-encoded non-coding RNAs as potential regulators involved in the microbial iron-responsive stimulon.

The FUR (ferric uptake regulator) superfamily: diversity and versatility of key transcriptional regulators

Control of metal homeostasis is essential for life in all kingdoms. In most prokaryotic organisms the FUR (ferric uptake regulator) family of transcriptional regulators is involved in the regulation of iron and zinc metabolism through control by Fur and Zur proteins. A third member of this family, the peroxide-stress response PerR, is present in most Gram-positives, establishing a tight functional interaction with the global regulator Fur. These proteins play a pivotal role for microbial survival under adverse conditions and in the expression of virulence in most pathogens. In this paper we present the current state of the art in the knowledge of the FUR family, including those members only present in more reduced numbers of bacteria, namely Mur, Nur and Irr. The huge amount of work done in the two last decades shows that FUR proteins present considerable diversity in their regulatory mechanisms and interesting structural differences. However, much work needs to be done to obtain a more complete picture of this family, especially in connection with the roles of some members as gas and redox sensors as well as to fully characterize their participation in bacterial adaptative responses.

The FurA regulon in Anabaena sp. PCC 7120: in silico prediction and experimental validation of novel target genes

In the filamentous cyanobacterium Anabaena sp. PCC 7120, the ferric uptake regulator FurA functions as a global transcriptional regulator. Despite several analyses have focused on elucidating the FurA-regulatory network, the number of target genes described for this essential transcription factor is limited to a handful of examples. In this article, we combine an in silico genome-wide predictive approach with experimental determinations to better define the FurA regulon. Predicted FurA-binding sites were identified upstream of 215 genes belonging to diverse functional categories including iron homeostasis, photosynthesis and respiration, heterocyst differentiation, oxidative stress defence and light-dependent signal transduction mechanisms, among others. The probabilistic model proved to be effective at discerning FurA boxes from non-cognate sequences, while subsequent electrophoretic mobility shift assay experiments confirmed the in vitro specific binding of FurA to at least 20 selected predicted targets. Gene-expression analyses further supported the dual role of FurA as transcriptional modulator that can act both as repressor and as activator. In either role, the in vitro affinity of the protein to its target sequences is strongly dependent on metal co-regulator and reducing conditions, suggesting that FurA couples in vivo iron homeostasis and the response to oxidative stress to major physiological processes in cyanobacteria.

Sequential binding of FurA from Anabaena sp. PCC 7120 to iron boxes: exploring regulation at the nanoscale

Fur (ferric uptake regulator) proteins are involved in the control of a variety of processes in most prokaryotes. Although it is assumed that this regulator binds its DNA targets as a dimer, the way in which this interaction occurs remains unknown. We have focused on FurA from the cyanobacterium Anabaena sp. PCC 7120. To assess the molecular mechanism by which FurA specifically binds to "iron boxes" in PfurA, we examined the topology arrangement of FurA-DNA complexes by atomic force microscopy. Interestingly, FurA-PfurA complexes exhibit several populations, in which one is the predominant and depends clearly on the regulator/promoter ratio on the environment. Those results together with EMSA and other techniques suggest that FurA binds PfurA using a sequential mechanism: (i) a monomer specifically binds to an "iron box" and bends PfurA; (ii) two situations may occur, that a second FurA monomer covers the free "iron box" or that joins to the previously used forming a dimer which would maintain the DNA kinked; (iii) trimerization in which the DNA is unbent; and (iv) finally undergoes a tetramerization; the next coming molecules cover the DNA strands unspecifically. In summary, the bending appears when an "iron box" is bound to one or two molecules and decreases when both "iron boxes" are covered. These results suggest that DNA bending contributes at the first steps of FurA repression promoting the recruitment of new molecules resulting in a fine regulation in the Fur-dependent cluster associated genes.

ChIP analysis unravels an exceptionally wide distribution of DNA binding sites for the NtcA transcription factor in a heterocyst-forming cyanobacterium

Background

The CRP-family transcription factor NtcA, universally found in cyanobacteria, was initially discovered as a regulator operating N control. It responds to the N regime signaled by the internal 2-oxoglutarate levels, an indicator of the C to N balance of the cells. Canonical NtcA-activated promoters bear an NtcA-consensus binding site (GTAN₈TAC) centered at about 41.5 nucleotides upstream from the transcription start point. In strains of the Anabaena/Nostoc genera NtcA is pivotal for the differentiation of heterocysts in response to N stress.

Results

In this study, we have used chromatin immunoprecipitation followed by high-throughput sequencing to identify the whole catalog of NtcA-binding sites in cells of the filamentous, heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 three hours after the withdrawal of combined N. NtcA has been found to bind to 2,424 DNA regions in the genome of Anabaena, which have been ascribed to 2,153 genes. Interestingly, only a small proportion of those genes are involved in N assimilation and metabolism, and 65% of the binding regions were located intragenically.

Conclusions

The distribution of NtcA-binding sites identified here reveals the largest bacterial regulon described to date. Our results show that NtcA has a much wider role in the physiology of the cell than it has been previously thought, acting both as a global transcriptional regulator and possibly also as a factor influencing the superstructure of the chromosome (and plasmids).

FurA influences heterocyst differentiation in Anabaena sp. PCC 7120

In Anabaena sp. PCC 7120, FurA is a global transcriptional regulator whose expression is strongly induced by NtcA in proheterocysts and remains stably expressed in mature heterocysts. In the present study, overexpression of furA partially suppressed heterocyst differentiation by impairing morphogenesis at an early stage. Recombinant purified FurA specifically bound in vitro to the promoter regions of ntcA, while quantitative RT-PCR analyses indicated that furA overexpression strongly affected the transient increase of ntcA expression that occurs shortly after nitrogen step-down. Overall, the results suggest a connection between iron homeostasis and heterocyst differentiation via FurA, by modulating the expression of ntcA.

Expression of fur and its antisense α-fur from Microcystis aeruginosa PCC7806 as response to light and oxidative stress

Ferric uptake regulation (Fur) proteins are prokaryotic transcriptional regulators that integrate signaling of iron metabolism and oxidative stress responses with several environmental stresses. In photosynthetic organisms, Fur proteins regulate many genes involved in photosynthesis, nitrogen metabolism and other key processes. Also, Fur triggers the expression of virulence factors in many bacterial pathogens, and Fur from Microcystis aeruginosa has been shown to bind promoter regions of the microcystin synthesis gene cluster. In this work, we studied transcriptional responses of fur genes under different light intensities and oxidative stress. An antisense of fur, the α-fur RNA, plays an important role in regulating fur expression under oxidative stress, affecting levels of Fur protein in cells. Importantly, an active photosynthetic electron chain is required for the expression of the fur gene.

The CyAbrB transcription factor CalA regulates the iron superoxide dismutase in Nostoc sp. strain PCC 7120

In the present investigation the results of induced over-production of the CyAbrB transcription factor CalA (Cyanobacterial AbrB-like, annotated as Alr0946) in the cyanobacterium Nostoc sp. PCC 7120 were analysed. The CalA overexpression strain showed a bleaching phenotype with lower growth rate and truncated filaments 2 days after induction of overexpression. The phenotype was even more pronounced when illumination was increased from 35 to 125 µmol m(-2) s(-1). Using gel-based quantitative proteomics, the induced overexpression of CalA was shown to downregulate the abundance of FeSOD, one of two types of superoxide dismutases in Nostoc sp. PCC 7120. The change in protein abundance was also accompanied by lower transcript as well as activity levels. Purified recombinant CalA from Nostoc sp. PCC 7120 was shown to interact with the promoter region of alr2938, encoding FeSOD, indicating a transcriptional regulation of FeSOD by CalA. The bleaching phenotype is in line with a decreased tolerance against oxidative stress and indicates that CalA is involved in regulation of cellular responses in which FeSOD has an important and specific function in the filamentous cyanobacterium Nostoc sp. PCC 7120.

Iron uptake and toxin synthesis in the bloom-forming Microcystis aeruginosa under iron limitation

Toxin production during cyanobacterial blooms poses a significant public health threat in water bodies globally and requires the development of effective bloom management strategies. Previously, synthesis of the hepatotoxin microcystin has been proposed to be regulated by iron availability, but the contribution of the toxin to the adaptation of cyanobacteria to environmental stresses, such as changing light intensity and nutrient limitation, remains unclear. The aim of this study was to compare the iron stress response in toxic and non-toxic strains of Microcystis aeruginosa subjected to moderate and severe iron limitation. The transcription of a number of genes involved in iron uptake, oxidative stress response, toxin synthesis and transcriptional control of these processes was accessed by quantitative real-time PCR (qRT-PCR). The process of adaptation of M. aeruginosa to iron stress was found to be highly dynamic and strain-specific. Toxin production in PCC 7806 increased in an iron-dependent manner and appeared to be regulated by FurA. The inability to produce microcystin, either due to natural mutations in the mcy gene cluster or due to insertional inactivation of mcyH, affected the remodelling of the photosynthetic machinery in iron-stressed cells, the transport of Fe(II) and transcription of the Fur family of transcriptional regulators. The presence of the toxin appears to give an advantage to microcystin-producing cyanobacteria in the early stages of exposure to severe iron stress and may protect the cell from reactive oxygen species-induced damage.

Bacterial antisense RNAs: how many are there, and what are they doing?

Antisense RNAs encoded on the DNA strand opposite another gene have the potential to form extensive base-pairing interactions with the corresponding sense RNA. Unlike other smaller regulatory RNAs in bacteria, antisense RNAs range in size from tens to thousands of nucleotides. The numbers of antisense RNAs reported for different bacteria vary extensively, but hundreds have been suggested in some species. If all of these reported antisense RNAs are expressed at levels sufficient to regulate the genes encoded opposite them, antisense RNAs could significantly impact gene expression in bacteria. Here, we review the evidence for these RNA regulators and describe what is known about the functions and mechanisms of action for some of these RNAs. Important considerations for future research as well as potential applications are also discussed.

Quantitative overview of N2 fixation in Nostoc punctiforme ATCC 29133 through cellular enrichments and iTRAQ shotgun proteomics

Nostoc punctiforme ATCC 29133 is a photoautotrophic cyanobacterium with the capacity to fix atmospheric N 2. Its ability to mediate this process is similar to that described for Nostoc sp. PCC 7120, where vegetative cells differentiate into heterocysts. Quantitative proteomic investigations at both the filament level and the heterocyst level are presented using isobaric tagging technology (iTRAQ), with 721 proteins at the 95% confidence interval quantified across both studies. Observations from both experiments yielded findings confirmatory of both transcriptional studies, and published Nostoc sp. PCC 7120 iTRAQ data. N. punctiforme exhibits similar metabolic trends, though changes in a number of metabolic pathways are less pronounced than in Nostoc sp. PCC 7120. Results also suggest a number of proteins that may benefit from future investigations. These include ATP dependent Zn-proteases, N-reserve degraders and also redox balance proteins. Complementary proteomic data sets from both organisms present key precursor knowledge that is important for future cyanobacterial biohydrogen research.

New insights into the role of Fur proteins: FurB (All2473) from Anabaena protects DNA and increases cell survival under oxidative stress

Fur (ferric uptake regulator) is a prokaryotic transcriptional regulator that controls a large number of genes mainly related to iron metabolism. Several Fur homologues with different physiological roles are frequently found in the same organism. The genome of the filamentous cyanobacterium Anabaena (Nostoc) sp. PCC 7120 codes for three different fur genes. FurA is an essential protein involved in iron homoeostasis that also modulates dinitrogen fixation. FurA interacts with haem, impairing its DNA-binding ability. To explore functional differences between Fur homologues in Anabaena, factors affecting their regulation, as well as some biochemical characteristics, have been investigated. Although incubation of FurB with haem severely hinders its ability to interact with DNA, binding of haem to FurC could not be detected. Oxidative stress enhances the transcription of the three fur genes, especially that of furB and furC. In addition, overexpression of FurA and FurB in Escherichia coli increases survival when the cells are challenged with H2O2 or Methyl Viologen (paraquat), a superoxide-anion-generating reagent. When present in saturating concentrations, FurB exhibits unspecific DNA-binding activity and protects DNA from cleavage produced by hydroxyl radicals or DNaseI. On the basis of these results, we suggest that, whereas at low concentrations FurB would act as a member of the Fur family, at saturating concentrations FurB protects DNA, showing a DNA-protection-during-starvation-like behaviour.

Role of NtcA, a cyanobacterial global nitrogen regulator, in the regulation of sucrose metabolism gene expression in Anabaena sp. PCC 7120

In the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120), it has been shown that spsB and susA, the genes coding for proteins related to sucrose synthesis and cleavage, respectively, exhibit converse expression regarding the nitrogen source. In the nitrogen-fixing filament, spsB expression is mostly localized to the heterocysts and susA is only expressed in vegetative cells. The aim of this work was to investigate the participation of NtcA, a global nitrogen regulator that operates in cyanobacteria, in the regulation of sucrose metabolism genes in Anabaena sp. PCC 7120. The induction of spsB expression observed in the filaments upon combined-nitrogen depletion was abolished in an NtcA-deficient mutant. In vitro experiments showed that NtcA binds specifically but with different affinities to two sites in the spsB promoter region. When susA expression was analyzed after a combined-nitrogen starvation, the levels of mRNA, polypeptide and activity increased in the mutant in comparison with the wild-type strain. Also, NtcA interacted with one site in the promoter region of susA. We conclude that sucrose metabolism is coordinated at the transcriptional level with nitrogen metabolism, suggesting a global metabolism regulating role for NtcA.

Transcription of hupSL in Anabaena variabilis ATCC 29413 is regulated by NtcA and not by hydrogen

Nitrogen-fixing cyanobacteria such as Anabaena variabilis ATCC 29413 use an uptake hydrogenase, encoded by hupSL, to recycle hydrogen gas that is produced as an obligate by-product of nitrogen fixation. The regulation of hupSL in A. variabilis is likely to differ from that of the closely related Anabaena sp. strain PCC 7120 because A. variabilis lacks the excision element-mediated regulation that characterizes hupSL regulation in strain PCC 7120. An analysis of the hupSL transcript in a nitrogenase mutant of A. variabilis that does not produce any detectable hydrogen indicated that neither nitrogen fixation nor hydrogen gas was required for the induction of hupSL. Furthermore, exogenous addition of hydrogen gas did not stimulate hupSL transcription. Transcriptional reporter constructs indicated that the accumulation of hupSL transcript after nitrogen step-down was restricted primarily to the microaerobic heterocysts. Anoxic conditions were not sufficient to induce hupSL transcription. The induction of hupSL after nitrogen step-down was reduced in a mutant in the global nitrogen regulator NtcA, but was not reduced in a mutant unable to form heterocysts. A consensus NtcA-binding site was identified upstream of hupSL, and NtcA was found to bind to this region. Thus, while neither hydrogen gas nor anoxia controlled the expression of hupSL, its expression was controlled by NtcA. Heterocyst differentiation was not required for hupSL induction in response to nitrogen step-down, but heterocyst-localized cues may add an additional level of regulation to hupSL.

Mutations in four regulatory genes have interrelated effects on heterocyst maturation in Anabaena sp. strain PCC 7120

Regulatory genes hepK, hepN, henR, and hepS are required for heterocyst maturation in Anabaena sp. strain PCC 7120. They presumptively encode two histidine kinases, a response regulator, and a serine/threonine kinase, respectively. To identify relationships between those genes, we compared global patterns of gene expression, at 14 h after nitrogen step-down, in corresponding mutants and in the wild-type strain. Heterocyst envelopes of mutants affected in any of those genes lack a homogeneous, polysaccharide layer. Those of a henR mutant also lack a glycolipid layer. patA, which encodes a positive effector of heterocyst differentiation, was up-regulated in all mutants except the hepK mutant, suggesting that patA expression may be inhibited by products related to heterocyst development. hepS and hepK were up-regulated if mutated and so appear to be negatively autoregulated. HepS and HenR regulated a common set of genes and so appear to belong to one regulatory system. Some nontranscriptional mechanism may account for the observation that henR mutants lack, and hepS mutants possess, a glycolipid layer, even though both mutations down-regulated genes involved in formation of the glycolipid layer. HepK and HepN also affected transcription of a common set of genes and therefore appear to share a regulatory pathway. However, the transcript abundance of other genes differed very significantly from expression in the wild-type strain in either the hepK or hepN mutant while differing very little from wild-type expression in the other of those two mutants. Therefore, hepK and hepN appear to participate also in separate pathways.