FurC (PerR) contributes to the regulation of peptidoglycan remodeling and intercellular molecular transfer in the cyanobacterium Anabaena sp. strain PCC 7120.

Microbial extracellular proteins and metabolites provide valuable information concerning how microbes adapt to changing environments. In cyanobacteria, dynamic acclimation strategies involve a variety of regulatory mechanisms, being ferric uptake regulator proteins as key players in this process. In the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120, FurC (PerR) is a global regulator that modulates the peroxide response and several genes involved in photosynthesis and nitrogen metabolism. To investigate the possible role of FurC in shaping the extracellular environment of Anabaena, the analysis of the extracellular metabolites and proteins of a furC-overexpressing variant was compared to that of the wild-type strain. There were 96 differentially abundant proteins, 78 of which were found for the first time in the extracellular fraction of Anabaena. While these proteins belong to different functional categories, most of them are predicted to be secreted or have a peripheral location. Several stress-related proteins, including PrxA, flavodoxin, and the Dps homolog All1173, accumulated in the exoproteome of furC-overexpressing cells, while decreased levels of FurA and a subset of membrane proteins, including several export proteins and amiC gene products, responsible for nanopore formation, were detected. Direct repression by FurC of some of those genes, including amiC1 and amiC2, could account for odd septal nanopore formation and impaired intercellular molecular transfer observed in the furC-overexpressing variant. Assessment of the exometabolome from both strains revealed the release of two peptidoglycan fragments in furC-overexpressing cells, namely 1,6-anhydro-N-acetyl-ß-D-muramic acid (anhydroMurNAc) and its associated disaccharide (ß-D-GlcNAc-(1-4)-anhydroMurNAc), suggesting alterations in peptidoglycan breakdown and recycling.IMPORTANCECyanobacteria are ubiquitous photosynthetic prokaryotes that can adapt to environmental stresses by modulating their extracellular contents. Measurements of the organization and composition of the extracellular milieu provide useful information about cyanobacterial adaptive processes, which can potentially lead to biomimetic approaches to stabilizing biological systems to adverse conditions. Anabaena sp. strain PCC 7120 is a multicellular, nitrogen-fixing cyanobacterium whose intercellular molecular exchange is mediated by septal junctions that traverse the septal peptidoglycan through nanopores. FurC (PerR) is an essential transcriptional regulator in Anabaena, which modulates the response to several stresses. Here, we show that furC-overexpressing cells result in a modified exoproteome and the release of peptidoglycan fragments. Phenotypically, important alterations in nanopore formation and cell-to-cell communication were observed. Our results expand the roles of FurC to the modulation of cell-wall biogenesis and recycling, as well as in intercellular molecular transfer.

Expanding the FurC (PerR) regulon in Anabaena (Nostoc) sp. PCC 7120: Genome-wide identification of novel direct targets uncovers FurC participation in central carbon metabolism regulation.

FurC (PerR, Peroxide Response Regulator) from Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120) is a master regulator engaged in the modulation of relevant processes including the response to oxidative stress, photosynthesis and nitrogen fixation. Previous differential gene expression analysis of a furC-overexpressing strain (EB2770FurC) allowed the inference of a putative FurC DNA-binding consensus sequence. In the present work, more data concerning the regulon of the FurC protein were obtained through the searching of the putative FurC-box in the whole Anabaena sp. PCC 7120 genome. The total amount of novel FurC-DNA binding sites found in the promoter regions of genes with known function was validated by electrophoretic mobility shift assays (EMSA) identifying 22 new FurC targets. Some of these identified targets display relevant roles in nitrogen fixation (hetR and hgdC) and carbon assimilation processes (cmpR, glgP1 and opcA), suggesting that FurC could be an additional player for the harmonization of carbon and nitrogen metabolisms. Moreover, differential gene expression of a selection of newly identified FurC targets was measured by Real Time RT-PCR in the furC-overexpressing strain (EB2770FurC) comparing to Anabaena sp. PCC 7120 revealing that in most of these cases FurC could act as a transcriptional activator.

Responses of Anabaena sp. PCC7120 to lindane: Physiological effects and differential expression of potential lin genes.

Lindane (γ-HCH) is an organochlorine pesticide that causes huge environmental concerns worldwide due to its recalcitrance and toxicity. The use of the cyanobacterium Anabaena sp. PCC 7120 in aquatic lindane bioremediation has been suggested but information relative to this process is scarce. In the present work, data relative to the growth, pigment composition, photosynthetic/respiration rate, and oxidative stress response of Anabaena sp. PCC 7120 in the presence of lindane at its solubility limit in water are shown. In addition, lindane degradation experiments revealed almost a total disappearance of lindane in the supernatants of Anabaena sp. PCC 7120 culture after 6 days of incubation. The diminishing in lindane concentration was in concordance with an increase in the levels of trichlorobenzene inside the cells. Furthermore, to identify potential orthologs of the linA, linB, linC, linD, linE, and linR genes from Sphingomonas paucimobilis B90A in Anabaena sp. PCC 7120, a whole genome screening was performed allowing the identification of five putative lin orthologs (all1353 and all0193 putative orthologs of linB, all3836 putative orthologs of linC, and all0352 and alr0353 putative orthologs of linE and linR, respectively) which could be involved in the lindane degradation pathway. Differential expression analysis of these genes in the presence of lindane revealed strong upregulation of one of the potential lin genes of Anabaena sp. PCC 7120.

Unbalancing Zur (FurB)-mediated homeostasis in Anabaena sp. PCC7120: Consequences on metal trafficking, heterocyst development and biofilm formation.

Zinc is required for the activity of many enzymes and plays an essential role in gene regulation and redox homeostasis. In Anabaena (Nostoc) sp. PCC7120, the genes involved in zinc uptake and transport are controlled by the metalloregulator Zur (FurB). Comparative transcriptomics of a zur mutant (Δzur) with the parent strain unveiled unexpected links between zinc homeostasis and other metabolic pathways. A notable increase in the transcription of numerous desiccation tolerance-related genes, including genes involved in the synthesis of trehalose and the transference of saccharide moieties, among many others, was detected. Biofilm formation analysis under static conditions revealed a reduced capacity of Δzur filaments to form biofilms compared to the parent strain, and such capacity was enhanced when Zur was overexpressed. Furthermore, microscopy analysis revealed that zur expression is required for the correct formation of the envelope polysaccharide layer in the heterocyst, as Δzur cells showed reduced staining with alcian blue compared to Anabaena sp. PCC7120. We suggest that Zur is an important regulator of the enzymes involved in the synthesis and transport of the envelope polysaccharide layer, influencing heterocyst development and biofilm formation, both relevant processes for cell division and interaction with substrates in its ecological niche.

Metal binding and oligomerization properties of FurC (PerR) from Anabaena sp. PCC7120: an additional layer of regulation?

Metal and redox homeostasis in cyanobacteria is tightly controlled to preserve the photosynthetic machinery from mismetallation and minimize cell damage. This control is mainly taken by FUR (ferric uptake regulation) proteins. FurC works as the PerR (peroxide response) paralog in Anabaena sp. PCC7120. Despite its importance, this regulator remained poorly characterized. Although FurC lacks the typical CXXC motifs present in FUR proteins, it contains a tightly bound zinc per subunit. FurC: Zn stoichiometrically binds zinc and manganese in a second site, manganese being more efficient in the binding of FurC: Zn to its DNA target PprxA. Oligomerization analyses of FurC: Zn evidence the occurrence of different aggregates ranging from dimers to octamers. Notably, intermolecular disulfide bonds are not involved in FurC: Zn dimerization, dimer being the most reduced form of the protein. Oligomerization of dimers occurs upon oxidation of thiols by H2O2 or diamide and can be reversed by 1,4-Dithiothreitol (DTT). Irreversible inactivation of the regulator occurs by metal catalyzed oxidation promoted by ferrous iron. However, inactivation upon oxidation with H2O2 in the absence of iron was reverted by addition of DTT. Comparison of models for FurC: Zn dimers and tetramers obtained using AlphaFold Colab and SWISS-MODEL allowed to infer the residues forming both metal-binding sites and to propose the involvement of Cys86 in reversible tetramer formation. Our results decipher the existence of two levels of inactivation of FurC: Zn of Anabaena sp. PCC7120, a reversible one through disulfide-formed FurC: Zn tetramers and the irreversible metal catalyzed oxidation. This additional reversible regulation may be specific of cyanobacteria.

Identification of Early Salt-Stress-Responsive Proteins in In Vitro Prunus Cultured Excised Roots.

Fruit-tree rootstock selection is a challenge under a scenario of growing environmental stresses in which the soil and climate are greatly affected. Salinization is an increasing global process that severely affects soil fertility. The selection of rootstocks with the ability to tolerate salt stress is essential. Excised root cultures may be an excellent experimental approach to study stress physiology and a predictive tool to assess possible tolerance. In this study, we show how protein changes in response to salt stress evaluated in excised root cultures of Prunus cerasus (moderate salt-sensitive cultivar) could be representative of these changes in the roots of whole plants. The 2D electrophoresis of root extracts and subsequent spot identification by MALDI-TOF/TOF-MS show 16 relevant proteins differentially expressed in roots as a response to 60 mM NaCl. Cytoplasmic isozyme fructose 1,6-bisphosphate aldolase shows relevant changes in its relative presence of isoforms as a response to saline stress, while the total level of enzymes remains similar. Ferredoxin-NADP+ reductase increases as a response to salinity, even though the measured activity is not significantly different. The observed changes are congruent with previous proteomic studies on the roots of whole plants that are involved in protection mechanisms against salt 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.

Thioredoxin Dependent Changes in the Redox States of FurA from Anabaena sp. PCC 7120.

FurA is a multifunctional regulator in cyanobacteria that contains five cysteines, four of them arranged into two CXXC motifs. Lack of a structural zinc ion enables FurA to develop disulfide reductase activity. In vivo, FurA displays several redox isoforms, and the oxidation state of its cysteines determines its activity as regulator and its ability to bind different metabolites. Because of the relationship between FurA and the control of genes involved in oxidative stress defense and photosynthetic metabolism, we sought to investigate the role of type m thioredoxin TrxA as a potential redox partner mediating dithiol-disulfide exchange reactions necessary to facilitate the interaction of FurA with its different ligands. Both in vitro cross-linking assays and in vivo two-hybrid studies confirmed the interaction between FurA and TrxA. Light to dark transitions resulted in reversible oxidation of a fraction of the regulator present in Anabaena sp. PCC7120. Reconstitution of an electron transport chain using E. coli NADPH-thioredoxin-reductase followed by alkylation of FurA reduced cysteines evidenced the ability of TrxA to reduce FurA. Furthermore, the use of site-directed mutants allowed us to propose a plausible mechanism for FurA reduction. These results point to TrxA as one of the redox partners that modulates FurA performance.

Fur-like proteins: Beyond the ferric uptake regulator (Fur) paralog.

Proteins belonging to the FUR (ferric uptake regulator) family are the cornerstone of metalloregulation in most prokaryotes. Although numerous reviews have been devoted to these proteins, these reports are mainly focused on the Fur paralog that gives name to the family. In the last years, the increasing knowledge on the other, less ubiquitous members of this family has evidenced their importance in bacterial metabolism. As the Fur paralog, the major regulator of iron homeostasis, Zur, Irr, BosR and PerR are tightly related to stress defenses and host-pathogen interaction being in many cases essential for virulence. Furthermore, the Nur and Mur paralogs largely contribute to control nickel and manganese homeostasis, which are cofactors of pivotal proteins for host colonization and bacterial redox homeostasis. The present review highlights the main features of FUR proteins that differ to the canonical Fur paralog either in the coregulatory metal, such as Zur, Nur and Mur, or in the action mechanism to control target genes, such as PerR, Irr and BosR.

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.

Regulation by FurC in Anabaena Links the Oxidative Stress Response to Photosynthetic Metabolism.

The FUR (Ferric Uptake Regulator) family in Anabaena sp. PCC 7120 consists of three paralogs named FurA (Fur), FurB (Zur) and FurC (PerR). furC seems to be an essential gene in the filamentous nitrogen-fixing strain Anabaena sp. PCC 7120, suggesting that it plays a fundamental role in this organism. In order to better understand the functions of FurC in Anabaena, the phenotype of a derivative strain that overexpresses this regulator (EB2770FurC) has been characterized. The furC-overexpressing variant presented alterations in growth rate, morphology and ultrastructure, as well as higher sensitivity to peroxide than Anabaena sp. PCC 7120. Interestingly, the overexpression of furC led to reduced photosynthetic O2 evolution, increased respiratory activity, and had a significant influence in the composition and efficiency of both photosystems. Comparative transcriptional analyses, together with electrophoretic mobility shift assays allowed the identification of different genes directly controlled by FurC, and involved in processes not previously related to PerR proteins, such as the cell division gene ftsZ and the major thylakoid membrane protease ftsH. The rise in the transcription of ftsH in EB2770FurC cells correlated with reduced levels of the D1 protein, which is involved in the PSII repair cycle. Deregulation of the oxidative stress response in EB2770FurC cells led to the identification of novel FurC targets involved in the response to H2O2 through different mechanisms. These results, together with the effect of furC overexpression on the composition, stability and efficiency of the photosynthetic machinery of Anabaena, disclose novel links between PerR proteins, cell division and photosynthesis in filamentous cyanobacteria.

Redox-Based Transcriptional Regulation in Prokaryotes: Revisiting Model Mechanisms.

SIGNIFICANCE: The successful adaptation of microorganisms to ever-changing environments depends, to a great extent, on their ability to maintain redox homeostasis. To effectively maintain the redox balance, cells have developed a variety of strategies mainly coordinated by a battery of transcriptional regulators through diverse mechanisms. Recent Advances: This comprehensive review focuses on the main mechanisms used by major redox-responsive regulators in prokaryotes and their relationship with the different redox signals received by the cell. An overview of the corresponding regulons is also provided. CRITICAL ISSUES: Some regulators are difficult to classify since they may contain several sensing domains and respond to more than one signal. We propose a classification of redox-sensing regulators into three major groups. The first group contains one-component or direct regulators, whose sensing and regulatory domains are in the same protein. The second group comprises the classical two-component systems involving a sensor kinase that transduces the redox signal to its DNA-binding partner. The third group encompasses a heterogeneous group of flavin-based photosensors whose mechanisms are not always fully understood and are often involved in more complex regulatory networks. FUTURE DIRECTIONS: Redox-responsive transcriptional regulation is an intricate process as identical signals may be sensed and transduced by different transcription factors, which often interplay with other DNA-binding proteins with or without regulatory activity. Although there is much information about some key regulators, many others remain to be fully characterized due to the instability of their clusters under oxygen. Understanding the mechanisms and the regulatory networks operated by these regulators is essential for the development of future applications in biotechnology and medicine.

Differential expression of the three Alcanivorax borkumensis SK2 genes coding for the P450 cytochromes involved in the assimilation of hydrocarbons.

Alcanivorax borkumensis, a marine bacterium highly specialized in degrading linear and branched alkanes, plays a key ecological role in the removal of marine oil spills. It contains several alternative enzyme systems for terminal hydroxylation of alkanes, including three P450 cytochromes (P450-1, P450-2 and P450-3). The present work shows cytochrome P450-1 to be expressed from the promoter of the upstream gene fdx. Promoter Pfdx was more active when C8 -C18 n-alkanes or pristane were assimilated than when pyruvate was available. The product of ABO_0199 (named CypR) was identified as a transcriptional activator of Pfdx . The inactivation of cypR impaired growth on tetradecane, showing the importance of the fdx-P450-1 and/or cypR genes. P450-2 expression was low-level and constitutive under all conditions tested, while that of P450-3 from promoter P450-3 was much higher when cells assimilated pristane than when n-alkanes or pyruvate were available. However, the inactivation of P450-3 had no visible impact on pristane assimilation. Cyo terminal oxidase, a component of the electron transport chain, was found to stimulate promoter PP450-3 activity, but it did not affect promoters Pfdx or PP450-2 . A. borkumensis, therefore, appears to carefully coordinate the expression of its multiple hydrocarbon degradation genes using both specific and global regulatory systems.

Marine hydrocarbonoclastic bacteria as whole-cell biosensors for n-alkanes.

Whole-cell biosensors offer potentially useful, cost-effective systems for the in-situ monitoring of seawater for hydrocarbons derived from accidental spills. The present work compares the performance of a biosensor system for the detection of alkanes in seawater, hosted in either Escherichia coli (commonly employed in whole-cell biosensors but not optimized for alkane assimilation) or different marine bacteria specialized in assimilating alkanes. The sensor system was based on the Pseudomonas putida AlkS regulatory protein and the PalkB promoter fused to a gene encoding the green fluorescent protein. While the E. coli sensor provided the fastest response to pure alkanes (25-fold induction after 2 h under the conditions used), a sensor based on Alcanivorax borkumensis was slower, requiring 3-4 h to reach similar induction values. However, the A. borkumensis sensor showed a fourfold lower detection threshold for octane (0.5 µM), and was also better at sensing the alkanes present in petrol. At petrol concentrations of 0.0125%, the A. borkumensis sensor rendered a sevenfold induction, while E. coli sensor showed no response. We discuss possible explanations to this behaviour in terms of the cellular adaptations to alkane uptake and the basal fluorescence produced by each bacterial strain, which was lowest for A. borkumensis.

The Pseudomonas putida†HskA hybrid sensor kinase responds to redox signals and contributes to the adaptation of the electron transport chain composition in response to oxygen availability.

Pseudomonas putida has a branched aerobic electron transport that includes five terminal oxidases, each of which has different properties. The relative expression of each oxidase is carefully regulated to assemble the most suitable electron transport chain for the prevailing conditions. The HskA hybrid sensor kinase participates in this control, but the signals to which HskA responds were unknown. Here, the influence of HskA on the mRNA abundance of genes coding for all terminal oxidases and for the bc1 complex was analysed in cells growing under controlled aerobic, semiaerobic or microaerobic conditions. The results indicate that the influence of HskA on the expression of each terminal oxidase and the bc1 complex varies depending on oxygen availability. This effect was more pronounced under aerobic or semiaerobic conditions, but decreased under microaerobic conditions. The expression of hskA was regulated by oxygen availability. We show that HskA autophosphorylation is inhibited by ubiquinone but not by ubiquinol, its reduced derivative. This suggests that HskA could sense the oxidation state of the respiratory ubiquinones, which may be a key factor in HskA activity. Inactivation of hskA reduced growth rate and oxygen consumption, stressing the importance of HskA for the assembly of an efficient electron transport chain.

The Pseudomonas putida HskA hybrid sensor kinase controls the composition of the electron transport chain.

Sensor kinases play a key role in sensing and responding to environmental and physiological signals in bacteria. In this study we characterized a previously unknown orphan hybrid sensor kinase from Pseudomonas putida, which is conserved in several Pseudomonads. Inactivation of the gene coding for this sensor kinase, which we have named HskA, modified the expression of at least 85 genes in cells growing in a complete medium. HskA showed a strong influence on the composition of the electron transport chain. In cells growing exponentially in a complete medium, the absence of HskA led to a significant reduction in the expression of the genes coding for the bc1 complex and for the CIO and Cbb3-1 terminal oxidases. In stationary phase cells, however, lack of HskA caused a higher expression of the Cyo terminal oxidase and a lower expression of the Aa3 terminal oxidase. The HskA polypeptide shows two PAS (signal-sensing) domains, a transmitter domain containing the invariant phosphorylatable histidine and an ATP binding site, and a receiver domain containing the conserved aspartate capable of transphosphorylation, but lacks an Hpt module. It is therefore a hybrid sensor kinase. Phosphorylation assays showed that purified HskA undergoes autophosphorylation in the presence of ATP.

Phosphate deficiency (N/P 40:1) induces mcyD transcription and microcystin synthesis in Microcystis aeruginosa PCC7806.

A real-time RT-PCR analysis of the transcriptional response to phosphate availability of the mcyD gene and microcystin-LR synthesis in Microcystis aeruginosa PCC7806 revealed that no significant changes were observed in the relative quantification of mcyD under excess phosphate (N/P = 1:1), whereas in deficiency of this nutrient (N/P = 40:1), a steady increase of mcyD during the exponential growth phase was detected, showing a maximal level on the 7th day of growth with a 6.8-fold increase over the control cells. The microcystin content in phosphate deficient cells correlates with the trend of mcyD transcription observed. Also, in this work we demonstrate that under phosphate deficiency conditions with a ratio of 40:1 N/P, the growth of M. aeruginosa PCC7806 was not affected when compared to control and phosphate excess samples. When blooms occur, the nutrients become exhausted and therefore phosphate availability will be scarce. In such a complex scenario, microcystin synthesis could be a response to phosphate deficiency, among other stress parameters.

An active photosynthetic electron transfer chain required for mcyD transcription and microcystin synthesis in Microcystis aeruginosa PCC7806.

In this study, quantitative real time RT-PCR has been used to monitor changes in the levels of transcripts encoding mcyD in Microcystis aeruginosa PCC7806 under oxidative agents and different conditions of light intensity. Microcystin content has also been determined in the same stressed cell aliquots. Our results corroborate the fact that changes in light intensities are able to induce mcyD gene transcription, but our data show that this is an early and short-term event. mcyD transcription requires an active photosynthetic electron transfer chain and the increased transcript level as a consequence of light is not related to oxidative stress. Indeed, oxidative stress leads to a general trend of a decrease of mcyD trancript. Microcystin amount found in the cells follows a tendency consistent with the mcyD transcript level. In summary, the data indicate that the synthesis of microcystin is dependent on photosynthesis, and also show that oxidative stress decreases the microcystin synthesis in toxigenic Microcystis.

Identification of three novel antisense RNAs in the fur locus from unicellular cyanobacteria.

The interplay between Fur (ferric uptake regulator) proteins and small, non-coding RNAs has been described as a key regulatory loop in several bacteria. In the filamentous cyanobacterium Anabaena sp. PCC 7120, a large dicistronic transcript encoding the putative membrane protein Alr1690 and an α-furA RNA is involved in the modulation of the global regulator FurA. In this work we report the existence of three novel antisense RNAs in cyanobacteria and show that a cis α-furA RNA is conserved in very different genomic contexts, namely in the unicellular cyanobacteria Microcystis aeruginosa PCC 7806 and Synechocystis sp. PCC 6803. Syα-fur RNA covers only part of the coding sequence of the fur orthologue sll0567, whose flanking genes encode two hypothetical proteins. Transcriptional analysis of fur and its adjacent genes in Microcystis unravels a highly compact organization of this locus involving overlapping transcripts. Maα-fur RNA spans the whole Mafur CDS and part of the flanking dnaJ and sufE sequences. In addition, Mafur seems to be part of a dicistronic operon encoding this regulator and an α-sufE RNA. These results allow new insights into the transcriptomes of two unicellular cyanobacteria and suggest that in M. aeruginosa PCC 7806, the α-fur and α-sufE RNAs might participate in a regulatory connection between the genes of the dnaJ-fur-sufE locus.

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.