Molecular cloning of the cyanobacterial adenylate cyclase gene from the filamentous cyanobacterium Anabaena cylindrica

The mechanism of iron homeostasis in the unicellular cyanobacterium synechocystis sp. PCC 6803 and its relationship to oxidative stress

In this article, we demonstrate the connection between intracellular iron storage and oxidative stress response in cyanobacteria. Iron is essential for the survival of all organisms. However, the redox properties that make iron a valuable cofactor also lead to oxidative interactions, resulting in the formation of harmful radicals. Therefore, iron accumulation in cells should be tightly regulated, a process in which ferritin family proteins play an important role. Synechocystis sp. PCC 6803 contains two ferritin-type storage complexes, bacterioferritin and MrgA. Previous studies demonstrated the role of bacterioferritin and MrgA in iron storage. In addition, MrgA was found to play a key role in oxidative stress response. Here, we examined the dual role of the ferritin family proteins using physiological and transcriptomic approaches. Microarray analysis of iron-limited wild-type and DeltamrgA cultures revealed a substantial up-regulation of oxidative stress-related genes in mutant cells. The PerR regulator was found to play an important role in that process. Furthermore, we were able to demonstrate the connection between internal iron quota, the presence of the two storage complexes, and the sensitivity to externally applied oxidative stress. These data suggest a pivotal role for the ferritin-type proteins of Synechocystis sp. PCC 6803 in coordinating iron homeostasis and in oxidative stress response. The combined action of the two complexes allows for the safe accumulation and release of iron from storage by minimizing damage resulting from interactions between reduced iron and the oxygen radicals that are produced in abundance by the photosynthetic apparatus.

A phytochrome-like protein AphC triggers the cAMP signaling induced by far-red light in the cyanobacterium Anabaena sp. strain PCC7120

In the filamentous, nitrogen-fixing cyanobacterium Anabaena sp. PCC7120, red light (630 nm) decreased, whereas far-red light (720 nm) increased cellular adenosine 3',5'-cyclic monophosphate (cAMP) content. To find a red and far-red light photoreceptor that triggers the cAMP signal cascade, we disrupted 10 open reading frame having putative chromophore-binding GAF domains. The response of the cellular cAMP concentration to red and far-red light in each open reading frame disruptant was determined. It was found that only the mutant of the gene all2699 failed to respond to far-red light. The open reading frame named as aphC encoded a protein with 920 amino acids including GAF domains similar to those involved in Cph2, a photoreceptor of Synechocystis sp. PCC6803. To determine which adenylate cyclase (AC) is responsible for far-red light signal, we disrupted all AC genes and found that CyaC was the candidate. The enzymatic activity of CyaC might be controlled by a far-red light photoreceptor through the phosphotransfer reaction. The site-specific mutant of the Asp59 residue of the receiver (R1) domain of CyaC lost its light-response capability. It was suggested that the far-red light signal was received by AphC and then transferred to the N-terminal response regulator domain of CyaC. Then its catalytic activity was stimulated, which increased the cellular cAMP concentration and drove the subsequent signal transduction cascade.

Blue light stimulates cyanobacterial motility via a cAMP signal transduction system

The participation of cAMP in photosignal transduction in cyanobacteria was investigated. When cells of the cyanobacterium Synechocystis sp. PCC 6803 were exposed to light, cellular cAMP contents increased within a few minutes. Among incident monochromatic lights, blue light (450 nm) markedly increased cellular cAMP content, while red (630 nm) and far-red (720 nm) lights did not. Disruption of the cya1 gene encoding an adenylate cyclase caused the insensitivity of cellular cAMP level to blue light. Treatment of wild-type cells with the flavin antagonist phenylacetic acid inhibited this blue light effect. The motility of wild-type cells was enhanced by blue light, whereas that of cya1 mutant cells was not. Based on these results, we concluded that a blue light-cAMP signal transduction system stimulates the motility of Synechocystis sp. PCC 6803.

Regulation of cAMP-mediated photosignaling by a phytochrome in the cyanobacterium Anabaena cylindrica

Changes in cellular adenosine 3',5'-cyclic monophosphate (cAMP) content induced by monochromatic light of various wavelengths were determined in the cyanobacterium Anabaena cylindrica. Irradiation with monochromatic red light caused a rapid decrease in cAMP content. In contrast, far-red light caused a rapid increase in its content. The effects of red and far-red light were reversible, suggesting the involvement of a prototype phytochrome as the photoreceptor for cAMP-mediated light-responsive signal transduction.

A blue-light-activated adenylyl cyclase mediates photoavoidance in Euglena gracilis

Blue light regulates processes such as the development of plants and fungi and the behaviour of microbes. Two types of blue-light receptor flavoprotein have been identified: cryptochromes, which have partial similarity to photolyases, and phototropins, which are photoregulated protein kinases. The former have also been found in animals with evidence of essential roles in circadian rhythms. Euglena gracilis, a unicellular flagellate, abruptly changes its swimming direction after a sudden increase or decrease in incident blue light intensity, that is, step-up or step-down photophobic responses, resulting in photoavoidance or photoaccumulation, respectively. Although these photobehaviours of Euglena have been studied for a century, the photoreceptor molecules mediating them have remained unknown. Here we report the discovery and biochemical characterization of a new type of blue-light receptor flavoprotein, photoactivated adenylyl cyclase, in the photoreceptor organelle of Euglena gracilis, with molecular genetic evidence that it mediates the step-up photophobic response.

Screening for the target gene of cyanobacterial cAMP receptor protein SYCRP1

The target genes for SYCRP1, a cyanobacterial cAMP receptor protein, were surveyed using a DNA microarray method. Total RNAs were extracted from a wild-type strain and a sycrp1 disruptant of Synechocystis sp. PCC 6803, and the respective gene expression levels were compared. The expression levels of six genes (slr1667, slr1168, slr2015, slr2016, slr2017 and slr2018) were clearly decreased by the disruption of the sycrp1 gene. The data suggest that slr1667 and slr1668 constitute one operon and the other four genes constitute another operon. Transcription start points for the first genes of these putative operons, which are slr1667 and slr2015, were determined by primer extension experiments. Gel mobility shift assays and DNase 1 footprint analyses were carried out to explore the binding of SYCRP1 to the putative promoter regions of slr1667 and slr2015. SYCRP1 bound to the specific site in the 5' upstream region of slr1667 from positions -170 to -155 relative to the transcription start point, while it did not bind to the 5' upstream region of slr2015. It was concluded that SYCRP1 regulates the expression of the slr1667 gene directly by binding to a specific site in its promoter.

cAMP acts as a second messenger in pollen tube growth and reorientation

Pollen tube growth and reorientation is a prerequisite for fertilization and seed formation. Here we report imaging of cAMP distribution in living pollen tubes microinjected with the protein kinase A-derived fluorosensor. Growing tubes revealed a uniform distribution of cAMP with a resting concentration of approximately 100-150 nM. Modulators of adenylyl cyclase (AC), forskolin, and dideoxyadenosine could alter these values. Transient elevations in the apical region could be correlated with changes in the tube-growth axis, suggesting a role for cAMP in polarized growth. Changes in cAMP arise through the activity of a putative AC identified in pollen. This signaling protein shows homology to functional motifs in fungal AC. Expression of the cDNA in Escherichia coli resulted in cAMP increase and complemented a catabolic defect in the fermentation of carbohydrates caused by the absence of cAMP in a cyaA mutant. Antisense assays performed with oligodeoxynucleotide probes directed against conserved motifs perturbed tip growth, suggesting that modulation of cAMP concentration is vital for tip growth.

Activation of a cyanobacterial adenylate cyclase, CyaC, by autophosphorylation and a subsequent phosphotransfer reaction

The CyaC protein, a cyanobacterial adenylate cyclase, has a unique primary structure composed of the catalytic domain of adenylate cyclase and the conserved domains of bacterial two-component regulatory systems, one transmitter domain and two receiver domains. In the present work, CyaC was produced in Escherichia coli as a histidine-tagged recombinant protein and purified to homogeneity. CyaC showed ability to autophosphorylate in vitro with the gamma-phosphate of [gamma-32P]ATP. CyaC derivatives were constructed by site-directed mutagenesis in which the highly conserved phosphorylation sites in the transmitter domain (His572) and receiver domains (Asp60 or Asp895) were replaced by glutamine and alanine residues, respectively. After autophosphorylation of the CyaC derivatives, the chemical stabilities of the phosphoryl groups bound to the derivatives were determined. It was found that His572 is the initial phosphorylation site and that the phosphoryl group once bound to His572 is transferred to Asp895. The enzyme activities of the CyaC derivatives defective in His572 or Asp895 were considerably reduced. Asp895 is phosphorylated by acetyl [32P]phosphate, a small phosphoryl molecule, but Asp60 is not. Acetyl phosphate stimulates adenylate cyclase activity only when Asp895 is intact. These results suggest that the phosphorylation of Asp895 is essential for the activation of adenylate cyclase and that Asp60 functions differently from Asp895 in regulating the enzyme activity.

Isolation and characterization of multiple adenylate cyclase genes from the cyanobacterium Anabaena sp. strain PCC 7120

Adenylate cyclase genes, designated cyaA, cyaB1, cyaB2, cyaC, and cyaD, were isolated from the filamentous cyanobacterium Anabaena sp. strain PCC 7120 by complementation of a strain of Escherichia coli defective for the presence of cya. These genes encoded polypeptides consisting of 735, 859, 860, 1,155, and 546 amino acid residues, respectively. Deduced amino acid sequences of the regions near the C-terminal ends of these cya genes were similar to those of catalytic domains of eukaryotic adenylate cyclases. The remaining part of each cya gene towards its N-terminal end showed a characteristic structure. CyaA had two putative membrane-spanning regions. Both CyaB1 and CyaB2 had regions that were very similar to the cyclic GMP (cGMP)-binding domain of cGMP-stimulated cGMP phosphodiesterase. CyaC consisted of four distinct domains forming sequentially from the N terminus: a response regulator-like domain, a histidine kinase-like domain, a response regulator-like domain, and the catalytic domain of adenylate cyclase. CyaD contained the forkhead-associated domain in its N-terminal region. Expression of these genes was examined by reverse transcription-PCR. The transcript of cyaC was shown to be predominant in this cyanobacterium. The cellular cyclic AMP level in the disruptant of the cyaC mutant was much lower than that in the wild type.

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.

Molecular characterization of an adenylate cyclase gene of the cyanobacterium Spirulina platensis

A cyaA gene, encoding an adenylate cyclase, was isolated from a filamentous cyanobacterium, Spirulina platensis, by functional complementation of a cya mutant of Escherichia coli, defective in adenylate cyclase activity. The predicted gene product of cyaA contains a signal peptide-like domain, a putative sensor domain similar to the gene product of vsrA of Pseudomonas solanacearum, a putative membrane-spanning domain and an adenylate cyclase-like catalytic domain. Two other positive clones that complemented the E. coli mutant were isolated from the same cyanobacterium, suggesting that several cya genes are functioning in S. platensis.