The coxBAC operon encodes a cytochrome c oxidase required for heterotrophic growth in the cyanobacterium Anabaena variabilis strain ATCC 29413

Heterotrophy among Cyanobacteria

Cyanobacteria have been studied in recent decades to investigate the principle mechanisms of plant-type oxygenic photosynthesis, as they are the inventors of this process, and their cultivation and research is much easier compared to land plants. Nevertheless, many cyanobacterial strains possess the capacity for at least some forms of heterotrophic growth. This review demonstrates that cyanobacteria are much more than simple photoautotrophs, and their flexibility toward different environmental conditions has been underestimated in the past. It summarizes the strains capable of heterotrophy known by date structured by their phylogeny and lists the possible substrates for heterotrophy for each of them in a table in the Supporting Information. The conditions are discussed in detail that cause heterotrophic growth for each strain in order to allow for reproduction of the results. The review explains the importance of this knowledge for the use of new methods of cyanobacterial cultivation, which may be advantageous under certain conditions. It seeks to stimulate other researchers to identify new strains capable of heterotrophy that have not been known so far.

Phylogenetic and functional analysis of cyanobacterial Cytochrome c6-like proteins

All known photosynthetic cyanobacteria carry a cytochrome c 6 protein that acts transferring electrons from cytochrome b 6 f complex to photosystem I, in photosynthesis, or cytochrome c oxidase, in respiration. In most of the cyanobacteria, at least one homologue to cytochrome c 6 is found, the so-called cytochrome c 6B or cytochrome c 6C. However, the function of these cytochrome c 6-like proteins is still unknown. Recently, it has been proposed a common origin of these proteins as well as the reclassification of the cytochrome c 6C group as c 6B, renaming the new joint group as cytochrome c 6BC. Another homologue to cytochrome c 6 has not been classified yet, the formerly called cytochrome c 6-3, which is present in the heterocyst-forming filamentous cyanobacteria Nostoc sp. PCC 7119. In this work, we propose the inclusion of this group as an independent group in the genealogy of cytochrome c 6-like proteins with significant differences from cytochrome c 6 and cytochrome c 6BC, with the proposed name cytochrome c 6D. To support this proposal, new data about phylogeny, genome localisation and functional properties of cytochrome c 6-like proteins is provided. Also, we have analysed the interaction of cytochrome c 6-like proteins with cytochrome f by isothermal titration calorimetry and by molecular docking, concluding that c 6-like proteins could interact with cytochrome b 6 f complex in a similar fashion as cytochrome c 6. Finally, we have analysed the reactivity of cytochrome c 6-like proteins with membranes enriched in terminal oxidases of cyanobacteria by oxygen uptake experiments, concluding that cytochrome c 6D is able to react with the specific copper-oxidase of the heterocysts, the cytochrome c oxidase 2.

The Synthesis and Assembly of a Truncated Cyanophage Genome and Its Expression in a Heterogenous Host

Cyanophages play an important role in regulating the dynamics of cyanobacteria communities in the hydrosphere, representing a promising biological control strategy for cyanobacterial blooms. Nevertheless, most cyanophages are host-specific, making it difficult to control blooming cyanobacteria via single or multiple cyanophages. In order to address the issue, we explore the interaction between cyanophages and their heterologous hosts, with the aim of revealing the principles of designing and constructing an artificial cyanophage genome towards multiple cyanobacterial hosts. In the present study, we use synthetic biological approaches to assess the impact of introducing a fragment of cyanophage genome into a heterologous cyanobacterium under a variety of environmental conditions. Based on a natural cyanophage A-4L genome (41,750 bp), a truncated cyanophage genome Syn-A-4-8 is synthesized and assembled in Saccharomyces cerevisiae. We found that a 351-15,930 bp area of the A-4L genome has a fragment that is lethal to Escherichia coli during the process of attempting to assemble the full-length A-4L genome. Syn-A-4-8 was successfully introduced into E. coli and then transferred into the model cyanobacterium Synechococcus elongatus PCC 7942 (Syn7942) via conjugation. Although no significant phenotypes of Syn7942 carrying Syn-A-4-8 (LS-02) could be observed under normal conditions, its growth exhibited a prolonged lag phase compared to that of the control strain under 290-millimolar NaCl stress. Finally, the mechanisms of altered salt tolerance in LS-02 were revealed through comparative transcriptomics, and ORF25 and ORF26 on Syn-A-4-8 turned out to be the key genes causing the phenotype. Our research represents an important attempt in designing artificial cyanophages towards multiple hosts, and offers new future insights into the control of cyanobacterial blooms.

Large-scale protein level comparison of Deltaproteobacteria reveals cohesive metabolic groups

Deltaproteobacteria, now proposed to be the phyla Desulfobacterota, Myxococcota, and SAR324, are ubiquitous in marine environments and play essential roles in global carbon, sulfur, and nutrient cycling. Despite their importance, our understanding of these bacteria is biased towards cultured organisms. Here we address this gap by compiling a genomic catalog of 1 792 genomes, including 402 newly reconstructed and characterized metagenome-assembled genomes (MAGs) from coastal and deep-sea sediments. Phylogenomic analyses reveal that many of these novel MAGs are uncultured representatives of Myxococcota and Desulfobacterota that are understudied. To better characterize Deltaproteobacteria diversity, metabolism, and ecology, we clustered ~1 500 genomes based on the presence/absence patterns of their protein families. Protein content analysis coupled with large-scale metabolic reconstructions separates eight genomic clusters of Deltaproteobacteria with unique metabolic profiles. While these eight clusters largely correspond to phylogeny, there are exceptions where more distantly related organisms appear to have similar ecological roles and closely related organisms have distinct protein content. Our analyses have identified previously unrecognized roles in the cycling of methylamines and denitrification among uncultured Deltaproteobacteria. This new view of Deltaproteobacteria diversity expands our understanding of these dominant bacteria and highlights metabolic abilities across diverse taxa.

The regulation of oxidative phosphorylation pathway on Vibrio alginolyticus adhesion under adversities

Vibrio alginolyticus is one of the most important pathogens in mariculture and leading to heavy losses. After treatment with Cu²⁺, Pb²⁺, and low pH, the expression of oxidative phosphorylation pathway genes, including coxA, coxB, coxC, ccoN, ccoO, and ccoQ, was found commonly downregulated by RNA‐seq as well as quantitative real‐time PCR. RNAi significantly reduced the expression of coxA, coxB, coxC, ccoN, ccoO, and ccoQ in V. alginolyticus. Compared with the wild‐type strain, the adhesion abilities of RNAi strains of V. alginolyticus were significantly impaired, as well as their cytochrome C oxidase activity. ccoQ appeared to be more important in the regulation of bacterial adhesion in these target genes, while ccoO was relatively weak in the regulation of the adhesion. Meanwhile, the changes of temperature, salinity, pH, and starvation affected coxA, coxB, coxC, ccoN, ccoO, and ccoQ expression remarkably. These findings indicated that: the oxidative phosphorylation pathway is a critical regulator of adhesion in V. alginolyticus; coxA, coxB, coxC, ccoN, ccoO, and ccoQ regulate the bacterial adhesion in response to environmental changes such as temperature, salinity, pH, and starvation.

Cytochrome c6 is the main respiratory and photosynthetic soluble electron donor in heterocysts of the cyanobacterium Anabaena sp. PCC 7120

Cytochrome c₆ is a soluble electron carrier, present in all known cyanobacteria, that has been replaced by plastocyanin in plants. Despite their high structural differences, both proteins have been reported to be isofunctional in cyanobacteria and green algae, acting as alternative electron carriers from the cytochrome b₆-f complex to photosystem I or terminal oxidases. We have investigated the subcellular localization of both cytochrome c₆ and plastocyanin in the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 grown in the presence of combined nitrogen and under diazotrophic conditions. Our studies conclude that cytochrome c₆ is expressed at significant levels in heterocysts, even in the presence of copper, condition in which it is strongly repressed in vegetative cells. However, the copper-dependent regulation of plastocyanin is not altered in heterocysts. In addition, in heterocysts, cytochrome c₆ has shown to be the main soluble electron carrier to cytochrome c oxidase-2 in respiration. A cytochrome c₆ deletion mutant is unable to grow under diazotrophic conditions in the presence of copper, suggesting that cytochrome c₆ plays an essential role in the physiology of heterocysts that cannot be covered by plastocyanin.

The Complex Transcriptional Response of Acaryochloris marina to Different Oxygen Levels

Ancient oxygenic photosynthetic prokaryotes produced oxygen as a waste product, but existed for a long time under an oxygen-free (anoxic) atmosphere, before an oxic atmosphere emerged. The change in oxygen levels in the atmosphere influenced the chemistry and structure of many enzymes that contained prosthetic groups that were inactivated by oxygen. In the genome of Acaryochloris marina, multiple gene copies exist for proteins that are normally encoded by a single gene copy in other cyanobacteria. Using high throughput RNA sequencing to profile transcriptome responses from cells grown under microoxic and hyperoxic conditions, we detected 8446 transcripts out of the 8462 annotated genes in the Cyanobase database. Two-thirds of the 50 most abundant transcripts are key proteins in photosynthesis. Microoxic conditions negatively affected the levels of expression of genes encoding photosynthetic complexes, with the exception of some subunits. In addition to the known regulation of the multiple copies of psbA, we detected a similar transcriptional pattern for psbJ and psbU, which might play a key role in the altered components of photosystem II. Furthermore, regulation of genes encoding proteins important for reactive oxygen species-scavenging is discussed at genome level, including, for the first time, specific small RNAs having possible regulatory roles under varying oxygen levels.

Identifying the Metabolic Differences of a Fast-Growth Phenotype in Synechococcus UTEX 2973

The photosynthetic capabilities of cyanobacteria make them interesting candidates for industrial bioproduction. One obstacle to large-scale implementation of cyanobacteria is their limited growth rates as compared to industrial mainstays. Synechococcus UTEX 2973, a strain closely related to Synechococcus PCC 7942, was recently identified as having the fastest measured growth rate among cyanobacteria. To facilitate the development of 2973 as a model organism we developed in this study the genome-scale metabolic model iSyu683. Experimental data were used to define CO₂ uptake rates as well as the biomass compositions for each strain. The inclusion of constraints based on experimental measurements of CO₂ uptake resulted in a ratio of the growth rates of Synechococcus 2973 to Synechococcus 7942 of 2.03, which nearly recapitulates the in vivo growth rate ratio of 2.13. This identified the difference in carbon uptake rate as the main factor contributing to the divergent growth rates. Additionally four SNPs were identified as possible contributors to modified kinetic parameters of metabolic enzymes and candidates for further study. Comparisons against more established cyanobacterial strains identified a number of differences between the strains along with a correlation between the number of cytochrome c oxidase operons and heterotrophic or diazotrophic capabilities.

Diazotrophic specific cytochrome c oxidase required to overcome light stress in the cyanobacterium Nostoc muscorum

Diazotrophic, filamentous and heterocystous cyanobacterium Nostoc muscorum perform photosynthesis in vegetative whereas nitrogen fixation occurs in heterocyst only. However, despite their metabolic plasticity, respiration takes place both in vegetative cells and heterocysts. The role of the respiratory electron transport system and terminal oxidases under light stress is not evident so far. As compared to the diazotrophically grown cultures, the non-diazotrophically grown cultures of the N. muscorum show a slight decrease in their growth, chlorophyll a contents and photosynthetic O2 evolution under light stress. Whereas respiratory O2 uptake under identical stress condition increases several fold. Likewise, nitrogen fixing enzyme i.e. nitrogenase over-expresses itself under light stress condition. The terminal enzyme of respiratory electron transport chain i.e. cytochrome c oxidase shows more activity under light stress, whilst light stress has no impact on Ca(++)-dependent ATPase activity. This leads to the conclusion that under light stress, cytochrome c oxidase plays a vital role in mitigating given light stress.

Influence of mixotrophic growth on rhythmic oscillations in expression of metabolic pathways in diazotrophic cyanobacterium Cyanothece sp. ATCC 51142

This study investigates the influence of mixotrophy on physiology and metabolism by analysis of global gene expression in unicellular diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 (henceforth Cyanothece 51142). It was found that Cyanothece 51142 continues to oscillate between photosynthesis and respiration in continuous light under mixotrophy with cycle time of ∼ 13 h. Mixotrophy is marked by an extended respiratory phase compared with photoautotrophy. It can be argued that glycerol provides supplementary energy for nitrogen fixation, which is derived primarily from the glycogen reserves during photoautotrophy. The genes of NDH complex, cytochrome c oxidase and ATP synthase are significantly overexpressed in mixotrophy during the day compared to autotrophy with synchronous expression of the bidirectional hydrogenase genes possibly to maintain redox balance. However, nitrogenase complex remains exclusive to nighttime metabolism concomitantly with uptake hydrogenase. This study throws light on interrelations between metabolic pathways with implications in design of hydrogen producer strains.

Atrazine affects the circadian rhythm of Microcystis aeruginosa

This study provides original data regarding the effects of atrazine (Atr) on the circadian rhythm of the cyanobacterium Microcystis aeruginosa. The results reveal that the circadian rhythms of the central circadian oscillator genes reached their peaks from 1 to 2.5 h after the light was switched on, and the circadian rhythms of physiologically related genes were highly synchronized with the central circadian oscillator genes. These circadian rhythms were consistent with cell growth at the physiological level. The circadian rhythms of the central circadian oscillator genes were altered, and their peaks disappeared or were delayed by the Atr treatment. Therefore, the rhythms of the physiologically related genes in this study also changed to synchronize the new circadian rhythms. And the physiological parameters were tightly correlated with the gene circadian rhythm in the Atr treatment, suggesting that Atr affects M. aeruginosa growth by possibly altering the circadian expression patterns of the clock. Furthermore, this influence is related to the exposure time point of Atr. Thus, chemicals treated in the suitable exposure time point can exert their fullest effects against cell growth.

Chemoheterotrophic growth of the Cyanobacterium Anabaena sp. strain PCC 7120 dependent on a functional cytochrome c oxidase

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium commonly used as a model organism for studying cyanobacterial cell differentiation and nitrogen fixation. For many decades, this cyanobacterium was considered an obligate photo-lithoautotroph. We now discovered that this strain is also capable of mixotrophic, photo-organoheterotrophic, and chemo-organoheterotrophic growth if high concentrations of fructose (at least 50 mM and up to 200 mM) are supplied. Glucose, a substrate used by some facultatively organoheterotrophic cyanobacteria, is not effective in Anabaena sp. PCC 7120. The gtr gene from Synechocystis sp. PCC 6803 encoding a glucose carrier was introduced into Anabaena sp. PCC 7120. Surprisingly, the new strain containing the gtr gene did not grow on glucose but was very sensitive to glucose, with a 5 mM concentration being lethal, whereas the wild-type strain tolerated 200 mM glucose. The Anabaena sp. PCC 7120 strain containing gtr can grow mixotrophically and photo-organoheterotrophically, but not chemo-organoheterotrophically with fructose. Anabaena sp. PCC 7120 contains five respiratory chains ending in five different respiratory terminal oxidases. One of these enzymes is a mitochondrial-type cytochrome c oxidase. As in almost all cyanobacteria, this enzyme is encoded by three adjacent genes called coxBAC1. When this locus was disrupted, the cells lost the capability for chemo-organoheterotrophic growth.

Synthetic biology in cyanobacteria engineering and analyzing novel functions

Cyanobacteria are the only prokaryotes capable of using sunlight as their energy, water as an electron donor, and air as a source of carbon and, for some nitrogen-fixing strains, nitrogen. Compared to algae and plants, cyanobacteria are much easier to genetically engineer, and many of the standard biological parts available for Synthetic Biology applications in Escherichia coli can also be used in cyanobacteria. However, characterization of such parts in cyanobacteria reveals differences in performance when compared to E. coli, emphasizing the importance of detailed characterization in the cellular context of a biological chassis. Furthermore, cyanobacteria possess special characteristics (e.g., multiple copies of their chromosomes, high content of photosynthetically active proteins in the thylakoids, the presence of exopolysaccharides and extracellular glycolipids, and the existence of a circadian rhythm) that have to be taken into account when genetically engineering them. With this chapter, the synthetic biologist is given an overview of existing biological parts, tools and protocols for the genetic engineering, and molecular analysis of cyanobacteria for Synthetic Biology applications.

RNA processing of nitrogenase transcripts in the cyanobacterium Anabaena variabilis

Little is known about the regulation of nitrogenase genes in cyanobacteria. Transcription of the nifH1 and vnfH genes, encoding dinitrogenase reductases for the heterocyst-specific Mo-nitrogenase and the alternative V-nitrogenase, respectively, was studied by using a lacZ reporter. Despite evidence for a transcription start site just upstream of nifH1 and vnfH, promoter fragments that included these start sites did not drive the transcription of lacZ and, for nifH1, did not drive the expression of nifHDK1. Further analysis using larger regions upstream of nifH1 indicated that a promoter within nifU1 and a promoter upstream of nifB1 both contributed to expression of nifHDK1, with the nifB1 promoter contributing to most of the expression. Similarly, while the region upstream of vnfH, containing the putative transcription start site, did not drive expression of lacZ, the region that included the promoter for the upstream gene, ava4055, did. Characterization of the previously reported nifH1 and vnfH transcriptional start sites by 5'RACE (5' rapid amplification of cDNA ends) revealed that these 5' ends resulted from processing of larger transcripts rather than by de novo transcription initiation. The 5' positions of both the vnfH and nifH1 transcripts lie at the base of a stem-loop structure that may serve to stabilize the nifHDK1 and vnfH specific transcripts compared to the transcripts for other genes in the operons providing the proper stoichiometry for the Nif proteins for nitrogenase synthesis.

Heme-copper oxidases and their electron donors in cyanobacterial respiratory electron transport

Cyanobacteria are the paradigmatic organisms of oxygenic (plant-type) photosynthesis and aerobic respiration. Since there is still an amazing lack of knowledge on the role and mechanism of their respiratory electron transport, we have critically analyzed all fully or partially sequenced genomes for heme-copper oxidases and their (putative) electron donors cytochrome c(6), plastocyanin, and cytochrome c(M). Well-known structure-function relationships of the two branches of heme-copper oxidases, namely cytochrome c (aa(3)-type) oxidase (COX) and quinol (bo-type) oxidase (QOX), formed the base for a critical inspection of genes and ORFs found in cyanobacterial genomes. It is demonstrated that at least one operon encoding subunits I-III of COX is found in all cyanobacteria, whereas many non-N(2)-fixing species lack QOX. Sequence analysis suggests that both cyanobacterial terminal oxidases should be capable of both the four-electron reduction of dioxygen and proton pumping. All diazotrophic organisms have at least one operon that encodes QOX. In addition, the highly refined specialization in heterocyst forming Nostocales is reflected by the presence of two paralogs encoding COX. The majority of cyanobacterial genomes contain one gene or ORF for plastocyanin and cytochrome c(M), whereas 1-4 paralogs for cytochrome c(6) were found. These findings are discussed with respect to published data about the role of respiration in wild-type and mutated cyanobacterial strains in normal metabolism, stress adaptation, and nitrogen fixation. A model of the branched electron-transport pathways downstream of plastoquinol in cyanobacteria is presented.

Heterocyst development and diazotrophic metabolism in terminal respiratory oxidase mutants of the cyanobacterium Anabaena sp. strain PCC 7120

Heterocyst development was analyzed in mutants of the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 bearing inactivated cox2 and/or cox3 genes, encoding heterocyst-specific terminal respiratory oxidases. At the morphological level, the cox2 cox3 double mutant (strain CSAV141) was impaired in membrane reorganization involving the so-called honeycomb system that in the wild-type strain is largely or exclusively devoted to respiration, accumulated glycogen granules at conspicuously higher levels than the wild type (in both vegetative cells and heterocysts), and showed a delay in carboxysome degradation upon combined nitrogen deprivation. Consistently, chemical analysis confirmed higher accumulation of glycogen in strain CSAV141 than in the wild type. No impairment was observed in the formation of the glycolipid or polysaccharide layers of the heterocyst envelope, consistent with the chemical detection of heterocyst-specific glycolipids, or in the expression of the heterocyst-specific genes nifHDK and fdxH. However, nitrogenase activity under oxic conditions was impaired in strain CSAV135 (cox3) and undetectable in strain CSAV141 (cox2 cox3). These results show that these dedicated oxidases are required for normal development and performance of the heterocysts and indicate a central role of Cox2 and, especially, of Cox3 in the respiratory activity of the heterocysts, decisively contributing to protection of the N(2) fixation machinery against oxygen. However, in contrast to the case for other diazotrophic bacteria, expression of nif genes in Anabaena seems not to be affected by oxygen.

Roles for heme–copper oxidases in extreme high-light and oxidative stress response in the cyanobacterium Synechococcus sp. PCC 7002

The ctaCIDIEI and ctaCIIDIIEII gene clusters that encode heme-copper cytochrome oxidases have been characterized in the marine cyanobacterium Synechococcus sp. PCC 7002 and the inactivation of ctaDI was shown to affect high-light adaptation. In this study, Synechococcus sp. PCC 7002 wild-type, ctaDI, ctaDII, and ctaDI-ctaDII double mutants were grown under extreme high-light and oxidative stress to further assess the roles of cytochrome oxidases in cyanobacteria. Cells of the ctaDI mutant strain barely grew under extreme high-light illumination of 4.5 mE m(-2) s(-1), suggesting that CtaDI is required for high-light acclimation in Synechococcus sp. PCC 7002. The ctaDI-ctaDII double mutant cells unexpectedly tolerated extreme high-light intensity, indicating that the disruption of ctaDII gene suppresses the high-light sensitivity phenotype of the ctaDI single mutant. The ctaDII mutant cells also exhibited higher tolerance to the oxidative stress compound, methyl viologen, in the growth media. The ctaDII mutant and the ctaDI-ctaDII double mutant cells had approximately twofold higher levels of superoxide dismutase (SOD) activity, indicating that the disruption of ctaDII gene increased the capacity to decompose active oxygen species. These results suggest that the CtaII cytochrome oxidase may be involved with the oxidative stress response, including the control of SOD expression.

Characterization of two cytochrome oxidase operons in the marine cyanobacterium Synechococcus sp. PCC 7002: inactivation of ctaDI affects the PS I:PS II ratio

Cyanobacteria have versatile electron transfer pathways and many of the proteins involved are functional in both respiratory and photosynthetic electron transport. Examples of such proteins include the cytochrome b (6) f complex, NADH dehydrogenase and cytochrome oxidase complexes. In this study we have cloned and sequenced two gene clusters from the marine cyanobacterium Synechococcus sp. PCC 7002 that potentially encode heme-copper cytochrome oxidases. The ctaCIDIEI and ctaCIIDIIEII gene clusters are most similar to two related gene clusters found in the freshwater cyanobacterial strain Synechocystis sp. PCC 6803. Unlike Synechocystis sp. PCC 6803, Synechococcus sp. PCC 7002 does not have a cydAB-like gene cluster which encodes a quinol oxidase. The ctaCIDIEI and ctaCIIDIIEII gene clusters were transcribed polycistronically, although the levels of transcripts for the ctaCIIDIIEII gene cluster were lower than those of the ctaCIDIEI gene cluster. The ctaDI and ctaDII coding sequences were interrupted by interposon mutagenesis and full segregants were isolated and characterized for both single and double mutants. Growth rates, chlorophyll and carotenoid contents, oxygen consumption and oxygen evolution were examined in the wild type and mutant strains. Differences between the wild type and mutant strains observed in 77 K fluorescence spectra and in pulse-amplified modulated (PAM) fluorescence studies suggest that the cyanobacterial oxidases play a role in photoinhibition and high light tolerance in Synechococcus sp. PCC 7002.

Respiratory terminal oxidases in the facultative chemoheterotrophic and dinitrogen fixing cyanobacterium Anabaena variabilis strain ATCC 29413: characterization of the cox2 locus

Upon nitrogen step-down, some filamentous cyanobacteria differentiate heterocysts, cells specialized for dinitrogen fixation, a highly oxygen sensitive process. Aerobic respiration is one of the mechanisms responsible for a microaerobic environment in heterocysts and respiratory terminal oxidases are the key enzymes of the respiratory chains. We used Anabaena variabilis strain ATCC 29413, because it is one of the few heterocyst-forming facultatively chemoheterotrophic cyanobacteria amenable to genetic manipulation. Using PCR with degenerate primers, we found four gene loci for respiratory terminal oxidases, three of which code for putative cytochrome c oxidases and one whose genes are homologous to cytochrome bd-type quinol oxidases. One cytochrome c oxidase, Cox2, was the only enzyme whose expression, tested by RT-PCR, was evidently up-regulated in diazotrophy, and therefore cloned, sequenced, and characterized. Up-regulation of Cox2 was corroborated by Northern and primer extension analyses. Strains were constructed lacking Cox1 (a previously characterized cytochrome c oxidase), Cox2, or both, which all grew diazotrophically. In vitro cytochrome c oxidase and respiratory activities were determined in all strains, allowing for the first time to estimate the relative contributions to total respiration of the different respiratory electron transport branches under different external conditions. Especially adding fructose to the growth medium led to a dramatic enhancement of in vitro cytochrome c oxidation and in vivo respiratory activity without significantly influencing gene expression.

Cytochrome c oxidase genes required for nitrogenase activity and diazotrophic growth in Anabaena sp. PCC 7120

N2 fixation is an O2-sensitive process and some filamentous diazotrophic cyanobacteria that grow performing oxygenic photosynthesis confine their N2 fixation machinery to heterocysts, specialized cells that maintain a reducing environment adequate for N2 fixation. Respiration is thought to contribute to the diazotrophic metabolism of heterocysts and the genome of the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 bears three gene clusters putatively encoding cytochrome c oxidases. Transcript analysis of these cox gene clusters through RNA/DNA hybridization identified two cox operons, cox2 and cox3, that are induced after nitrogen step-down in an NtcA- and HetR-dependent manner and appear to be expressed specifically in heterocysts. In contrast, cox1 was expressed only in vegetative cells. Expression of cox2 and cox3 occurred at an intermediate stage (about 9 h) during the process of heterocyst development following nitrogen step-down. Inactivation of genes in the two inducible cox operons, but not separately in either of them, strongly reduced nitrogenase activity and prevented diazotrophic growth in aerobic conditions. These results show that the nitrogen-regulated cytochrome c oxidase-type respiratory terminal oxidases Cox2 and Cox3 are essential for heterocyst function in Anabaena sp. PCC 7120.

Newly identified cytochrome c oxidase operon in the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 specifically induced in heterocysts

Two operons have been cloned from Anabaena sp. strain PCC 7120 DNA, each of which encodes the three core subunits of distinct mitochondrial-type cytochrome c oxidases. The two operons are only 72 to 85% similar to one another at the nucleotide level in the most conserved subunit. One of these, coxBACII, is induced >20-fold in the middle to late stages of heterocyst differentiation. Analysis of green fluorescent protein reporters indicates that this operon is expressed specifically in proheterocysts and heterocysts. The other operon, coxBACI, is induced only 2.5-fold following nitrogen step-down and is expressed in all cells. Surprisingly, a disruption mutant of coxAII, the gene encoding subunit I of the heterocyst-specific oxidase, grows normally in the absence of combined nitrogen. It is likely that coxBACI and/or two other putative terminal oxidases present in the Anabaena sp. strain PCC 7120 genome are able to compensate for the loss of the heterocyst-specific oxidase in providing ATP for nitrogen fixation and maintaining a low oxygen level in heterocysts.