H2 Uptake Activity Research Articles

Synergistic enhancement of pulsed light-induced H2 photoproduction in Chlamydomonas cells by optimal sulfite concentration and light waveform

Pulsed light illumination stands out as a noteworthy technique for photosynthetic H2 production, playing a crucial role in eliminating O2 and activating hydrogenase enzymes. However, further improvements are essential to make H2 photoproduction suitable for future commercial applications. In our study, we observed a distinct enhancement in pulsed light-induced H2 photoproduction in the unicellular green alga Chlamydomonas reinhardtii when treated with the optimal concentration of the mild O2 scavenger Na2SO3. This improvement was a result of reduced O2 content, increased hydrogenase enzyme activity, and suppressed H2-uptake activity. Furthermore, our findings indicate that exposing Na2SO3-treated C. reinhardtii to optimal light waveform continues to significantly boost pulsed light-induced H2 photoproduction, attributed to the alleviation of impaired photosystem II activity. Altogether, the combined application of optimal sulfite concentration and light waveform effectively enhances pulsed light-induced photosynthetic H2 production in the green alga C. reinhardtii.

La2O3‐Promoted Ni/Al2O3 Catalyst for CO Methanation: Enhanced Catalytic Activity and Stability

To simultaneously inhibit Ni sintering and coke formation, while maintaining high catalytic activity, Ni–La2O3/Al2O3 catalysts are prepared by a modified deposition–precipitation (DP) method and applied for the CO methanation reaction. The activity of the catalyst prepared by the DP method is much higher than those prepared by the traditional impregnation and co‐impregnation methods due to the promotion of La2O3 as well as its unique dispersion on the surface of the catalyst. In the 100 h‐lifetime test under the conditions of 450 °C, 0.1 MPa, a weight hourly space velocity (WHSV) of 120 000 mL g−1 h−1, the Ni–La2O3/Al2O3 catalyst prepared by the DP method shows high catalytic stability. The characterization results show that La2O3 species can be deposited on the surface of the catalyst during the preparation process and can act as a physical barrier to prevent sintering of Ni particles and coke formation during high‐temperature reduction and reaction. La2O3 species can improve the reducibility of Ni particles and decrease the Ni particle sizes, resulting in larger H2 uptake and higher catalytic activity. Furthermore, the La3+/La2+ redox can also decrease coke formation by increasing the active oxygen species on the Ni surface.

Site-directed mutagenesis of Campylobacter concisus respiratory genes provides insight into the pathogen\u2019s growth requirements

Campylobacter concisus is an emerging human pathogen found throughout the entire human oral-gastrointestinal tract. The ability of C. concisus to colonize diverse niches of the human body indicates the pathogen is metabolically versatile. C. concisus is able to grow under both anaerobic conditions and microaerophilic conditions. Hydrogen (H2) has been shown to enhance growth and may even be required. Analysis of several C. concisus genome sequences reveals the presence of two sets of genes encoding for distinct hydrogenases: a H2-uptake-type (“Hyd”) complex and a H2-evolving hydrogenase (“Hyf”). Whole cells hydrogenase assays indicate that the former (H2-uptake) activity is predominant in C. concisus, with activity among the highest we have found for pathogenic bacteria. Attempts to generate site-directed chromosomal mutants were partially successful, as we could disrupt hyfB, but not hydB, suggesting that H2-uptake, but not H2-evolving activity, is an essential respiratory pathway in C. concisus. Furthermore, the tetrathionate reductase ttrA gene was inactivated in various C. concisus genomospecies. Addition of tetrathionate to the medium resulted in a ten-fold increase in cell yield for the WT, while it had no effect on the ttrA mutant growth. To our knowledge, this is the first report of mutants in C. concisus.

Novel hydrogenases from deep-sea hydrothermal vent metagenomes identified by a recently developed activity-based screen.

Hydrogen is one ofthe most common elements on Earth. The enzymes converting molecular hydrogen into protons and electrons are the hydrogenases. Hydrogenases are ubiquitously distributed in all three domains of life where they play a central role in cell metabolism. So far, the recovery of hydrogenases has been restricted to culture-dependent and sequence-based approaches. We have recently developed the only activity-based screen for seeking H2-uptake enzymes from metagenomes without having to rely on enrichment and isolation of hydrogen-oxidizing microorganisms or prior metagenomic sequencing. When screening 14,400 fosmid clones from three hydrothermal vent metagenomes using this solely activity-based approach, four clones with H2-uptake activity were identified with specific activities of up to 258 ± 19 nmol H2/min/mg protein of partially purified membrane fractions. The respective metagenomic fragments exhibited mostly very low or no similarities to sequences in the public databases. A search with hidden Markov models for different hydrogenase groups showed no hits for three of the four metagenomic inserts, indicating that they do not encode for classical hydrogenases. Our activity-based screen serves as a powerful tool for the discovery of (novel) hydrogenases which would not have been identified by the currently available techniques. This screen can be ideally combined with culture- and sequence-based approaches to investigate the tremendous hydrogen-converting potential in the environment.

The hydrogen metabolism of sulfur deprived Chlamydomonas reinhardtii cells involves hydrogen uptake activities

Several species of unicellular microalgae such as the model species Chlamydomonas reinhardtii possess plastid-localized [FeFe]-hydrogenases which, via ferredoxin, can accept electrons from photosynthetic electron transport. Thereby, under specific conditions, these algae light-dependently produce molecular hydrogen (H2), which offers a sustainable way to generate a “green” and efficient fuel. Until today, the most common way to induce sustained H2 production is to deprive Chlamydomonas of macronutrients such as sulfur (S) which results in a downregulation of photosynthetic production of molecular oxygen (O2) and of assimilatory processes. These acclimation responses allow the O2 sensitive algal [FeFe]-hydrogenases to become active and serve as an alternative electron sink of photosynthesis. Despite much progress in the field and a general understanding of the underlying mechanisms, many basic and applied aspects of the photosynthetic H2 metabolism of eukaryotic algae remain to be elucidated. One rarely investigated factor is that microalgae have also been reported to consume H2, especially as a response to high H2 concentrations. Here, we analyzed the H2 uptake activities of S-deprived Chlamydomonas cells incubated in different PBRs providing different gas phase volumes, either in continuous light or in the dark. We show that H2 uptake occurs after prolonged incubation in the light as well as in sudden darkness. Dark-induced H2 uptake can be delayed adding the phosphoribulose kinase inhibitor glycolaldehyde, suggesting a connection to carbohydrate metabolism. The results indicate that PBR setups as well as envisioned outdoor cultivation systems with natural light-dark cycles have to be carefully designed to prevent efficiency losses.

Detection and isolation of plant-associated bacteria scavenging atmospheric molecular hydrogen.

High-affinity hydrogen (H2 )-oxidizing bacteria possessing group 5 [NiFe]-hydrogenase genes are important contributors to atmospheric H2 uptake in soil environments. Although previous studies reported the occurrence of a significant H2 uptake activity in vegetation, there has been no report on the identification and diversity of the responsible microorganisms. Here, we show the existence of plant-associated bacteria with the ability to consume atmospheric H2 that may be a potential energy source required for their persistence in plants. Detection of the gene hhyL - encoding the large subunit of group 5 [NiFe]-hydrogenase - in plant tissues showed that plant-associated high-affinity H2 -oxidizing bacteria are widely distributed in herbaceous plants. Among a collection of 145 endophytic isolates, seven Streptomyces strains were shown to possess hhyL gene and exhibit high- or intermediate-affinity H2 uptake activity. Inoculation of Arabidopsis thaliana (thale cress) and Oryza sativa (rice) seedlings with selected isolates resulted in an internalization of the bacteria in plant tissues. H2 uptake activity per bacterial cells was comparable between plant and soil, demonstrating that both environments are favourable for the H2 uptake activity of streptomycetes. This study first demonstrated the occurrence of plant-associated high-affinity H2 -oxidizing bacteria and proposed their potential contribution as atmospheric H2 sink.

Breathing air to save energy--new insights into the ecophysiological role of high-affinity [NiFe]-hydrogenase in Streptomyces avermitilis.

The Streptomyces avermitilis genome encodes a putative high-affinity [NiFe]-hydrogenase conferring the ability to oxidize tropospheric H2 in mature spores. Here, we used a combination of transcriptomic and mutagenesis approaches to shed light on the potential ecophysiological role of the enzyme. First, S. avermitilis was either exposed to low or hydrogenase-saturating levels of H2 to investigate the impact of H2 on spore transcriptome. In total, 1293 genes were differentially expressed, with 1127 and 166 showing lower and higher expression under elevated H2 concentration, respectively. High H2 exposure lowered the expression of the Sec protein secretion pathway and ATP-binding cassette-transporters, with increased expression of genes encoding proteins directing carbon metabolism toward sugar anabolism and lower expression of NADH dehydrogenase in the respiratory chain. Overall, the expression of relA responsible for the synthesis of the pleiotropic alarmone ppGpp decreased upon elevated H2 exposure, which likely explained the reduced expression of antibiotic synthesis and stress response genes. Finally, deletion of hhySL genes resulted in a loss of H2 uptake activity and a dramatic loss of viability in spores. We propose that H2 is restricted to support the seed bank of Streptomyces under a unique survival-mixotrophic energy mode and discuss important ecological implications of this finding.

Purification and Molecular Characterization of an [FeFe]-Hydrogenase from <i>Thermotoga hypogea</i>

Thermotoga hypogea is a hyperthermophilic bacterium growing on carbohydrates and peptides, producing acetate, CO2 and H2 as major end products. Hydrogenase activity was detected in the cell-free extract of T. hypogea and a hydrogenase was purified to homogeneity. The purified enzyme was homotetrameric with a subunit of 65 kDa, and contained 16 atoms of Fe and 11.6 atoms of acid labile sulfur per mole subunit. The gene encoding the hydrogenase was sequenced, and four iron-sulfur clusters including the H-cluster were found to be present in the gene sequence, indicating it is an [FeFe]-hydrogenase. The enzyme was oxygen sensitive with a t1/2 of 3 min when exposed to air. It was thermostable with t1/2 values of approximately 40 and 15 min when incubated at 70°C and 85°C, respectively. The optimal temperature for the hydrogenase activity was about 85 °C. Both benzyl viologen (BV) and methyl viologen (MV) could serve as electron acceptors, and the apparent Vmax values were determined to be 1,142 and 607 μmol H2 oxidized min-1 mg-1, respectively. For H2 uptake activity, the apparent Km values for MV and BV were 0.17 and 0.24 mM, respectively. For H2 evolution activity, the apparent Km value for MV and apparent Vmax value were determined to be 1.1 mM and 192 μmol min-1 mg-1, respectively. Ferredoxin purified from T. hypogea served as electron carrier. The enzyme exhibited pH optima of 10.0 and 8.0 for the H2 uptake and evolution activities, respectively. The physiological function of the enzyme is proposed to catalyze the H2 evolution. Keywords: [FeFe]-hydrogenase, ferredoxin, iron-sulfur clusters, ferredoxin, hydrogen production, hyperthermophiles, Thermotoga hypogea.

Nano-TiO2 Enhanced Photofermentative Hydrogen Produced from the Dark Fermentation Liquid of Waste Activated Sludge

After anaerobic dark fermentation of waste activated sludge (WAS) for hydrogen production, there are a large number of organic compounds including protein, polysaccharide, and volatile fatty acids left in the dark fermentation liquid, which can be further bioconverted to hydrogen by photofermentation techniquea. In this study, the enhancement of photofermentative hydrogen produced from WAS dark fermentation liquid by using nano-TiO2 is reported. First, high concentration of NH(4)(+)-N in the dark fermentation liquid was observed to inhibit the photofermentative hydrogen production, and its removal was essential. Then the effect of nano-TiO2 on photofermentative hydrogen generation was investigated, and the addition of 100 mg/L nano-TiO2 increased hydrogen by 46.1%. Finally, the mechanisms for nano-TiO2 improving hydrogen production were investigated. It was found that nano-TiO2 improved the decomposition of protein and polysaccharide to small-molecule organic compounds and promoted the growth of photosynthetic bacteria and the activity of nitrogenase but decreased the H2-uptake hydrogenase activity.

Thermal deactivation of high-affinity H2 uptake activity in soils

Abstract Soils are the most important sink for atmospheric hydrogen, which is assumed to be oxidized by abiontic soil hydrogenases or by putative high-affinity hydrogenases of microbial origin. The activity of soil hydrogenases has been found to change with soil temperature as it changes during the day and the season. However, it is unclear whether and to which extent the soil hydrogenases are deactivated by increased temperature. Therefore, we incubated soils from different climates and different ecosystems (forest, agricultural, arid, hyper-arid, paddy, peat) at elevated temperature and measured the residual activity at 25 °C after different incubation times. We found that at least part of the soil hydrogenase is deactivated irreversibly already at relatively low temperatures (>30 °C) and short exposure times (>10 min) and that the deactivation was more pronounced in soil from cold versus hot climate. The deactivation kinetics could be fitted to a biexponential model, but were complex with respect to soil type and deactivation temperature. The results show that new hydrogenase activity has to be generated in the soil to compensate for activity loss by diurnal and seasonally increased temperature.

Streptomycetes contributing to atmospheric molecular hydrogen soil uptake are widespread and encode a putative high‐affinity [NiFe]‐hydrogenase

Uptake of molecular hydrogen (H2) by soil is a biological reaction responsible for approximately 80% of the global loss of atmospheric H2. Indirect evidence obtained over the last decades suggests that free soil hydrogenases with an unusually high affinity for H2 are carrying out the reaction. This assumption has recently been challenged by the isolation of Streptomyces sp. PCB7, displaying the high-affinity H2 uptake activity previously attributed to free soil enzymes. While this finding suggests that actinobacteria could be responsible for atmospheric H2 soil uptake, the ecological importance of H2-oxidizing streptomycetes remains to be investigated. Here, we show that high-affinity H2 uptake activity is widespread among the streptomycetes. Among 14 streptomycetes strains isolated from temperate forest and agricultural soils, six exhibited a high-affinity H2 uptake activity. The gene encoding the large subunit of a putative high-affinity [NiFe]-hydrogenase (hydB-like gene sequence) was detected exclusively in the isolates exhibiting high-affinity H2 uptake. Catalysed reporter deposition-fluorescence in situ hybridization (CARD-FISH) experiments targeting hydB-like gene transcripts and H2 uptake assays performed with strain PCB7 suggested that streptomycetes spores catalysed the H2 uptake activity. Expression of the activity in term of biomass revealed that 10(6)-10(7) H2-oxidizing bacteria per gram of soil should be sufficient to explain in situ H2 uptake by soil. We propose that specialized H2-oxidizing actinobacteria are responsible for the most important sink term in the atmospheric H2 budget.

Alumina‐Supported Ni–Au: Surface Synergistic Effects in Catalytic Hydrodechlorination

AbstractCatalytic gas‐phase hydrodechlorination (HDC) of 2,4‐dichlorophenol (2,4‐DCP) has been investigated over Ni/Al2O3 and Au/Al2O3 prepared by impregnation, and Au–Ni/Al2O3 prepared by reductive deposition of Au onto Ni. Catalyst activation by temperature‐programmed reduction is examined and the activated catalysts are characterized in terms of H2 chemisorption, XRD and TEM‐energy dispersive X‐ray (EDX) measurements. Ni/Al2O3 (<1–10 nm) and Au/Al2O3 (<1–15 nm) exhibit a relatively narrow metal size distribution while Au–Ni/Al2O3 bore larger particles (1–30 nm) with variable surface Ni/Au ratios. Au/Al2O3 exhibits low H2 uptake and low HDC activity to generate 2‐chlorophenol (2‐CP) as the sole product. H2 chemisorption on Au–Ni/Al2O3 was approximately five times lower than that recorded for Ni/Al2O3 but both catalysts delivered equivalent initial HDC activities. Ni/Al2O3 exhibits an irreversible temporal deactivation where partial dechlorination to 2‐CP is increasingly favored over full dechlorination to phenol. In contrast, thermal treatment of Au–Ni/Al2O3 in H2 after reaction elevates HDC activity with a preferential full HDC to phenol. This response is linked to a surface reconstruction resulting in a more homogeneous combination of Ni and Au. This result was also achieved by a direct treatment of Au–Ni/Al2O3 with HCl. A parallel/ consecutive kinetic model is used to quantify the catalytic HDC response.

Dissecting the roles ofEscherichia colihydrogenases in biohydrogen production

Escherichia coli can perform at least two modes of anaerobic hydrogen metabolism and expresses at least two types of hydrogenase activity. Respiratory hydrogen oxidation is catalysed by two 'uptake' hydrogenase isoenzymes, hydrogenase -1 and -2 (Hyd-1 and -2), and fermentative hydrogen production is catalysed by Hyd-3. Harnessing and enhancing the metabolic capability of E. coli to perform anaerobic mixed-acid fermentation is therefore an attractive approach for bio-hydrogen production from sugars. In this work, the effects of genetic modification of the genes encoding the uptake hydrogenases, as well as the importance of preculture conditions, on hydrogen production and fermentation balance were examined. In suspensions of resting cells pregrown aerobically with formate, deletions in Hyd-3 abolished hydrogen production, whereas the deletion of both uptake hydrogenases improved hydrogen production by 37% over the parent strain. Under fermentative conditions, respiratory H2 uptake activity was absent in strains lacking Hyd-2. The effect of a deletion in hycA on H2 production was found to be dependent upon environmental conditions, but H2 uptake was not significantly affected by this mutation.

Hydrogen Production by Termite Gut Protists: Characterization of Iron Hydrogenases of Parabasalian Symbionts of the Termite Coptotermes formosanus

Cellulolytic flagellated protists in the guts of termites produce molecular hydrogen (H(2)) that is emitted by the termites; however, little is known about the physiology and biochemistry of H(2) production from cellulose in the gut symbiotic protists due to their formidable unculturability. In order to understand the molecular basis for H(2) production, we here identified two genes encoding proteins homologous to iron-only hydrogenases (Fe hydrogenases) in Pseudotrichonympha grassii, a large cellulolytic symbiont in the phylum Parabasalia, in the gut of the termite Coptotermes formosanus. The two Fe hydrogenases were phylogenetically distinct and had different N-terminal accessory domains. The long-form protein represented a phylogenetic lineage unique among eukaryotic Fe hydrogenases, whereas the short form was monophyletic with those of other parabasalids. Active recombinant enzyme forms of these two Fe hydrogenases were successfully obtained without the specific auxiliary maturases. Although they differed in their extent of specific activity and optimal pH, both enzymes preferentially catalyzed H(2) evolution rather than H(2) uptake. H(2) evolution, at least that associated with the short-form enzyme, was still active even under high hydrogen partial pressure. H(2) evolution activity was detected in the hydrogenosomal fraction of P. grassii cells; however, the vigorous H(2) uptake activity of the endosymbiotic bacteria compensated for the strong H(2) evolution activity of the host protists. The results suggest that termite gut symbionts are a rich reservoir of novel Fe hydrogenases whose properties are adapted to the gut environment and that the potential of H(2) production in termite guts has been largely underestimated.

Analysis of the hupSL Operon of the Nonheterocystous Cyanobacterium Lyngbya majuscula CCAP 1446/4: Regulation of Transcription and Expression under a Light-Dark Regimen

This work presents the characterization of an uptake hydrogenase from a marine filamentous nonheterocystous cyanobacterium, Lyngbya majuscula CCAP 1446/4. The structural genes encoding the uptake hydrogenase (hupSL) were isolated and characterized, and regulatory sequences were identified upstream of hupS. In silico analysis highlighted various sets of long repetitive sequences within the hupSL intergenic region and downstream of hupL. The transcriptional regulator that operates global nitrogen control in cyanobacteria (NtcA) was shown to bind to the promoter region, indicating its involvement in the transcriptional regulation of hupSL. Under N2-fixing conditions and a 12-h light/12-h dark regime, H2 uptake activity was shown to follow a daily pattern with a clear maximum towards the end of the dark period, preceded by an increase in the transcript levels initiated in the end of the light phase. Novel antibodies directed against HupL of Lyngbya majuscula CCAP 1446/4 were used to monitor the protein levels throughout the 24-h period. The results suggest that protein turnover occurs, with degradation taking place during the light phase and de novo synthesis occurring during the dark phase, coinciding with the pattern of H2 uptake. Taking into account our results and the established correlation between the uptake hydrogenase activity and N2 fixation in cyanobacteria, it seems probable that both processes are confined to the dark period in aerobically grown cells of Lyngbya majuscula CCAP 1446/4.

A simplified method for assay of hydrogenase activities of H2 evolution and uptake in Enterobacter aerogenes

Assay of hydrogenase activity pertaining to H2 production needs anaerobic conditions. To establish a simplified method for assay of hydrogenase activities by using intact cells of Enterobater aerogenes, different chemicals capable of enhancing the cell-wall permeability to electron mediators were examined. As a result, Triton X-100 and CTAB were found to be appropriate for H2 uptake and evolution activities of the intact cells, respectively. This method enabled H2 uptake and evolution activities of the intact cells to be easily detected. This is also the first report of the presence of H2 uptake hydrogenase activity in E. aerogenes.

Oxygen Tolerance of the H2-sensing [NiFe] Hydrogenase from Ralstonia eutropha H16 Is Based on Limited Access of Oxygen to the Active Site

Hydrogenases, abundant proteins in the microbial world, catalyze cleavage of H2 into protons and electrons or the evolution of H2 by proton reduction. Hydrogen metabolism predominantly occurs in anoxic environments mediated by hydrogenases, which are sensitive to inhibition by oxygen. Those microorganisms, which thrive in oxic habitats, contain hydrogenases that operate in the presence of oxygen. We have selected the H2-sensing regulatory [NiFe] hydrogenase of Ralstonia eutropha H16 to investigate the molecular background of its oxygen tolerance. Evidence is presented that the shape and size of the intramolecular hydrophobic cavities leading to the [NiFe] active site of the regulatory hydrogenase are crucial for oxygen insensitivity. Expansion of the putative gas channel by site-directed mutagenesis yielded mutant derivatives that are sensitive to inhibition by oxygen, presumably because the active site has become accessible for oxygen. The mutant proteins revealed characteristics typical of standard [NiFe] hydrogenases as described for Desulfovibrio gigas and Allochromatium vinosum. The data offer a new strategy how to engineer oxygen-tolerant hydrogenases for biotechnological application.

Interaction between the H2 sensor HupUV and the histidine kinase HupT controls HupSL hydrogenase synthesis in Rhodobacter capsulatus.

The photosynthetic bacterium Rhodobacter capsulatus contains two [NiFe]hydrogenases: an energy-generating hydrogenase, HupSL, and a regulatory hydrogenase, HupUV. The synthesis of HupSL is specifically activated by H(2) through a signal transduction cascade comprising three proteins: the H(2)-sensing HupUV protein, the histidine kinase HupT, and the transcriptional regulator HupR. Whereas a phosphotransfer between HupT and HupR was previously demonstrated, interaction between HupUV and HupT was only hypothesized based on in vivo analyses of mutant phenotypes. To visualize the in vitro interaction between HupUV and HupT proteins, a six-His (His(6))-HupU fusion protein and the HupV protein were coproduced by using a homologous expression system. The two proteins copurified as a His(6)-HupUHupV complex present in dimeric and tetrameric forms, both of which had H(2) uptake activity. We demonstrated that HupT and HupUV interact and form stable complexes that could be separated on a native gel. Interaction was also monitored with surface plasmon resonance technology and was shown to be insensitive to salt concentration and pH changes, suggesting that the interactions involve hydrophobic residues. As expected, H(2) affects the interaction between HupUV and HupT, leading to a weakening of the interaction, which is independent of the phosphate status of HupT. Several forms of HupT were tested for their ability to interact with HupUV and to complement hupT mutants. Strong interaction with HupUV was obtained with the isolated PAS domain of HupT and with inactive HupT mutated in the phosphorylable histidine residue, but only the wild-type HupT protein was able to restore normal H(2) regulation.

Effect of redox mediators on nitrogenase and hydrogenase activities in Azotobacter vinelandii.

In bioelectrochemical studies, redox mediators such as methylene blue, natural red, and thionine are used to studying the redox characteristics of enzymes in the living cell. Here we show that nitrogenase activity in Azotobacter vinelandii is completely inhibited by oxidized methylene blue (MBo) when the concentration of this mediator in the medium is increased up to 72 microM. This activity in A. vinelandii is somewhat inhibited by a coenzyme, ascorbic acid (AA). However, the nitrogenase activity within the A. vinelandii cell is unchanged even for a high concentration of oxidized natural red (NRo) alone. Interestingly, these mediators and AA do not have the capacity to inhibit the H2 uptake activity of the hydrogenase in A. vinelandii. Average active rates of 66 nM H2 evolved/mg cell protein/min from the nitrogenase and 160 nM H2-uptake/mg cell protein/min from the hydrogenase in A. vinelandii are found in aid of the activities of the enzymes for H2 evolution and for H2 uptake are compared. The activities of both enzymes in A. vinelandii are strongly inhibited by thionine having high oxidative potential. Mechanisms of various mediators acting in vivo for both enzymes in A. vinelandii are discussed.

Transcriptional regulation of Nostoc uptake hydrogenase.

Filamentous, heterocystous cyanobacteria may contain both an uptake hydrogenase (encoded by hupSL) and a bidirectional enzyme (encoded by hoxFUYH). The present study identifies three strains (Anabaena variabilis, Nostoc muscorum and Nostoc sp. strain PCC 73102) with a contiguous hupL in both vegetative cells and heterocysts. The two Nostoc strains differ in either containing a bidirectional enzyme (N. muscorum) or lacking this enzyme (N. PCC 73102). Transcriptional studies, using reverse transcriptase-polymerase chain reaction, demonstrated an induction of a hupL transcript approximately 24 h after a shift from non-nitrogen-fixing to nitrogen-fixing conditions (in parallel with the induction of an in vivo light-dependent H2-uptake activity) in N. muscorum. However, the level of hoxH transcripts did not change significantly during the induction of the H2-uptake activity.