J. Balogh

Hydrogenases, accessory genes and the regulation of (NiFe) hydrogenase biosynthesis in Thiocapsa roseopersicina

The purple (Sulphur) phototrophic bacterium, Thiocapsa roseopersicina BBS contains several [NiFe] hydrogenases, of which two are membrane bound. Mutant T . roseopersicina cells, carrying deletions in both gene clusters showed hydrogenase activity. This activity was located in the cytoplasm. The structural gene cluster hoxEFUYH was identi:ed and sequenced. In addition, genes homologous to hupUV=hoxBC, the hydrogen sensing hydrogenase have been identi:ed and sequenced. Regulation of hydrogenase biosynthesis was studied in detail for HydSL (renamed HynSL). A random mutagenesis system was optimised for T. roseopersicina. One of the mutations was in a gene similar to that coding for the HypF proteins in other organisms. Inactivation of the hypF gene resulted in a 60-fold increase in hydrogen evolution under nitrogen :xing conditions. In addition to hypF, the following accessory genes were identi:ed: hydD, hupK, hypC1, hypC2, hypDE. Characterisation of the corresponding gene products and search for additional accessory genes are in progress. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

Microscopic study of the magnetic coupling in a nanocrystalline soft magnet

The magnetic behaviour of nanosize ferromagnetic bcc granules embedded in an amorphous tissue (i.e., partially crystallized amorphous alloy) was studied by Mossbauer spectroscopy. The results are compared with the bulk counterparts: bcc-Fe and amorphous . Size dependent enhancement of the Curie point of the nanosize amorphous phase was not observed. At temperatures well above the Curie point of the amorphous phase superparamagnetic relaxation of the bcc crystallites is observed opening new possibilities to study the anisotropy energy of nanosize ferromagnetic grains.

The hydrogenases of Thiocapsa roseopersicina.

The purple sulphur phototrophic bacterium, Thiocapsa roseopersicina BBS, contains several NiFe hydrogenases. One of these enzymes (HynSL) is membrane associated, remarkably stable and can be used for practical applications. HupSL is also located in the photosynthetic membrane, its properties are similar to other known Hup-type NiFe hydrogenases. A third hydrogenase activity was located in the soluble fraction and was analogous to the NAD-reducing hydrogenases of cyanobacteria. The hoxEFUYH genes are transcribed together. HoxE is needed for the in vivo electron flow to and from the soluble hydrogenase. Some of the accessory genes were identified using random mutagenesis, and sequencing of the T. roseopersicina genome is in progress. The HupD, HynD and HoxW gene products corresponded to the proteases processing the C-termini of the three NiFe hydrogenases respectively. HypF and HupK mutants displayed significant in vivo H(2) evolution, which could be linked to the nitrogenase activity for the DeltahypF and to the bidirectional Hox activity in the DeltahupK strain. Both HypC proteins are needed for the biosynthesis of each NiFe hydrogenase. The hydrogenase expression is regulated at the transcriptional level through distinct mechanisms. The expression of hynSL is up-regulated under anaerobic conditions with the participation of an FNR (fumarate and nitrate reduction regulator)-type protein, FnrT. Although the genes encoding a typical H(2) sensor (hupUV) and a two-component regulator (hupR and hupT) are present in T. roseopersicina, the system is cryptic in the wild-type BBS strain. The hupR gene was identified in the gene cluster downstream from hupSL. Introduction of actively expressed hupT repressed the hupSL gene expression as expected by analogy with other bacteria.

Amorphous alloy formation by mechanical alloying and consecutive heat treatment in Fe50B50 powder mixture

Equiatomic composition powder mixture of pure Fe and B was mechanically alloyed in a controlled atmosphere vibration mill. Amorphous alloy phase formation was established by Mossbauer spectroscopy and by transmission electron microscopy. The variation of the average boron concentration and the homogeneity of the amorphous phase during the alloying process was followed by Mossbauer spectroscopy. At the early stage of the milling process a broad concentration distribution (from 20 to 50 at. % B) was observed. Upon further milling the average B concentration of the amorphous component was gradually enhanced and the concentration heterogeneity decreased. The B concentration of the amorphous phase could be further enhanced by heat treating the milled powder mixture. The highest boron content alloy attained in this study is Fe60B40.

Sonochemical synthesis of iron nitride nanoparticles

A method for the preparation of nanoparticles of iron nitride powders is reported. Iron nitride particles have been synthesised by two methods. In the first, Fe(CO)5 was sonicated in a decane solution under a gaseous mixture of NH3 and H2 (3.5:1 molar ratio) at ca. 0 °C. The second method was based on nitriding the sonochemically prepared amorphous iron at ca. 400 °C for 4 h under a mixed stream of NH3–H2(3.5:1 molar ratio). Different products were obtained in the two cases. The product of the sonication of Fe(CO)5 was amorphous Fe2–3N and a small quantity of iron oxide. The X-ray diffraction patterns in the second case showed Fe4N as a main product. The magnetic properties of both products were measured. The coercive force HC of the Fe4N is 190 Oe, and the saturation magnetization sigma;s is 170 emu g–1 .

Magnetic properties of FeZr metastable phases

Abstract Amorphous Fe 100− x Zr x (7 x 3 Zr were found in the fully crystallized samples; the Fe 2 Zr intermetallic phase of the ingots was not present.

Hydrogen independent expression of hupSL genes in Thiocapsa roseopersicina BBS

The expression of many membrane bound [NiFe] hydrogenases is regulated by their substrate molecule, hydrogen. The HupSL hydrogenase, encoded in the hupSLCDHIR operon, probably plays a role in hydrogen recycling in the phototrophic purple bacterium, Thiocapsa roseopersicina BBS. RpoN, coding for sigma factor 54, was shown to be important for expression, suggesting a regulated biosynthsis from the hup gene cluster. The response regulator gene, hupR, has been identified in the hup operon and expression of hupSL was reduced in a chromosomal hupR mutant, which indicated that HupR was implicated in the activation process. The hupT and hupUV genes were isolated, and show similarity to the histidine kinase element of the H2‐driven signal transduction system and to the regulatory hydrogenases of Ralstonia eutropha and Rhodobacter capsulatus, respectively. Although the genes of the entire H2 sensing and regulation system were present, the expression of the hupSL genes was not affected by the presence or absence of H2. Using reverse transcription PCR, we could not detect any mRNA specific to the hupTUV genes in cells grown under diverse conditions. The hupT and hupUV mutant strains had the same phenotype as the wild‐type strains. The hupT gene product, expressed from a plasmid, repressed HupSL synthesis as expected while introduction of actively expressed hupTUV genes together derepressed the HupSL activity in T. roseopersicina. The gene product of hupUV behaves similarly to other regulatory hydrogenases and shows H–D exchange activity.

Structure and magnetic properties of nanocrystalline (Fe1−xCox)90Zr7B2Cu1 (0⩽x⩽0.6)

Nanocrystalline (Fe1−xCox)90Zr7B2Cu1 (0⩽x⩽0.6) alloys were investigated by x-ray diffraction, 57Fe Mossbauer spectroscopy, and magnetic measurements. The grain size did not change significantly with composition. Co was preferentially partitioned to the residual amorphous phase, and the bcc grains were accordingly enriched by Fe. The room-temperature coercive field increased with the Co addition, which is attributed to the increasing magnetostriction constant.

Magnetic and transport properties of Fe-Ag granular multilayers

Results of magnetization, magnetotransport and Mossbauer spectroscopy measurements of sequentially evaporated Fe-Ag granular composites are presented. The strong magnetic scattering of the conduction electrons is reflected in the sublinear temperature dependence of the resistance and in the large negative magnetoresistance. The simultaneous analysis of the magnetic properties and the transport behavior suggests a bimodal grain size distribution. A detailed quantitative description of the unusual features observed in the transport properties is given.

Anaerobic regulation of hydrogenase transcription in different bacteria

Hydrogen metabolism is closely related to other important metabolic and energetic processes of bacterial cells, such as photosynthesis, anaerobic respiration and sulphur metabolism. Even small environmental changes influence these networks through different regulatory systems. The presence or absence of oxygen is one of the most important signals; how the cascades evolved to transmit this signal in different bacteria is summarized. In many instances, hydrogen is released only under anoxic conditions, because of bioenergetic considerations. Most [NiFe] hydrogenases are inactivated by oxygen, but many of them can be re-activated under reducing conditions. In addition to direct inactivation of the hydrogenases, oxygen can also regulate their expression. The global regulatory systems [FNR (fumarate and nitrate reduction regulator), ArcAB (aerobic respiratory control) and RegAB], which respond to alterations in oxygen content and redox conditions of the environment, have an important role in hydrogenase regulation of several bacteria. FNR-like proteins were shown to be important for the regulation of hydrogenases in Escherichia coli, Thiocapsa roseopersicina and Rhizobium leguminosarum, whereas RegA protein modulates the expression of hupSL genes in Rhodobacter capsulatus.