Pyrococcus furiosus: large-scale cultivation and enzyme purification

An unprecedented function for a tungsten-containing oxidoreductase

Five tungstopterin-containing oxidoreductases were characterized from the hyperthermophile Pyrococcus furiosus. Each enzyme catalyzes the reversible conversion of one or more aldehydes to the corresponding carboxylic acid, but they have different specificities. The physiological functions of only two of these enzymes are known: one, termed GAPOR, is a glycolytic enzyme that oxidizes glyceraldehyde-3-phosphate, while the other, termed AOR, oxidizes multiple aldehydes generated during peptide fermentation. Two of the enzymes have known structures (AOR and FOR). Herein, we focus on WOR5, the fifth tungstopterin enzyme to be discovered in P. furiosus. Expression of WOR5 was previously shown to be increased during cold shock (growth at 72 ℃), although the physiological substrate is not known. To gain insight into WOR5 function, we sought to determine both its structure and identify its intracellular substrate. Crystallization experiments were performed with a concentrated cytoplasmic extract of P. furiosus grown at 72 ℃ and the structure of WOR5 was deduced from the crystals that were obtained. In contrast to a previous report, WOR5 is heterodimeric containing an additional polyferredoxin-like subunit with four [4Fe-4S] clusters. The active site structure of WOR5 is substantially different from that of AOR and FOR and the significant electron density observed adjacent to the tungsten cofactor of WOR5 was modeled as an aliphatic sulfonate. Biochemical assays and product analysis confirmed that WOR5 is an aliphatic sulfonate ferredoxin oxidoreductase (ASOR). A catalytic mechanism for ASOR is proposed based on the structural information and the potential role of ASOR in the cold-shock response is discussed.

Cryoelectron microscopy structure and mechanism of the membrane-associated electron-bifurcating flavoprotein Fix/EtfABCX

The electron-transferring flavoprotein-menaquinone oxidoreductase ABCX (EtfABCX), also known as FixABCX for its role in nitrogen-fixing organisms, is a member of a family of electron-transferring flavoproteins that catalyze electron bifurcation. EtfABCX enables endergonic reduction of ferredoxin (E°' ∼-450 mV) using NADH (E°' -320 mV) as the electron donor by coupling this reaction to the exergonic reduction of menaquinone (E°' -80 mV). Here we report the 2.9 Å structure of EtfABCX, a membrane-associated flavin-based electron bifurcation (FBEB) complex, from a thermophilic bacterium. EtfABCX forms a superdimer with two membrane-associated EtfCs at the dimer interface that contain two bound menaquinones. The structure reveals that, in contrast to previous predictions, the low-potential electrons bifurcated from EtfAB are most likely directly transferred to ferredoxin, while high-potential electrons reduce the quinone via two [4Fe-4S] clusters in EtfX. Surprisingly, EtfX shares remarkable structural similarity with mammalian [4Fe-4S] cluster-containing ETF ubiquinone oxidoreductase (ETF-QO), suggesting an unexpected evolutionary link between bifurcating and nonbifurcating systems. Based on this structure and spectroscopic studies of a closely related EtfABCX, we propose a detailed mechanism of the catalytic cycle and the accompanying structural changes in this membrane-associated FBEB system.

The catalytic mechanism of electron-bifurcating electron transfer flavoproteins (ETFs) involves an intermediary complex with NAD<sup/>

Electron bifurcation plays a key role in anaerobic energy metabolism, but it is a relatively new discovery, and only limited mechanistic information is available on the diverse enzymes that employ it. Herein, we focused on the bifurcating electron transfer flavoprotein (ETF) from the hyperthermophilic archaeon Pyrobaculum aerophilum The EtfABCX enzyme complex couples NADH oxidation to the endergonic reduction of ferredoxin and exergonic reduction of menaquinone. We developed a model for the enzyme structure by using nondenaturing MS, cross-linking, and homology modeling in which EtfA, -B, and -C each contained FAD, whereas EtfX contained two [4Fe-4S] clusters. On the basis of analyses using transient absorption, EPR, and optical titrations with NADH or inorganic reductants with and without NAD+, we propose a catalytic cycle involving formation of an intermediary NAD+-bound complex. A charge transfer signal revealed an intriguing interplay of flavin semiquinones and a protein conformational change that gated electron transfer between the low- and high-potential pathways. We found that despite a common bifurcating flavin site, the proposed EtfABCX catalytic cycle is distinct from that of the genetically unrelated bifurcating NADH-dependent ferredoxin NADP+ oxidoreductase (NfnI). The two enzymes particularly differed in the role of NAD+, the resting and bifurcating-ready states of the enzymes, how electron flow is gated, and the two two-electron cycles constituting the overall four-electron reaction. We conclude that P. aerophilum EtfABCX provides a model catalytic mechanism that builds on and extends previous studies of related bifurcating ETFs and can be applied to the large bifurcating ETF family.

Recombinant expression library of Pyrococcus furiosus constructed by high-throughput cloning: a useful tool for functional and structural genomics

Hyperthermophile Pyrococcus furiosus grows optimally near 100°C and is an important resource of many industrial and molecular biological enzymes. To study the structure and function of P. furiosus proteins at whole genome level, we constructed expression plasmids of each P. furiosus gene using a ligase-independent cloning method, which was based on amplifying target gene and vector by PCR using phosphorothioate-modified primers and digesting PCR products by λ exonuclease. Our cloning method had a positive clone percentage of ≥ 80% in 96-well plate cloning format. Small-scale expression experiment showed that 55 out of 80 genes were efficiently expressed in Escherichia coli Strain Rosetta 2(DE3)pLysS. In summary, this recombinant expression library of P. furiosus provides a platform for functional and structural studies, as well as developing novel industrial enzymes. Our cloning scheme is adaptable to constructing recombinant expression library of other sequenced organisms.

Proteomics of Pyrococcus furiosus (Pfu): Identification of Extracted Proteins by Three Independent Methods

Pyrococcus furiosus (Pfu) is an excellent organism to generate reference samples for proteomics laboratories because of its moderately sized genome and very little sequence duplication within the genome. We demonstrated a stable and consistent method to prepare proteins in bulk that eliminates growth and preparation as a source of uncertainty in the standard. We performed several proteomic studies in different laboratories using each laboratory's specific workflow as well as separate and integrated data analysis. This study demonstrated that a Pfu whole cell lysate provides suitable protein sample complexity to not only validate proteomic methods, work flows, and benchmark new instruments but also to facilitate comparison of experimental data generated over time and across instruments or laboratories.

Anaerobic high-throughput cultivation method for isolation of thermophiles using biomass-derived substrates

Flow cytometry (FCM) techniques have been developed for sorting mesophilic organisms, but the difficulty increases if the target microbes are thermophilic anaerobes. We demonstrate a reliable, high-throughput method of screening thermophilic anaerobic organisms using FCM and 96-well plates for growth on biomass-relevant substrates. The method was tested using the cellulolytic thermophiles Clostridium thermocellum (T(opt) = 55 °C), Caldicellulosiruptor obsidiansis (T(opt) = 78 °C) and the fermentative hyperthermophiles, Pyrococcus furiosus (T(opt) = 100 °C) and Thermotoga maritima (T(opt) = 80 °C). Multi-well plates were incubated at various temperatures for approximately 72-120 h and then tested for growth. Positive growth resulting from single cells sorted into individual wells containing an anaerobic medium was verified by OD(600). Depending on the growth substrate, up to 80 % of the wells contained viable cultures, which could be transferred to fresh media. This method was used to isolate thermophilic microbes from Rabbit Creek, Yellowstone National Park (YNP), Wyoming. Substrates for enrichment cultures including crystalline cellulose (Avicel), xylan (from Birchwood), pretreated switchgrass and Populus were used to cultivate organisms that may be of interest to lignocellulosic biofuel production.

Engineering hyperthermophilic archaeon Pyrococcus furiosus to overproduce its cytoplasmic [NiFe]-hydrogenase

The cytoplasmic hydrogenase (SHI) of the hyperthermophilic archaeon Pyrococcus furiosus is an NADP(H)-dependent heterotetrameric enzyme that contains a nickel-iron catalytic site, flavin, and six iron-sulfur clusters. It has potential utility in a range of bioenergy systems in vitro, but a major obstacle in its use is generating sufficient amounts. We have engineered P. furiosus to overproduce SHI utilizing a recently developed genetic system. In the overexpression (OE-SHI) strain, transcription of the four-gene SHI operon was under the control of a strong constitutive promoter, and a Strep-tag II was added to the N terminus of one subunit. OE-SHI and wild-type P. furiosus strains had similar rates of growth and H(2) production on maltose. Strain OE-SHI had a 20-fold higher transcription of the polycistronic hydrogenase mRNA encoding SHI, and the specific activity of the cytoplasmic hydrogenase was ∼10-fold higher when compared with the wild-type strain, although the expression levels of genes encoding processing and maturation of SHI were the same in both strains. Overexpressed SHI was purified by a single affinity chromatography step using the Strep-tag II, and it and the native form had comparable activities and physical properties. Based on protein yield per gram of cells (wet weight), the OE-SHI strain yields a 100-fold higher amount of hydrogenase when compared with the highest homologous [NiFe]-hydrogenase system previously reported (from Synechocystis). This new P. furiosus system will allow further engineering of SHI and provide hydrogenase for efficient in vitro biohydrogen production.

Homologous expression of a subcomplex of Pyrococcus furiosus hydrogenase that interacts with pyruvate ferredoxin oxidoreductase

Hydrogen gas is an attractive alternative fuel as it is carbon neutral and has higher energy content per unit mass than fossil fuels. The biological enzyme responsible for utilizing molecular hydrogen is hydrogenase, a heteromeric metalloenzyme requiring a complex maturation process to assemble its O(2)-sensitive dinuclear-catalytic site containing nickel and iron atoms. To facilitate their utility in applied processes, it is essential that tools are available to engineer hydrogenases to tailor catalytic activity and electron carrier specificity, and decrease oxygen sensitivity using standard molecular biology techniques. As a model system we are using hydrogen-producing Pyrococcus furiosus, which grows optimally at 100°C. We have taken advantage of a recently developed genetic system that allows markerless chromosomal integrations via homologous recombination. We have combined a new gene marker system with a highly-expressed constitutive promoter to enable high-level homologous expression of an engineered form of the cytoplasmic NADP-dependent hydrogenase (SHI) of P. furiosus. In a step towards obtaining 'minimal' hydrogenases, we have successfully produced the heterodimeric form of SHI that contains only two of the four subunits found in the native heterotetrameric enzyme. The heterodimeric form is highly active (150 units mg(-1) in H(2) production using the artificial electron donor methyl viologen) and thermostable (t(1/2) ∼0.5 hour at 90°C). Moreover, the heterodimer does not use NADPH and instead can directly utilize reductant supplied by pyruvate ferredoxin oxidoreductase from P. furiosus. The SHI heterodimer and POR therefore represent a two-enzyme system that oxidizes pyruvate and produces H(2) in vitro without the need for an intermediate electron carrier.

Parallel evolution of transcriptome architecture during genome reorganization

Assembly of genes into operons is generally viewed as an important process during the continual adaptation of microbes to changing environmental challenges. However, the genome reorganization events that drive this process are also the roots of instability for existing operons. We have determined that there exists a statistically significant trend that correlates the proportion of genes encoded in operons in archaea to their phylogenetic lineage. We have further characterized how microbes deal with operon instability by mapping and comparing transcriptome architectures of four phylogenetically diverse extremophiles that span the range of operon stabilities observed across archaeal lineages: a photoheterotrophic halophile (Halobacterium salinarum NRC-1), a hydrogenotrophic methanogen (Methanococcus maripaludis S2), an acidophilic and aerobic thermophile (Sulfolobus solfataricus P2), and an anaerobic hyperthermophile (Pyrococcus furiosus DSM 3638). We demonstrate how the evolution of transcriptional elements (promoters and terminators) generates new operons, restores the coordinated regulation of translocated, inverted, and newly acquired genes, and introduces completely novel regulation for even some of the most conserved operonic genes such as those encoding subunits of the ribosome. The inverse correlation (r=-0.92) between the proportion of operons with such internally located transcriptional elements and the fraction of conserved operons in each of the four archaea reveals an unprecedented view into varying stages of operon evolution. Importantly, our integrated analysis has revealed that organisms adapted to higher growth temperatures have lower tolerance for genome reorganization events that disrupt operon structures.

Identification of membrane proteins in the hyperthermophilic archaeon pyrococcus furiosus using proteomics and prediction programs

Cell-free extracts from the hyperthermophilic archaeon Pyrococcus furiosus were separated into membrane and cytoplasmic fractions and each was analyzed by 2D-gel electrophoresis. A total of 66 proteins were identified, 32 in the membrane fraction and 34 in the cytoplasmic fraction. Six prediction programs were used to predict the subcellular locations of these proteins. Three were based on signal-peptides (SignalP, TargetP, and SOSUISignal) and three on transmembrane-spanning α-helices (TSEG, SOSUI, and PRED-TMR2). A consensus of the six programs predicted that 23 of the 32 proteins (72%) from the membrane fraction should be in the membrane and that all of the proteins from the cytoplasmic fraction should be in the cytoplasm. Two membrane-associated proteins predicted to be cytoplasmic by the programs are also predicted to consist primarily of transmembrane-spanning β-sheets using porin protein models, suggesting that they are, in fact, membrane components. An ATPase subunit homolog found in the membrane fraction, although predicted to be cytoplasmic, is most likely complexed with other ATPase subunits in the membrane fraction. An additional three proteins predicted to be cytoplasmic but found in the membrane fraction, may be cytoplasmic contaminants. These include a chaperone homolog that may have attached to denatured membrane proteins during cell fractionation. Omitting these three proteins would boost the membrane-protein predictability of the models to near 80%. A consensus prediction using all six programs for all 2242 ORFs in the P. furiosus genome estimates that 24% of the ORF products are found in the membrane. However, this is likely to be a minimum value due to the programs’ inability to recognize certain membrane-related proteins, such as subunits associated with membrane complexes and porin-type proteins.

Antimicrobial activity of the iron-sulfur nitroso compound Roussin's black salt [Fe4S3(NO)7] on the hyperthermophilic archaeon Pyrococcus furiosus

The iron-sulfur nitroso compound [Fe(4)S(3)(NO)(7)](-) is a broad-spectrum antimicrobial agent that has been used for more than 100 years to combat pathogenic anaerobes. Known as Roussin's black salt (RBS), it contains seven moles of nitric oxide, the release of which was always assumed to mediate its cytotoxicity. Using the hyperthermophilic archaeon Pyrococcus furiosus, it is demonstrated through growth studies, membrane analyses, and scanning electron microscopy that nitric oxide does not play a role in RBS toxicity; rather, the mechanism involves membrane disruption leading to cell lysis. Moreover, insoluble elemental sulfur (S(0)), which is reduced by P. furiosus to hydrogen sulfide, prevents cell lysis by RBS. It is proposed that S(0) also directly interacts with the membranes of P. furiosus during its transfer into the cell, ultimately for reduction by a cytosolic NADPH sulfur reductase. RBS is proposed to be a new class of inorganic antimicrobial agent that also has potential use as an inert cell-lysing agent.

Novel multiprotein complexes identified in the hyperthermophilic archaeon Pyrococcus furiosus by non-denaturing fractionation of the native proteome

Virtually all cellular processes are carried out by dynamic molecular assemblies or multiprotein complexes, the compositions of which are largely undefined. They cannot be predicted solely from bioinformatics analyses nor are there well defined techniques currently available to unequivocally identify protein complexes (PCs). To address this issue, we attempted to directly determine the identity of PCs from native microbial biomass using Pyrococcus furiosus, a hyperthermophilic archaeon that grows optimally at 100 degrees C, as the model organism. Novel PCs were identified by large scale fractionation of the native proteome using non-denaturing, sequential column chromatography under anaerobic, reducing conditions. A total of 967 distinct P. furiosus proteins were identified by mass spectrometry (nano LC-ESI-MS/MS), representing approximately 80% of the cytoplasmic proteins. Based on the co-fractionation of proteins that are encoded by adjacent genes on the chromosome, 106 potential heteromeric PCs containing 243 proteins were identified, only 20 of which were known or expected. In addition to those of unknown function, novel and uncharacterized PCs were identified that are proposed to be involved in the metabolism of amino acids (10), carbohydrates (four), lipids (two), vitamins and metals (three), and DNA and RNA (nine). A further 30 potential PCs were classified as tentative, and the remaining potential PCs (13) were classified as weakly interacting. Some major advantages of native biomass fractionation for PC identification are that it provides a road map for the (partial) purification of native forms of novel and uncharacterized PCs, and the results can be utilized for the recombinant production of low abundance PCs to provide enough material for detailed structural and biochemical analyses.

Insights into the metabolism of elemental sulfur by the hyperthermophilic archaeon Pyrococcus furiosus: characterization of a coenzyme A- dependent NAD(P)H sulfur oxidoreductase

The hyperthermophilic archaeon Pyrococcus furiosus uses carbohydrates as a carbon source and produces acetate, CO2, and H2 as end products. When S(0) is added to a growing culture, within 10 min the rate of H2 production rapidly decreases and H(2)S is detected. After 1 hour cells contain high NADPH- and coenzyme A-dependent S(0) reduction activity (0.7 units/mg, 85 degrees C) located in the cytoplasm. The enzyme responsible for this activity was purified to electrophoretic homogeneity (specific activity, 100 units/mg) and is termed NAD(P)H elemental sulfur oxidoreductase (NSR). NSR is a homodimeric flavoprotein (M(r), 100,000) and is encoded by PF1186. This designation was previously assigned to the gene encoding an enzyme that reduces coenzyme A disulfide, which is a side reaction of NSR. Whole-genome DNA microarray and quantitative PCR analyses showed that the expression of NSR is up-regulated up to sevenfold within 10 min of S(0) addition. This primary response to S(0) also involves the up-regulation (>16-fold) of a 13-gene cluster encoding a membrane-bound oxidoreductase (MBX). The cluster encoding MBX is proposed to replace the homologous 14-gene cluster that encodes the ferredoxin-oxidizing, H2-evolving membrane-bound hydrogenase (MBH), which is down-regulated >12-fold within 10 min of S(0) addition. Although an activity for MBX could not be demonstrated, it is proposed to conserve energy by oxidizing ferredoxin and reducing NADP, which is used by NSR to reduce S(0). A secondary response to S(0) is observed 30 min after S(0) addition and includes the up-regulation of genes encoding proteins involved in amino acid biosynthesis and iron metabolism, as well as two so-called sulfur-induced proteins termed SipA and SipB. This novel S(0)-reducing system involving NSR and MBX has been found so far only in the heterotrophic Thermococcales and is in contrast to the cytochrome- and quinone-based S(0)-reducing system in autotrophic archaea and bacteria.

Metabolic and evolutionary relationships among Pyrococcus Species: genetic exchange within a hydrothermal vent environment

Pyrococcus furiosus and Pyrococcus woesei grow optimally at temperatures near 100 degrees C and were isolated from the same shallow marine volcanic vent system. Hybridization of genomic DNA from P. woesei to a DNA microarray containing all 2,065 open reading frames (ORFs) annotated in the P. furiosus genome, in combination with PCR analysis, indicated that homologs of 105 ORFs present in P. furiosus are absent from the uncharacterized genome of P. woesei. Pulsed-field electrophoresis indicated that the sizes of the two genomes are comparable, and the results were consistent with the hypothesis that P. woesei lacks the 105 ORFs found in P. furiosus. The missing ORFs are present in P. furiosus mainly in clusters. These clusters include one cluster (Mal I, PF1737 to PF1751) involved in maltose metabolism and another cluster (PF0691 to PF0695) whose products are thought to remove toxic reactive nitrogen species. Accordingly, it was found that P. woesei, in contrast to P. furiosus, is unable to utilize maltose as a carbon source for growth, and the growth of P. woesei on starch was inhibited by addition of a nitric oxide generator. In P. furiosus the ORF clusters not present in P. woesei are bracketed by or are in the vicinity of insertion sequences or long clusters of tandem repeats (LCTRs). While the role of LCTRs in lateral gene transfer is not known, the Mal I cluster in P. furiosus is a composite transposon that undergoes replicative transposition. The same locus in P. woesei lacks any evidence of insertion activity, indicating that P. woesei is a sister or even the parent of P. furiosus. P. woesei may have acquired by lateral gene transfer more than 100 ORFs from other organisms living in the same thermophilic environment to produce the type strain of P. furiosus.

A simple energy-conserving system: proton reduction coupled to proton translocation

Oxidative phosphorylation involves the coupling of ATP synthesis to the proton-motive force that is generated typically by a series of membrane-bound electron transfer complexes, which ultimately reduce an exogenous terminal electron acceptor. This is not the case with Pyrococcus furiosus, an archaeon that grows optimally near 100 degrees C. It has an anaerobic respiratory system that consists of a single enzyme, a membrane-bound hydrogenase. Moreover, it does not require an added electron acceptor as the enzyme reduces protons, the simplest of acceptors, to hydrogen gas by using electrons from the cytoplasmic redox protein ferredoxin. It is demonstrated that the production of hydrogen gas by membrane vesicles of P. furiosus is directly coupled to the synthesis of ATP by means of a proton-motive force that has both electrochemical and pH components. Such a respiratory system enables rationalization in this organism of an unusual glycolytic pathway that was previously thought not to conserve energy. It is now clear that the use of ferredoxin in place of the expected NAD as the electron acceptor for glyceraldehyde 3-phosphate oxidation enables energy to be conserved by hydrogen production. In addition, this simple respiratory mechanism readily explains why the growth yields of P. furiosus are much higher than could be accounted for if ATP synthesis occurred only by substrate-level phosphorylation. The ability of microorganisms such as P. furiosus to couple hydrogen production to energy conservation has important ramifications not only in the evolution of respiratory systems but also in the origin of life itself.

Characterization of a fourth tungsten-containing enzyme from the hyperthermophilic archaeon Pyrococcus furiosus

Pyrococcus furiosus grows optimally near 100 degrees C using peptides and carbohydrates as carbon sources, and it reduces elemental sulfur (S(0)), if present, to H(2)S. Tungsten (W), an element rarely used in biology, is required for optimal growth, and three different tungsten-containing enzymes have been previously purified from this organism. They all oxidize aldehydes of various types and are thought to play primary roles in the catabolism of sugars or amino acids. Here, the purification of a fourth tungsten-containing enzyme, termed WOR 4, from cell extracts of P. furiosus grown with S(0) is described. This was achieved by monitoring through multiple chromatography steps the W that is not associated with the three characterized tungstoenzymes. The N-terminal sequence of WOR 4 and the approximate molecular weight of its subunit determined electrophoretically (69,000) correspond to the product of an ORF (PF1961, wor4) present in the complete genome sequence of P. furiosus. WOR 4 is a homodimer and contains approximately one W, three Fe, three or four acid-labile sulfide, and one Ca atom per subunit. The visible and electron paramagnetic resonance spectra of the oxidized and reduced enzyme indicate the presence of an unusual iron-sulfur chromophore. WOR 4 does not oxidize aliphatic or aromatic aldehydes or hydroxy acids, nor does it reduce keto acids. Consistent with prior microarray data, the protein could not be purified from P. furiosus cells grown in the absence of S(0), suggesting that it may have a role in S(0) metabolism.

A solution NMR molecular model for the aspartate-ligated, cubane cluster containing ferredoxin from the hyperthermophilic archeaon Pyrococcus furiosus

A solution molecular model for the conformationally dynamically heterogeneous Pyrococcus furiosus ferredoxin with an intact disulfide bond has been constructed on the basis of reported (1)H NMR spectral parameters using distance geometry and simulated annealing protocols. Conventional long-mixing time NOESY and H-bonding constraints have been augmented by previously reported short-mixing time NOESY, steady-state NOE, and cluster paramagnetism-induced relaxation. The family of 15 structures with inconsequential violations exhibited low rms deviations for backbone atoms for the overwhelming majority of the residues, including the cluster ligating loop with the unprecedented ligated Asp14. Larger rms deviations were observed across the disulfide bond, but closer inspection revealed that the 15 structures can be factored into 10 substructures exhibiting an "S" or right-handed disulfide orientation and 5 exhibiting an "R" or left-handed disulfide orientation. The remainder of the structure is indistinguishable for the two disulfide orientations but confirms stabilizing extensions of secondary structural elements in the lengthening of the long helix and both the lengthening and incorporation of a third strand into the beta-sheet involving the termini, with these extensions interacting strongly in a modular fashion through the rings of Tyr46 and Trp2. These extensions of stabilizing interactions in Pyrococcus furiosus Fd, however, lead to strong destabilization of the disulfide bond and destabilization of the highly conserved first and last beta-turns in the sequence. It is concluded that the structural alternations in Pyrococcus Fd relative to other hyperthermostable Fds are not to increase thermostability but to place "stress" on the disulfide bond and render it more reducible. The possible physiological implications of this unique reducible disulfide bond are discussed.

Key role for sulfur in peptide metabolism and in regulation of three hydrogenases in the hyperthermophilic archaeon Pyrococcus furiosus

The hyperthermophilic archaeon Pyrococcus furiosus grows optimally at 100 degrees C by the fermentation of peptides and carbohydrates. Growth of the organism was examined in media containing either maltose, peptides (hydrolyzed casein), or both as the carbon source(s), each with and without elemental sulfur (S(0)). Growth rates were highest on media containing peptides and S(0), with or without maltose. Growth did not occur on the peptide medium without S(0). S(0) had no effect on growth rates in the maltose medium in the absence of peptides. Phenylacetate production rates (from phenylalanine fermentation) from cells grown in the peptide medium containing S(0) with or without maltose were the same, suggesting that S(0) is required for peptide utilization. The activities of 14 of 21 enzymes involved in or related to the fermentation pathways of P. furiosus were shown to be regulated under the five different growth conditions studied. The presence of S(0) in the growth media resulted in decreases in specific activities of two cytoplasmic hydrogenases (I and II) and of a membrane-bound hydrogenase, each by an order of magnitude. The primary S(0)-reducing enzyme in this organism and the mechanism of the S(0) dependence of peptide metabolism are not known. This study provides the first evidence for a highly regulated fermentation-based metabolism in P. furiosus and a significant regulatory role for elemental sulfur or its metabolites.