The amino acid sequence of glutamate dehydrogenase fromPyrococcus furiosus, a hyperthermophilic archaebacterium

The complete amino acid sequence of glutamate dehydrogenase from the archaebacteriumPyrococcus furiosus has been determined. The sequence was reconstructed by automated sequence analysis of peptides obtained after cleavage with cyanogen bromide, Asp-N endoproteinase, trypsin, or pepsin. The enzyme subunit is composed of 420 amino acid residues yielding a molecular mass of 47,122 D. In the recently determined primary structure of glutamate dehydrogenase from another thermophilic archaebacterium,Sulfolobus solfataricus, the presence of some methylated lysines was detected and the possible role of this posttranslational modification in enhancing the thermostability of the enzyme was discussed (Maras, B., Consalvi, V., Chiaraluce, R., Politi, L., De Rosa, M., Bossa, F., Scandurra, R., and Barra, D. (1992),Eur. J. Biochem. 203, 81–87). In the primary structure reported here, such posttranslational modification has not been found, indicating that the role of lysine methylation should be revisited. Comparison of the sequence of glutamate dehydrogenase fromPyrococcus furiosus with that ofS. solfataricus shows a 43.7% similarity, thus indicating a common evolutionary pathway.     

DBF (Disulfide Bond Forming) Enzyme from the Hyperthermophilic Archaebacterium Sulfolobus Solfataricus Behaves Like a Molecular Chaperone

DBF enzyme from the hyperthermophilic archaebacterium Sulfolobus solfataricus greatly enhances the refolding at 30°C of denatured and reduced bovine pancreatic ribonuclease (Guagliardi et al., 1992). Here we show that DBF behaves like a molecular chaperone: it affects in an ATP-dependent manner the in vitro refolding at 50°C of two thermostable dehydrogenases, an alcohol dehydrogenase and a glutamate dehydrogenase from S. solfataricus. This paper also reports the complete amino acid sequence of DBF. The role of molecular chaperones from thermophilic microorganisms in applied biocatalysis is discussed.     

Characterization of Native and Recombinant Forms of an Unusual Cobalt-Dependent Proline Dipeptidase (Prolidase) from the Hyperthermophilic Archaeon Pyrococcus furiosus

Proline dipeptidase (prolidase) was purified from cell extracts of the proteolytic, hyperthermophilic archaeon Pyrococcus furiosus by multistep chromatography. The enzyme is a homodimer (39.4 kDa per subunit) and as purified contains one cobalt atom per subunit. Its catalytic activity also required the addition of Co²⁺ ions (K_d, 0.24 mM), indicating that the enzyme has a second metal ion binding site. Co²⁺ could be replaced by Mn²⁺ (resulting in a 25% decrease in activity) but not by Mg²⁺, Ca²⁺, Fe²⁺, Zn²⁺, Cu²⁺, or Ni²⁺. The prolidase exhibited a narrow substrate specificity and hydrolyzed only dipeptides with proline at the C terminus and a nonpolar amino acid (Met, Leu, Val, Phe, or Ala) at the N terminus. Optimal prolidase activity with Met-Pro as the substrate occurred at a pH of 7.0 and a temperature of 100°C. The N-terminal amino acid sequence of the purified prolidase was used to identify in the P. furiosus genome database a putative prolidase-encoding gene with a product corresponding to 349 amino acids. This gene was expressed in Escherichia coli and the recombinant protein was purified. Its properties, including molecular mass, metal ion dependence, pH and temperature optima, substrate specificity, and thermostability, were indistinguishable from those of the native prolidase from P. furiosus. Furthermore, the K_m values for the substrate Met-Pro were comparable for the native and recombinant forms, although the recombinant enzyme exhibited a twofold greater V_max value than the native protein. The amino acid sequence of P. furiosus prolidase has significant similarity with those of prolidases from mesophilic organisms, but the enzyme differs from them in its substrate specificity, thermostability, metal dependency, and response to inhibitors. The P. furiosus enzyme appears to be the second Co-containing member (after methionine aminopeptidase) of the binuclear N-terminal exopeptidase family.     

Bifunctional isocitrate-homoisocitrate dehydrogenase: a missing link in the evolution of beta-decarboxylating dehydrogenase

Beta-decarboxylating dehydrogenases comprise 3-isopropylmalate dehydrogenase, isocitrate dehydrogenase, and homoisocitrate dehydrogenase. They share a high degree of amino acid sequence identity and occupy equivalent positions in the amino acid biosynthetic pathways for leucine, glutamate, and lysine, respectively. Therefore, not only the enzymes but also the whole pathways should have evolved from a common ancestral pathway. In Pyrococcus horikoshii, only one pathway of the three has been identified in the genomic sequence, and PH1722 is the sole beta-decarboxylating dehydrogenase gene. The organism does not require leucine, glutamate, or lysine for growth; the single pathway might play multiple (i.e., ancestral) roles in amino acid biosynthesis. The PH1722 gene was cloned and expressed in Escherichia coli and the substrate specificity of the recombinant enzyme was investigated. It exhibited activities on isocitrate and homoisocitrate at near equal efficiency, but not on 3-isopropylmalate. PH1722 is thus a novel, bifunctional beta-decarboxylating dehydrogenase, which likely plays a dual role in glutamate and lysine biosynthesis in vivo.     

Hjc resolvase is a distantly related member of the type II restriction endonuclease family

Hjc resolvase is an archaeal enzyme involved in homologous DNA recombination at the Holliday junction intermediate. However, the structure and the catalytic mechanism of the enzyme have not yet been identified. We performed database searching using the amino acid sequence of the enzyme from Pyrococcus furiosus as a query. We detected 59 amino acid sequences showing weak but significant sequence similarity to the Hjc resolvase. The detected sequences included DpnII, HaeII and Vsr endonuclease, which belong to the type II restriction endonuclease family. In addition, a highly conserved region was identified from a multiple alignment of the detected sequences, which was similar to an active site of the type II restriction endonucleases. We substituted three conserved amino acid residues in the highly conserved region of the Hjc resolvase with Ala residues. The amino acid replacements inactivated the enzyme. The experimental study, together with the results of the database searching, suggests that the Hjc resolvase is a distantly related member of the type II restriction endonuclease family. In addition, the results of our database searches suggested that the members of the RecB domain superfamily are evolutionarily related to the type II restriction endonuclease family.     

Engineering activity and stability of Thermotoga maritima glutamate dehydrogenase. II: construction of a 16-residue ion-pair network at the subunit interface.

The role of an 18-residue ion-pair network, that is present in the glutamate dehydrogenase from the hyperthermophilic archaeon Pyrococcus furiosus, in conferring stability to other, less stable homologous enzymes, has been studied by introducing four new charged amino acid residues into the subunit interface of glutamate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima. These two GDHs are 55 % identical in amino acid sequence, differ greatly in thermo-activity and stability and derive from microbes with different phylogenetic positions. Amino acid substitutions were introduced as single mutations as well as in several combinations. Elucidation of the crystal structure of the quadruple mutant S128R/T158E/N117R/S160E T. maritima glutamate dehydrogenase showed that all anticipated ion-pairs are formed and that a 16-residue ion-pair network is present. Enlargement of existing networks by single amino acid substitutions unexpectedly resulted in a decrease in resistance towards thermal inactivation and thermal denaturation. However, combination of destabilizing single mutations in most cases restored stability, indicating the need for balanced charges at subunit interfaces and high cooperativity between the different members of the network. Combination of the three destabilizing mutations in triple mutant S128R/T158E/N117R resulted in an enzyme with a 30 minutes longer half-life of inactivation at 85 degrees C, a 3 degrees C higher temperature optimum for catalysis, and a 0.5 degrees C higher apparent melting temperature than that of wild-type glutamate dehydrogenase. These findings confirm the hypothesis that large ion-pair networks do indeed stabilize enzymes from hyperthermophilic organisms.     

Identification and Molecular Characterization of a Novel Type of α-galactosidase from Pyrococcus furiosus

An α-galactosidase gene from Pyrococcus furiosus was identified, cloned and functionally expressed in Escherichia coli. It is the first α-galactosidase from a hyperthermophilic archaeon described to date. The gene encodes a unique amino acid sequence compared to other α-galactosidases. Highest homology was found with α-amylases classified in family 57 of glycoside hydrolases. The 364 amino acid protein had a calculated mass of 41.6 kDa. The recombinant α-galactosidase specifically catalyzed the hydrolysis of para-nitrophenyl-α-galactopyranoside, and to some extent that of melibiose and raffinose. The enzyme proved to be an extremely thermo-active and thermostable α-galactosidase with a temperature optimum of 115°C and a half-life time of 15 hours at 100°C. The pH optimum is between 5.0 and 5.5. Sequence analysis showed four conserved carboxylic residues. Site-directed mutagenesis was applied to identify the potential catalytic residues. Glu117Ala showed decreased enzyme activity, which could be rescued by the addition of azide or formate. It is concluded that glutamate 117 is the catalytic nucleophile, whereas the acid/base catalyst remains to be identified.     

Sequence of archaealMethanococcus jannaschii α-amylase contains features of families 13 and 57 of glycosyl hydrolases: A trace of their common ancestor?

Two sequentially different, seemingly unrelated α-amylase families exist, known as family-13 and family-57 glycosyl hydrolases. Despite the common enzyme activity, it has as yet been impossible to detect any sequence similarity between the two families. The detailed analysis of the recently determined sequence of the α-amylase from methanogenic archaeonMethanococcus jannaschii using the sensitiveHydrophobic Cluster Analysis method revealed that this α-amylase contains features of both families of α-amylases. Thus theM. jannaschii α-amylase is similar to thePyrococcus furiosus α-amylase from family 57 while it obviously contains most of the sequence fingerprints characteristic for α-amylase family 13. Importantly, a glutamic acid residue equivalent with the family-13 catalytic glutamate positioned in the β5-strand segment was identified in members of family 57. The results presented in this report indicate that the two families, 13 and 57, are either the products of a very distant common ancestor or have evolved from each other, although at present they can represent two different α-amylase families with evolved different catalytic mechanisms, catalytic machinery and folds.     

Mutation leading to gene fusion in the histidine operon of Salmonella typhimurium

A spontaneous polar mutation located in the region of an intercistronic border in the hiatidine operon of Salmonella was isolated in our laboratory. The mutant, R81, tests as a frameshift in reversion experiments but is prototrophic, capable of growth without histidine supplements despite lowered levels of certain histidine enzymes. The mutation affects the operator distal end of the D gene, causing production of an active histidinol dehydrogenase enzyme with an altered C-terminus. The mutation severely affects expression of the immediately succeeding gene in the translation sequence, hisC, suggesting either that the D–C border and possibly hisC are physically altered or that their normal function in translation is seriously impaired. We have previously described the fortuitous production from R81 of a non-polar derivative with fused D and C genes. This strain produces a bifunctional enzyme with normally separate dehydrogenase and aminotransferase activities present on dimers or multimers of a single fused polypeptide chain. We have now investigated in greater detail the R81 mutation by amino acid sequencing of the C-terminus of altered histidinol dehydrogenase. We find that the R81 mutation causes the addition of a “tail” of four amino acid residues to an otherwise normal dehydrogenase polypeptide chain. The results support our previous suggestion that the R81 mutation profoundly effects the D–C gene border and that this effect is prerequisite to gene fusion.     

A topA intein in Pyrococcus furiosus and its relatedness to the r-gyr intein of Methanococcus jannaschii

A new intein coding sequence was found in a topA (DNA topoisomerase I) gene by cloning and sequencing this gene from the hyperthermophilic Archaeon Pyrococcus furiosus. The predicted Pfu topA intein sequence is 373 amino acids long and located two residues away from the catalytic tyrosine of the topoisomerase. It contains putative intein sequence blocks (C, E, and H) associated with intein endonuclease activity, in addition to intein sequence blocks (A, B, F, and G) that are necessary for protein splicing. This DNA topoisomerase I intein is most related to a reverse gyrase intein from the methanogenic Archaeon Methanococcus jannaschii. These two inteins share 31% amino acid sequence identity and, more importantly, have the same insertion sites in their respective host proteins. It is suggested that these two inteins are homologous inteins present in structurally related, but functionally distinct, proteins, with implications on intein evolution and intein homing.     

Changes in the Catalytic Properties of Pyrococcus furiosus Thermostable Amylase by Mutagenesis of the Substrate Binding Sites

Pyrococcus furiosus thermostable amylase (TA) is a cyclodextrin (CD)-degrading enzyme with a high preference for CDs over maltooligosaccharides. In this study, we investigated the roles of four residues (His414, Gly415, Met439, and Asp440) in the function of P. furiosus TA by using site-directed mutagenesis and kinetic analysis. A variant form of P. furiosus TA containing two mutations (H414N and G415E) exhibited strongly enhanced α-(1,4)-transglycosylation activity, resulting in the production of a series of maltooligosaccharides that were longer than the initial substrates. In contrast, the variant enzymes with single mutations (H414N or G415E) showed a substrate preference similar to that of the wild-type enzyme. Other mutations (M439W and D440H) reversed the substrate preference of P. furiosus TA from CDs to maltooligosaccharides. Relative substrate preferences for maltoheptaose over β-CD, calculated by comparing k_cat/K_m ratios, of 1, 8, and 26 for wild-type P. furiosus TA, P. furiosus TA with D440H, and P. furiosus TA with M439W and D440H, respectively, were found. Our results suggest that His414, Gly415, Met439, and Asp440 play important roles in substrate recognition and transglycosylation. Therefore, this study provides information useful in engineering glycoside hydrolase family 13 enzymes.     

Purification and Molecular Characterization of the Tungsten-Containing Formaldehyde Ferredoxin Oxidoreductase from the Hyperthermophilic Archaeon Pyrococcus furiosus: the Third of a Putative Five-Member Tungstoenzyme Family

Pyrococcus furiosus is a hyperthermophilic archaeon which grows optimally near 100°C by fermenting peptides and sugars to produce organic acids, CO₂, and H₂. Its growth requires tungsten, and two different tungsten-containing enzymes, aldehyde ferredoxin oxidoreductase (AOR) and glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR), have been previously purified from P. furiosus. These two enzymes are thought to function in the metabolism of peptides and carbohydrates, respectively. A third type of tungsten-containing enzyme, formaldehyde ferredoxin oxidoreductase (FOR), has now been characterized. FOR is a homotetramer with a mass of 280 kDa and contains approximately 1 W atom, 4 Fe atoms, and 1 Ca atom per subunit, together with a pterin cofactor. The low recovery of FOR activity during purification was attributed to loss of sulfide, since the purified enzyme was activated up to fivefold by treatment with sulfide (HS⁻) under reducing conditions. FOR uses P. furiosus ferredoxin as an electron acceptor (K_m = 100 μM) and oxidizes a range of aldehydes. Formaldehyde (K_m = 15 mM for the sulfide-activated enzyme) was used in routine assays, but the physiological substrate is thought to be an aliphatic C₅ semi- or dialdehyde, e.g., glutaric dialdehyde (K_m = 1 mM). Based on its amino-terminal sequence, the gene encoding FOR (for) was identified in the genomic database, together with those encoding AOR and GAPOR. The amino acid sequence of FOR corresponded to a mass of 68.7 kDa and is highly similar to those of the subunits of AOR (61% similarity and 40% identity) and GAPOR (50% similarity and 23% identity). The three genes are not linked on the P. furiosus chromosome. Two additional (and nonlinked) genes (termed wor4 and wor5) that encode putative tungstoenzymes with 57% (WOR4) and 56% (WOR5) sequence similarity to FOR were also identified. Based on sequence motif similarities with FOR, both WOR4 and WOR5 are also proposed to contain a tungstobispterin site and one [4Fe-4S] cluster per subunit.     

Novel Bifunctional Hyperthermostable Carboxypeptidase/Aminoacylase from Pyrococcus horikoshii OT3

Genome sequencing of the thermophilic archaeon Pyrococcus horikoshii OT3 revealed a gene which had high sequence similarity to the gene encoding the carboxypeptidase of Sulfolobus solfataricus and also to that encoding the aminoacylase from Bacillus stearothermophilus. The gene from P. horikoshii comprises an open reading frame of 1,164 bp with an ATG initiation codon and a TGA termination codon, encoding a 43,058-Da protein of 387 amino acid residues. However, some of the proposed active-site residues for carboxypeptidase were not found in this gene. The gene was overexpressed in Escherichia coli with the pET vector system, and the expressed enzyme had high hydrolytic activity for both carboxypeptidase and aminoacylase at high temperatures. The enzyme was stable at 90°C, with the highest activity above 95°C. The enzyme contained one bound zinc ion per one molecule that was essential for the activity. The results of site-directed mutagenesis of Glu367, which corresponds to the essential Glu270 in bovine carboxypeptidase A and the essential Glu in other known carboxypeptidases, revealed that Glu367 was not essential for this enzyme. The results of chemical modification of the SH group and site-directed mutagenesis of Cys102 indicated that Cys102 was located at the active site and was related to the activity. From these findings, it was proven that this enzyme is a hyperthermostable, bifunctional, new zinc-dependent metalloenzyme which is structurally similar to carboxypeptidase but whose hydrolytic mechanism is similar to that of aminoacylase. Some characteristics of this enzyme suggested that carboxypeptidase and aminoacylase might have evolved from a common origin.     

Enzymological Characteristics of the Hyperthermostable NAD-Dependent Glutamate Dehydrogenase from the Archaeon Pyrobaculum islandicum and Effects of Denaturants and Organic Solvents

NAD-dependent glutamate dehydrogenase (l-glutamate:NAD oxidoreductase, deaminating; EC 1.4.1.2) was purified to homogeneity from a crude extract of the continental hyperthermophilic archaeon Pyrobaculum islandicum by two successive Red Sepharose CL-4B affinity chromatographies. The enzyme is the most thermostable NAD-dependent dehydrogenase found to date; the activity was not lost after incubation at 100°C for 2 h. The enzyme activity increased linearly with temperature, and the maximum was observed at ca. 90°C. The enzyme has a molecular mass of about 220 kDa and consists of six subunits with identical molecular masses of 36 kDa. The enzyme required NAD as a coenzyme for l-glutamate deamination and was different from the NADP-dependent glutamate dehydrogenase from other hyperthermophiles. The K_m values for NAD, l-glutamate, NADH, 2-oxoglutarate, and ammonia were 0.025, 0.17, 0.0050, 0.066, and 9.7 mM, respectively. The enzyme activity was significantly increased by the addition of denaturants such as guanidine hydrochloride and some water-miscible organic solvents such as acetonitrile and tetrahydrofuran. When fluorescence of the enzyme was measured in the presence of guanidine hydrochloride, a significant emission spectrum change and a shift in the maximum were observed but not in the presence of urea. These results indicate that this hyperthermophilic enzyme may have great potential in applications to biosensor and bioreactor processes.During the past decade, many anaerobic hyperthermophiles growing at a temperature near or above the boiling point of water have been isolated from marine and continental volcanic environments (1). The interest in hyperthermophiles has been rapidly expanding. In particular, interest is focused on understanding the adaptation mechanisms that allow the metabolism to function and the biomolecules, such as protein, enzyme, and DNA, to remain intact at extremely high temperature. Most hyperthermophiles belong to Archaea, the third domain of life (22), and evolutionary attention has been paid to their biomolecules because they may be the most slowly evolving or primitive group of microorganisms yet discovered. In addition, enzymes from the hyperthermophiles have a large biotechnological potential (2, 6). Of the enzymes from hyperthermophiles, glutamate dehydrogenase (GluDH) (EC 1.4.1.4., glutamate:NADP oxidoreductase) is one of the enzymes for which the most abundant information concerning enzymological properties and the relationships between structure and function has been obtained. Extremely thermostable NADP-dependent GluDHs have been purified from Pyrococcus furiosus (5, 18, 20), Pyrococcus woesei (18), Thermococcus litoralis (14, 19), and Thermococcus profundus (11). The gdhA gene of Pyrococcus furiosus (8, 9) has been cloned and sequenced, and the structural difference between the GluDHs of Pyrococcus furiosus, T. litoralis, and Clostridium symbiosum has been investigated to elucidate protein thermostability (3). In addition, a key role of the ion pair networks in maintaining the structure stability of Pyrococcus furiosus GluDH at an extremely high temperature has been indicated (24). However, information about hyperthermostable GluDH is limited so far to that regarding marine hyperthermophilic species of the order Thermococcales such as Pyrococcus and Thermococcus.In the course of investigating GluDH distribution in hyperthermophilic archaea, we found the activity of NAD-dependent GluDH (EC 1.4.1.2) in the cell extract of a continental hyperthermophilic archaeon, Pyrobaculum islandicum. This is the first example of the occurrence of NAD-dependent GluDH in anaerobic hyperthermophilic archaea. In general, the physiological function of NAD-dependent GluDH is known to be different from that of NADP-dependent GluDH (17). In addition, the NAD-dependent GluDH may be expected to be more preferable for application than the NADP-dependent enzyme, because NAD and NADH are much cheaper than NADP and NADPH, respectively (4, 23). Thus, we purified the enzyme from P. islandicum for characterization. We describe here the characteristics of this GluDH with emphasis on its high stability in some denaturants and organic solvents.     

Crystal structure and nucleic acid‐binding activity of the CRISPR‐associated protein Csx1 of Pyrococcus furiosus

In many prokaryotic organisms, chromosomal loci known as clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR‐associated (CAS) genes comprise an acquired immune defense system against invading phages and plasmids. Although many different Cas protein families have been identified, the exact biochemical functions of most of their constituents remain to be determined. In this study, we report the crystal structure of PF1127, a Cas protein of Pyrococcus furiosus DSM 3638 that is composed of 480 amino acids and belongs to the Csx1 family. The C‐terminal domain of PF1127 has a unique β‐hairpin structure that protrudes out of an α‐helix and contains several positively charged residues. We demonstrate that PF1127 binds double‐stranded DNA and RNA and that this activity requires an intact β‐hairpin and involve the homodimerization of the protein. In contrast, another Csx1 protein from Sulfolobus solfataricus P2 that is composed of 377 amino acids does not have the β‐hairpin structure and exhibits no DNA‐binding properties under the same experimental conditions. Notably, the C‐terminal domain of these two Csx1 proteins is greatly diversified, in contrast to the conserved N‐terminal domain, which appears to play a common role in the homodimerization of the protein. Thus, although P. furiosus Csx1 is identified as a nucleic acid‐binding protein, other Csx1 proteins are predicted to exhibit different individual biochemical activities. Proteins 2013. © 2012 Wiley Periodicals, Inc.     

Nucleotide sequence, cloning, and overexpression of the d-threonine dehydrogenase gene from Pseudomonas cruciviae

d-Threonine dehydrogenase (EC 1.1.1) catalyses the oxidation of the 3-hydroxyl group of d-threonine. The nucleotide sequence of the structural gene, dtdS, for this enzyme from Pseudomonas cruciviae IFO 12047 was determined. The dtdS gene encodes a 292 amino acid polypeptide. The enzyme was overproduced in Escherichia coli cells; the activity was found in cell extracts of the clone. The enzyme showed high sequence similarity to 3-hydroxyisobutyrate dehydrogenases. This is the first example showing the primary structure of an enzyme catalysing the NADP⁺-dependent dehydrogenation of d-threo-3-hydroxyamino acids.     

Glutamate dehydrogenase from the aerobic hyperthermophilic archaeon Aeropyrum pernix K1: enzymatic characterization, identification of the encoding gene, and phylogenetic implications

NADP-dependent glutamate dehydrogenase (l-glutamate: NADP oxidoreductase, deaminating, EC 1.4.1.4) from the aerobic hyperthermophilic archaeon Aeropyrum pernix K1 (JCM 9820) was purified to homogeneity for characterization. The enzyme retained its full activity on heating at 95°C for 30 min, and the maximum activity in l-glutamate deamination was obtained around 100°C. The enzyme showed a strict specificity for l-glutamate and NADP on oxidative deamination and for 2-oxoglutarate and NADPH on reductive amination. The K _m values for NADP, l-glutamate, NADPH, 2-oxoglutarate, and ammonia were 0.039, 3.3, 0.022, 1.7, and 83 mM, respectively. On the basis of the N-terminal amino acid sequence, the encoding gene was identified in the A. pernix K1 genome, cloned, and expressed in Escherichia coli. Analysis of the nucleotide sequence revealed an open reading frame of 1257 bp starting with a minor TTG codon and encoding a protein of 418 amino acids with a molecular weight of 46 170. Phylogenetic analysis revealed that the glutamate dehydrogenase from A. pernix K1 clustered with those from aerobic Sulfolobus solfataricus, Sulfolobus shibatae, and anaerobic Pyrobaculum islandicum in Crenarchaeota, and it separated from another cluster of the enzyme from Thermococcales in Euryarchaeota. The branching pattern of the enzymes from A. pernix K1, S. solfataricus, S. shibatae, and Pb. islandicum in the phylogenetic tree coincided with that of 16S rDNAs obtained from the same organisms. Received: April 24, 2000 / Accepted: August 10, 2000