A new acetotrophic marine methane-producing bacterium that was isolated from the methane-evolving sediments of a marine canyon is described. Exponential phase cultures grown with sodium acetate contained irregularly shaped cocci that aggregated in the early stationary phase and finally differentiated into communal cysts that released individual cocci when ruptured or transferred to fresh medium. The irregularly shaped cocci (1.9 + 0.2 mm in diameter) were gram negative and occurred singly or in pairs. Cells were nonmotile, but possessed a single fimbria-like structure. Micrographs of thin sections showed a monolayered cell wall approximately 10 nm thick that consisted of protein subunits. The cells in aggregates were separated by visible septation. The communal cysts contained several single cocci encased in a common envelope. An amorphous form of the communal cyst that had incomplete septation and internal membrane-like vesicles was also present in late exponential phase cultures. Sodium acetate, methanol, methylamine, dimethylamine, and trimethylamine were substrates for growth and methanogenesis; H2-CO2 (80:20) and sodium formate were not. The optimal growth temperature was 35 to 40°C. The optimal pH range was 6.5 to 7.0. Both NaCl and Mg2+ were required for growth, with maximum growth rates at 0.2 M NaCl and 0.05 M MgSO4. The DNA base composition was 41 ± 1% guanine plus cytosine. Methanosarcina acetivorans is the proposed species. C2A is the type strain (DSM 2834, ATCC 35395).
A cholate-inducible, NADH-dependent flavin oxidoreductase from the intestinal bacterium Eubacterium sp. strain VPI 12708 was purified 372-fold to apparent electrophoretic homogeneity. The subunit and native molecular weights were estimated to be 72,000 and 210,000, respectively, suggesting a homotrimeric organization. Three peaks of NADH:flavin oxidoreductase activity (forms I, H, and III) eluted from a DEAE-high-performance liquid chromatography column. Absorption spectra revealed that purified form III, but not form I, contained bound flavin, which dissociated during purification to generate form I. Enzyme activity was inhibited by sulfhydryl-reactive compounds, acriflavine, o-phenanthroline, and EDTA. Activity assays and Western blot (immunoblot) analysis confirmed that expression of the enzyme was cholate inducible. The first 25 N-terminal amino acid residues of purified NADH:flavin oxidoreductase were determined, and a corresponding oligonucleotide probe was synthesized for use in cloning of the associated gene, baiH. Restriction mapping, sequence data, and RNA blot analysis suggested that the baiH gene was located on a previously described, cholate-inducible operon 210 kb long. The baiH gene encoded a 72,006-Da polypeptide containing 661 amino acids. The deduced amino acid sequence of the baiH gene was homologous to that of NADH oxidase from Thermoanaerobium brockii, trimethylamine dehydrogenase from methylotrophic bacterium W3A1, Old Yellow Enzyme from Saccharomyces carisbergensis, and the product of the baiC gene ofEubacterium sp. strain VPI 12708, located upstream from the baiH gene in the cholate-inducible operon. Alignment of these five sequences revealed potential ligands for an iron-sulfur cluster, a putative fiavin adenine dinucleotide-binding domain, and two other well-conserved domains of unknown function.
Southern blot analysis indicated that the gene encoding the constitutive, NADP-linked bile acid 7a-hydroxysteroid dehydrogenase of Eubacterium sp. strain VPI 12708 was located on a 6.5-kb EcoRI fragment of the chromosomal DNA. This fragment was cloned into bacteriophage lambda gtll, and a 2.9-kb piece of this insert was subcloned into pUC19, yielding the recombinant plasmid pBH51. DNA sequence analysis of the 7a-hydroxysteroid dehydrogenase gene in pBH51 revealed a 798-bp open reading frame, coding for a protein with a calculated molecular weight of 28,500. A putative promoter sequence and ribosome binding site were identified. The 7a-hydroxysteroid dehydrogenase mRNA transcript in Eubacterium sp. strain VPI 12708 was about 0.94 kb in length, suggesting that it is monocistronic. An Escherichia coli DH5a transformant harboring pBH51 had approximately 30-fold greater levels of 7a-hydroxysteroid dehydrogenase mRNA, immunoreactive protein, and specific activity than Eubacterium sp. strain VPI 12708. The 7af-hydroxysteroid dehydrogenase purified from the pBH51 transformant was similar in subunit molecular weight, specific activity, and kinetic properties to that from Eubacterium sp. strain VPI 12708, and it reacted with antiserum raised against the authentic enzyme on Western immunoblots. Alignment of the amino acid sequence of the 7a-hydroxysteroid dehydrogenase with those of 10 other pyridine nucleotide-linked alcohol/polyol dehydrogenases revealed six conserved amino acid residues in the N-terminal regions thought to function in coenzyme binding.Many members of the human intestinal microflora can modify the glycine and taurine conjugates of the primary bile acids, cholic acid and chenodeoxycholic acid, producing up to 20 different bile acid metabolites (21). The predominant modifications include deconjugation of the glycine and taurine moieties by bile salt hydrolase, removal of 7-hydroxy groups (7-dehydroxylation), and dehydrogenation of a-and ,B-hydroxy groups at carbons 3, 7, and 12 by stereospecific bile acid hydroxysteroid dehydrogenases to yield oxo bile acids.Quantitatively, the most important of these transformations is the 7ux-dehydroxylation of cholic and chenodeoxycholic acids, producing deoxycholic and lithocholic acids, respectively (43). The human intestinal anaerobic bacterium, Eubacterium sp. strain VPI 12708, has a multistep 7a-dehydroxylation pathway which is induced by addition of cholic acid to the culture medium (6,9,11, 22,(46)(47)(48). Several bile acid-inducible (bai) genes encoding proteins involved in this pathway have been cloned and sequenced (10,11,18,31,49,50).7a-Hydroxysteroid dehydrogenase is the most common class of bile acid hydroxysteroid dehydrogenases found in nature (3), having been detected in numerous genera of bacteria (3, 14-16, 28, 30, 37) and in mammalian liver (1). 7a-Hydroxysteroid dehydrogenases have been purified from Bacteroides species (23,29,41), Escherichia coli (28,36,37), and rat liver microsomes (1). Recently, two 7a-hydroxysteroid dehydrogenases have been pu...
The membrane-associated coenzyme F420-reducing hydrogenase of Methanobacterium formicicum was purified 87-fold to electrophoretic homogeneity. The enzyme contained a, I, and fy subunits (molecular weights of 43,600, 36,700, and 28,800, respectively) and formed aggregates (molecular weight, 1,020,000) of a coenzyme F420-active a4,1yz, trimer (molecular weight, 109,000). The hydrogenase contained 1 mol of flavin adenine dinucleotide (FAD), 1 mol of nickel, 12 to 14 mol of iron, and 11 mol of acid-labile sulfide per mol of the 109,000-molecular-weight species, but no selenium. The isoelectric point was 5.6. The amino acid sequence I-N3-P-N2-R-N,-EGH-N6-V (where N is any amino acid) was conserved in the N-termini of the a subunits of the F420-hydrogenases from M. formicicum and Methanobacterium thermoautotrophicum and of the largest subunits of nickel-containing hydrogenases from Desulfovibrio baculatus, Desulfovibrio gigas, and Rhodobacter capsulatus. The purified F420-hydrogenase required reductive reactivation before assay. FAD dissociated from the enzyme during reactivation unless potassium salts were present, yielding deflavoenzyme that was unable to reduce coenzyme F420. Maximal coenzyme F420-reducing activity was obtained at 55°C and pH 7.0 to 7.5, and with 0.2 to 0.8 M KCI in the reaction mixture. The enzyme catalyzed H2 production at a rate threefold lower than that for H2 uptake and reduced coenzyme F420, methyl viologen, flavins, and 7,8-didemethyl-8-hydroxy-5-deazariboflavin. Specific antiserum inhibited the coenzyme F420-dependent but not the methyl viologen-dependent activity of the purified enzyme.
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