A previously unknown chemolithoautotrophic arsenite-oxidizing bacterium has been isolated from a gold mine in the Northern Territory of Australia. The organism, designated NT-26, was found to be a gram-negative motile rod with two subterminal flagella. In a minimal medium containing only arsenite as the electron donor (5 mM), oxygen as the electron acceptor, and carbon dioxide-bicarbonate as the carbon source, the doubling time for chemolithoautotrophic growth was 7.6 h. Arsenite oxidation was found to be catalyzed by a periplasmic arsenite oxidase (optimum pH, 5.5). Based upon 16S rDNA phylogenetic sequence analysis, NT-26 belongs to the Agrobacterium/Rhizobium branch of the ␣-Proteobacteria and may represent a new species. This recently discovered organism is the most rapidly growing chemolithoautotrophic arsenite oxidizer known.Arsenic is found in many environments and is toxic to life in some forms. When present in insoluble forms, which are nontoxic, arsenic is frequently found as a mineral in combination with sulfur (e.g., orpiment [As 2 S 3 ] and realgar [AsS]) and especially with iron and sulfur (e.g., arsenopyrite [FeAsS]). The oxidation of these insoluble forms, chemically and/or microbiologically, results in the formation of arsenite (As III [H 3 AsO 3 ]). In environments such as acid mine drainage, arsenite concentrations can be extremely high (2 to 13 mg/liter) (22). The arsenite formed in various environments can then be oxidized to arsenate (As V [H 2 AsO 4 Ϫ ϩ H ϩ ]). Both of these soluble forms of arsenic, arsenite and arsenate, are toxic to living organisms, with arsenite considered more toxic than arsenate (5, 22).Interestingly, the chemical oxidation of arsenite to arsenate is slow. Most arsenite is, therefore, oxidized to arsenate microbiologically (22). Arsenite-oxidizing bacteria were first described in 1918 (7). These organisms, as well as a number of others that have been isolated more recently, are almost all heterotrophic arsenite-oxidizing bacteria, as they require the presence of organic matter for growth (17,18,23,24). The most common organism found has been Alcaligenes faecalis (7,17,18,23,24). For these heterotrophic bacteria, the oxidation (equation 1) is considered a detoxification mechanism rather than one that can support growth, despite the fact that the reaction is exergonic.Only one bacterium has been described that is able to use the energy gained from this reaction for growth. This organism, Pseudomonas arsenitoxidans, was found to grow chemolithoautotrophically with arsenite, oxygen, and carbon dioxide. The fastest doubling time reported for growth with arsenite was, however, in the order of 2 days (8). This report describes a new bacterium, isolated from a gold mine in the Northern Territory of Australia, that can also grow chemolithoautotrophically with arsenite as the electron donor, oxygen as the electron acceptor, and carbon dioxide (CO 2 ) or bicarbonate (HCO 3 Ϫ ) as the carbon source. Growth was rapid, with a doubling time of 7.6 h for chemolithoautotrophic grow...

Thauera selenatis is one of two isolated bacterial species that can obtain energy by respiring anaerobically with selenate as the terminal electron acceptor. The reduction of selenate to selenite is catalyzed by a selenate reductase, previously shown to be located in the periplasmic space of the cell. This study describes the purification of the enzyme from T. selenatis grown anaerobically with selenate. The enzyme is a trimeric ␣␤␥ complex with an apparent M r of 180,000. The ␣, ␤, and ␥ subunits are 96 kDa, 40 kDa, and 23 kDa, respectively, in size. The selenate reductase contains molybdenum, iron, and acid-labile sulfur as prosthetic group constituents. UV-visible absorption spectroscopy also revealed the presence of one cytochrome b per ␣␤␥ complex. The K m for selenate was determined to be 16 M, and the V max was 40 mol/min/mg of protein. The enzyme is specific for the reduction of selenate; nitrate, nitrite, chlorate, and sulfate were not reduced at detectable rates. These studies constitute the first description of a selenate reductase, which represents a new class of enzymes. The significance of this enzyme in relation to cell growth and energy generation is discussed.

Chrysiogenes arsenatis is the only bacterium known that respires anaerobically using arsenate as the terminal electron acceptor and the respiratory substrate acetate as the electron donor. During growth, the arsenate is reduced to arsenite; the reduction is catalyzed by an arsenate reductase. This study describes the purification and characterization of a respiratory arsenate reductase (Arr). The enzyme consists of two subunits with molecular masses of 87 kDa (ArrA) and 29 kDa (ArrB), and is a heterodimer A 1 β 1 with a native molecular mass of 123 kDa. The arsenate reductase contains molybdenum, iron, acid-labile sulfur and zinc as cofactor constituents. The K m of the enzyme for arsenate is 0.3 mM and the Vmax is 7013 µmol arsenate reduced · min Ϫ1 · mg protein Ϫ1 . Nitrate, sulfate, selenate and fumarate cannot serve as alternative electron acceptors for the arsenate reductase. Synthesis of the protein is regulated, as arsenate must be present during growth for the enzyme to be fully induced. The N-terminus of ArrA is similar to a number of procaryotic molybdenum-containing polypeptides (e.g. the formate dehydrogenases H and N of Escherichia coli). The N-terminus of ArrB is similar to iron-sulfur proteins. The respiratory arsenate reductase of C. arsenatis is different from the non-respiratory arsenate reductases of E. coli and Staphylococcus aureus.Keywords : respiratory arsenate reductase; arsenate respiration ; Chrysiogenes arsenatis.Arsenic is naturally present in soil, water and air and can occur in the oxidation states ϩ5 (arsenate), ϩ3 (arsenite), 0 (elemental arsenic) and Ϫ3 (arsine) [1]. The two soluble forms, arsenate and arsenite, are commonly found in water and soil. Both forms are toxic, although arsenite more so than arsenate [1].Arsenic resistance appears to be widespread among bacteria [2]. The mechanism for arsenate and arsenite resistance has, however, been investigated in depth only in organisms where resistance is conferred by proteins encoded by similar ars operons. These operons are located on the chromosome of Escherichia coli [2], on plasmid R773 of E. coli [3], on the IncN plasmid R46 found originally in Salmonella typhimurium [4], on plasmid pI258 of Staphylococcus aureus [5] and on plasmid pSX267 of Staphylococcus xylosus [6]. With each of these systems, arsenate, that has entered the cell via the phosphate-transport system, is first reduced to arsenite by a soluble cytoplasmic arsenate reductase, and the arsenite is then transported out of the cell via an energy-dependent arsenite transporter [7,8]. The arsenate reductase of these arsenic-resistance systems does not appear to be involved in energy conservation when catalyzing the reduction of arsenate to arsenite [7,8].Arsenate is also reduced to arsenite by a group of bacteria that grow anaerobically using the non-respiratory substrate lactate as the electron donor ; the lactate is oxidised to acetate. Energy is conserved during this reduction (i.e. arsenate respiration),

Two sulfate-reducing bacteria, which also reduce arsenate, were isolated; both organisms oxidized lactate incompletely to acetate. When using lactate as the electron donor, one of these organisms, Desulfomicrobium strain Ben-RB, rapidly reduced (doubling time = 8 h) 5.1 mM arsenate at the same time it reduced sulfate (9.6 mM). Sulfate reduction was not inhibited by the presence of arsenate. Arsenate could act as the terminal electron acceptor in minimal medium (doubling time = 9 h) in the absence of sulfate. Arsenate was reduced by a membrane-bound enzyme that is either a c-type cytochrome or is associated with such a cytochrome; benzyl-viologen-dependent arsenate reductase activity was greater in cells grown with arsenate/sulfate than in cells grown with sulfate only. The second organism, Desulfovibrio strain Ben-RA, also grew (doubling time = 8 h) while reducing arsenate (3.1 mM) and sulfate (8.3 mM) concomitantly. No evidence was found, however, that this organism is able to grow using arsenate as the terminal electron acceptor. Instead, it appears that arsenate reduction by the Desulfovibrio strain Ben-RA is catalyzed by an arsenate reductase that is encoded by a chromosomally-borne gene shown to be homologous to the arsC gene of the Escherichia coli plasmid, R773 ars system.

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