Previously, we isolated a selenate- and arsenate-reducing bacterium, designated strain SF-1, from selenium-contaminated sediment and identified it as a novel species, Bacillus selenatarsenatis. B. selenatarsenatis strain SF-1 independently reduces selenate to selenite, arsenate to arsenite, and nitrate to nitrite by anaerobic respiration. To identify the genes involved in selenate reduction, 17 selenate reduction-defective mutant strains were isolated from a mutant library generated by random insertion of transposon Tn 916 . Tn 916 was inserted into the same genome position in eight mutants, and the representative strain SF-1AM4 did not reduce selenate but did reduce nitrate and arsenate to the same extent as the wild-type strain. The disrupted gene was located in an operon composed of three genes designated srdBCA , which were predicted to encode a putative oxidoreductase complex by the BLASTX program. The plasmid vector pGEMsrdBCA, containing the srdBCA operon with its own promoter, conferred the phenotype of selenate reduction in Escherichia coli DH5α, although E. coli strains containing plasmids lacking any one or two of the open reading frames from srdBCA did not exhibit the selenate-reducing phenotype. Domain structure analysis of the deduced amino acid sequence revealed that SrdBCA had typical features of membrane-bound and molybdopterin-containing oxidoreductases. It was therefore proposed that the srdBCA operon encoded a respiratory selenate reductase complex. This is the first report of genes encoding selenate reductase in Gram-positive bacteria.

A facultatively anaerobic, selenate- and arsenate-reducing bacterium, designated strain SF-1^T, was isolated from a selenium-contaminated sediment obtained from an effluent drain of a glass-manufacturing plant in Japan. The bacterium stained Gram-positive and was a motile, spore-forming rod capable of respiring with selenate, arsenate and nitrate as terminal electron acceptors. The major cellular fatty acids of the strain were iso-C_15 : 0, iso-C_17 : 1 ω10c and C_16 : 1 ω7c alcohol. The G+C content of the genomic DNA was 42.8 mol%. Though the nearest phylogenetic neighbour was Bacillus jeotgali JCM 10885^T, with a 16S rRNA gene sequence similarity of 99.6 %, DNA–DNA hybridization studies showed only 14 % relatedness between these strains, a level that is clearly below the value recommended to delimit different species. This, together with the phenotypic differences (utilization of electron acceptors, NaCl tolerance), suggests that strain SF-1^T represents a novel species of the genus Bacillus, for which the name Bacillus selenatarsenatis sp. nov. is proposed. The type strain is SF-1^T (=JCM 14380^T=DSM 18680^T).