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Arsenic is to be found throughout the environment, although it is not uniformly distributed. From the geochemical point of view, this element is usually associated with sulfur, iron and other metals in sulfurated minerals and the hydrothermal environments resulting from recent or more ancient volcanic activity. Arsenic is known to be present in more than 200 minerals, the most common of which is arsenopyrite (FeAsS). The inorganic forms of arsenic are highly toxic, since this metalloid impairs the metabolism of most higher organisms and inhibits their development. In addition, arsenic is known to be mutagenic, carcinogenic and teratogenic.
Many studies have dealt with the arsenic resistance mechanism consisting of the reduction of As[V] into As[III] in Proteobacteria. Recently, the genes involved in the oxidation or the methylation of this metalloid have been characterized. In addition, some strains have been shown to use the reduction of arseniate in anaerobic respiration. However, little is known so far about the physiological, enzymatic, and genetic aspects of arsenic metabolism and the underlying regulatory mechanisms. In the environments which harboured the first forms of life on earth, As[III] may have been one of the main mineral substrates providing chimiolithotrophic organisms with energy. Studies on these enzymes, focusing in particular on their diversity, depending on the physico-chemical characteristics of the environment, should help to understand their in the ecology of systems severely contaminated by arsenic. It is likely that by eliminating arsenic, the bacteria producing these enzymes are able to colonize hostile environments, and that this may even contribute to the survival and implantation of other micro-organisms by extending the conditions limiting their existence.
The NT-26 strain belongs to the Agrobacterium/Rhizobium branch of the α-Proteobacteria and probably represents a new species of Rhizobium. This strain was isolated from a gold mine in the Northern Territory, Australia . In contrast to the β-Proteobacteria Herminiimonas arsenicoxydans  and Thiomonas sp. previously sequenced by our laboratories, the NT-26 strain grows fast with As[III] chemolithotrophically, which means that it is able to gain energy mainly by using arsenite as the electron donor and CO2 as its sole carbon source.