| Summary: | <p>Abstract</p> <p>Background</p> <p>The <it>ars </it>gene system provides arsenic resistance for a variety of microorganisms and can be chromosomal or plasmid-borne. The <it>arsC </it>gene, which codes for an arsenate reductase is essential for arsenate resistance and transforms arsenate into arsenite, which is extruded from the cell. A survey of GenBank shows that <it>arsC </it>appears to be phylogenetically widespread both in organisms with known arsenic resistance and those organisms that have been sequenced as part of whole genome projects.</p> <p>Results</p> <p>Phylogenetic analysis of aligned <it>arsC </it>sequences shows broad similarities to the established 16S rRNA phylogeny, with separation of bacterial, archaeal, and subsequently eukaryotic <it>arsC </it>genes. However, inconsistencies between <it>arsC </it>and 16S rRNA are apparent for some taxa. Cyanobacteria and some of the γ-Proteobacteria appear to possess <it>arsC </it>genes that are similar to those of Low GC Gram-positive Bacteria, and other isolated taxa possess <it>arsC </it>genes that would not be expected based on known evolutionary relationships. There is no clear separation of plasmid-borne and chromosomal <it>arsC </it>genes, although a number of the Enterobacteriales (γ-Proteobacteria) possess similar plasmid-encoded <it>arsC </it>sequences.</p> <p>Conclusion</p> <p>The overall phylogeny of the arsenate reductases suggests a single, early origin of the <it>arsC </it>gene and subsequent sequence divergence to give the distinct <it>arsC </it>classes that exist today. Discrepancies between 16S rRNA and <it>arsC </it>phylogenies support the role of horizontal gene transfer (HGT) in the evolution of arsenate reductases, with a number of instances of HGT early in bacterial <it>arsC </it>evolution. Plasmid-borne <it>arsC </it>genes are not monophyletic suggesting multiple cases of chromosomal-plasmid exchange and subsequent HGT. Overall, <it>arsC </it>phylogeny is complex and is likely the result of a number of evolutionary mechanisms.</p>
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