In this article, a mechanism of arsenite [As(III)] resistance through methylation and subsequent volatization is described. Heterologous expression of arsM from Rhodopseudomonas palustris was shown to confer As(III) resistance to an arsenic-sensitive strain of Escherichia coli. ArsM catalyzes the formation of a number of methylated intermediates from As(III), with trimethylarsine as the end product. The net result is loss of arsenic, from both the medium and the cells. Because ArsM homologues are widespread in nature, this microbial-mediated transformation is proposed to have an important impact on the global arsenic cycle.As(III) ͉ ArsM ͉ methylation A s genomes are sequenced, it is becoming clear that nearly all bacteria and archaea have arsenic-resistance (ars) operons that confer resistance to arsenite [As(III)] and arsenate [As(V)] (1). The widespread occurrence of ars genes reflects the fact that arsenic is a ubiquitous environmental toxic metal. In most cases, these operons encode transport proteins that extrude As(III) from cells. In eukaryotes, As(III) detoxification involves glutathionylation coupled to removal of the As(GS) 3 complex from the cytosol by ABC transporters, such as the Saccharomyces cerevisiae Ycf1p vacuolar pump (2) or mammalian biliary extrusion pump MRP2 (3). In many mammals, including humans, an alternate metabolic fate of As(III) is methylation in the liver, followed by urinary excretion of the methylated species (4). In the past, this process was considered a detoxification mechanism (5), but more recent data suggest that the methylation actually increases toxicity by producing the more toxic monomethylarsenite [MMA(III)] and dimethylarsenite [DMA(III)], calling into question whether the process is, in fact, a detoxification process (6). An enzyme (termed Cyt19 or As3MT) that catalyzes As(III)-S-adenosylmethyltransferase activity has been identified recently in rats and humans (7-9). The enzyme has been characterized in vitro, but its physiological role is unknown.Bacteria and fungi are known to produce volatile and toxic arsines (10) but the physiological roles of arsenic methylation in microorganisms are likewise unclear, and the biochemical basis is unknown. While examining microbial genomes, we identified large number of genes for bacterial and archaeal homologues of Cyt19. We have termed a subset of these genes arsM and their protein product ArsM (As(III) S-adenosylmethyltransferase). What sets these arsM genes apart from genes for other homologues is that they are each downstream of an arsR gene, encoding the archetypal arsenic-responsive transcriptional repressor that controls expression of ars operons (11), suggesting that these ArsMs evolved to confer arsenic resistance.The gene for the 283-residue ArsM (29,656 Da) (accession no. NP948900.1) was cloned from Rhodopseudomonas palustris and expressed in an arsenic-hypersensitive strain of Escherichia coli. As(III)-resistance cells in E. coli expressing recombinant arsM correlated with conversion of medium arsenic to the methy...

Arsenic is the most common toxic substance in the environment, ranking first on the Superfund list of hazardous substances. It is introduced primarily from geochemical sources and is acted on biologically, creating an arsenic biogeocycle. Geothermal environments are known for their elevated arsenic content and thus provide an excellent setting in which to study microbial redox transformations of arsenic. To date, most studies of microbial communities in geothermal environments have focused on Bacteria and Archaea, with little attention to eukaryotic microorganisms. Here, we show the potential of an extremophilic eukaryotic alga of the order Cyanidiales to influence arsenic cycling at elevated temperatures. Cyanidioschyzon sp. isolate 5508 oxidized arsenite [As(III)] to arsenate [As(V)], reduced As(V) to As(III), and methylated As(III) to form trimethylarsine oxide (TMAO) and dimethylarsenate [DMAs(V)]. Two arsenic methyltransferase genes, CmarsM7 and CmarsM8, were cloned from this organism and demonstrated to confer resistance to As(III) in an arsenite hypersensitive strain of Escherichia coli. The 2 recombinant CmArsMs were purified and shown to transform As(III) into monomethylarsenite, DMAs(V), TMAO, and trimethylarsine gas, with a T opt of 60 -70°C. These studies illustrate the importance of eukaryotic microorganisms to the biogeochemical cycling of arsenic in geothermal systems, offer a molecular explanation for how these algae tolerate arsenic in their environment, and provide the characterization of algal methyltransferases.arsenic detoxification ͉ arsenic methylation ͉ As(III) S-adenosylmethyltransferase ͉ thermophile

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