Abstract
SummaryArsenic resistance is commonly clustered in ars operons in bacteria; main ars operon components encode an arsenate reductase, a membrane extrusion protein, and an As‐sensitive transcription factor. In the As‐resistant thermophile Thermus thermophilus HB27, genes encoding homologues of these proteins are interspersed in the chromosome. In this article, we show that two adjacent genes, TtsmtB, encoding an ArsR/SmtB transcriptional repressor and, TTC0354, encoding a Zn2+/Cd2+‐dependent membrane ATPase are involved in As resistance; differently from characterized ars operons, the two genes are transcribed from dedicated promoters upstream of their respective genes, whose expression is differentially regulated at transcriptional level. Mutants defective in TtsmtB or TTC0354 are more sensitive to As than the wild type, proving their role in arsenic resistance. Recombinant dimeric TtSmtB binds in vitro to both promoters, but its binding capability decreases upon interaction with arsenate and, less efficiently, with arsenite. In vivo and in vitro experiments also demonstrate that the arsenate reductase (TtArsC) is subjected to regulation by TtSmtB. We propose a model for the regulation of As resistance in T. thermophilus in which TtSmtB is the arsenate sensor responsible for the induction of TtArsC which generates arsenite exported by TTC0354 efflux protein to detoxify cells.
Highlights
Arsenic (As) is an ubiquitous metalloid naturally present in soil, water and air that adversely affects human and animal health
We propose a model for the regulation of As resistance in T. thermophilus in which TtSmtB is the arsenate sensor responsible for the induction of the involvement of a thioredoxin-coupled arsenate reductase (TtArsC) which generates arsenite exported by TTC0354 efflux protein to detoxify cells
The abundance of arsenic in the environment has guided the evolution of multiple defence strategies in almost all microorganisms (Contursi et al, 2013); for instance, despite being toxic, some microorganisms use arsenic as electron acceptor in anaerobic respiratory chains or as electron donor for chemolythotrophic growth and even for anoxigenic photosynthesis (Kulp et al, 2008; van Lis et al, 2013); other microorganisms are able to methylate inorganic arsenic or de-methylate the organic forms (Qin et al, 2006)
Summary
Arsenic (As) is an ubiquitous metalloid naturally present in soil, water and air that adversely affects human and animal health. Because of its abundance and toxicity, monitoring arsenic concentration in the environment and in several foodstuffs used for human consumption is very important. The highly toxic trivalent arsenite, As(III), is the more abundant form, whereas in oxygenated environments, the less-toxic and more stable pentavalent arsenate, As(V), dominates. Arsenite enters the cell through aquaglyceroporins; as it has a high affinity for sulfur, it exerts its toxicity through binding to dithiols in proteins, in glutathione (GSH) and in lipoic acid contributing to protein/enzyme inactivation (Liu et al, 2004; Meng et al, 2004). Arsenate enters the cells through phosphate transporters and its toxicity is mediated by replacing phosphate in essential biochemical reactions (Tawfik and Viola, 2011; Kamerlin et al, 2013). The abundance of arsenic in the environment has guided the evolution of multiple defence strategies in almost all microorganisms (Contursi et al, 2013); for instance, despite being toxic, some microorganisms use arsenic as electron acceptor in anaerobic respiratory chains or as electron donor for chemolythotrophic growth and even for anoxigenic photosynthesis (Kulp et al, 2008; van Lis et al, 2013); other microorganisms are able to methylate inorganic arsenic or de-methylate the organic forms (Qin et al, 2006)
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