Insights in the regulation of the arsenic resistance system in Thermus thermophilus

Microorganisms living in arsenic-rich geothermal environments act on arsenic with different biochemical strategies, but the molecular mechanisms responsible for the resistance to the harmful effects of the toxic metalloid have only partially been examined[1]. In a recent study, we started an investigation on the mechanisms of arsenic resistance in the thermophilic bacterium Thermus thermophilus HB27. This strain, originally isolated from a Japanese hot spring, exhibited tolerance to both arsenate (V) and arsenite (III) in a millimolar range and owns in its genome a chromosomal arsenate reductase (TtarsC) which is an important contributor in the arsenic resistance[2]. With the aim to study the arsenic resistance regulation we focused on a putative transcriptional regulator (TtSmtB) sharing high identity with the members of the ArsR/SmtB family. TtSmtB is part of an operon including five genes not related to arsenic resistance with the exception of a putative metal-efflux transporter. Since by bioinformatics we found in the operon some putative internal promoters, by means of an in vivo reporter system[3], we identified those responsive to arsenate and arsenite. TtSmtB was also cloned and expressed in Escherichia coli. The purification of the recombinant protein, proved that TtSmtB is a DNA binding protein of 123 amino acids interacting with the in vivo active promoters. Preliminary EMSA assays in the presence of arsenite suggested its role as a derepressor.