An ArsR/SmtB family member regulates arsenic resistance genes unusually arranged in Thermus thermophilus HB27

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.

Identification of a New Heavy-Metal-Resistant Strain of Geobacillus stearothermophilus Isolated from a Hydrothermally Active Volcanic Area in Southern Italy

“…For instance, gram-positive and gram-negative bacteria possess arsenic resistance systems encoded by operons carried either on plasmids or on the chromosome. Genetic determinants are usually the three genes arsR, arsB, and arsC [18,19], among which arsR encodes a trans-acting repressor of the ArsR/SmtB family involved in transcriptional regulation [20][21][22], arsB encodes an As(III) efflux transporter (ArsB/Acr3) [23], and arsC encodes a cytoplasmic arsenate reductase that converts As(V) to As(III), the latter extruded outside the cell [24][25][26]. Other organisms benefit of additional proteins that improve the arsenic resistance, such as the arsenite methyltransferases [27].…”

Characterization of a promiscuous cadmium and arsenic resistance mechanism in Thermus thermophilus HB27 and potential application of a novel bioreporter system

“…Thermus thermophilus HB27 is a thermophilic aerobic Gram-negative bacterium capable of growing in the presence of arsenic concentrations that are lethal for other microorganisms [ 21 ]. In recent studies we demonstrated that the arsenic resistance system is not clustered in a single ars operon as in other organisms, but the genes are spread in the genome: TTC1502 encoding a cytoplasmic arsenate reductase ( Tt ArsC) able to reduce arsenate to arsenite, TTC0354 , encoding a P 1B -type membrane ATPase responsible for the efflux (herein named Tt ArsX) and TTC0353 encoding a repressor ( Tt SmtB) sensitive to both As(V) and As(III) [ 22 – 24 ]. Tt SmtB is a member of the ArsR/SmtB family, sharing 50% identity with the well characterized SmtB of Synechococcus PCC7942 [ 25 ].…”

Metal-Tolerant Thermophiles: From the Analysis of Resistance Mechanisms to their Biotechnological Exploitation

“…Both in Gram-positive and Gram-negative bacteria the arsenic resistance system is composed by operons of 3-5 genes carried on plasmids or chromosomes; the two most common contain either five genes (arsRDABC), as in the plasmid R773 of the Escherichia coli, or three genes (arsRBC), as in the plasmid pI258 of Staphylococcus aureus [36,37]. The arsR gene encodes a trans-acting repressor of the ArsR/SmtB family involved in transcriptional regulation [30], arsB encodes an As(III) efflux transporter [38], and arsC encodes a cytoplasmic arsenate reductase that converts As(V) to As(III), which is extruded outside the cell [39]. Where present, ArsD is a metallochaperone that transfers trivalent metalloids to ArsA, the arsenite-stimulated efflux ATPase [31].…”

“…These systems are generally finely regulated by specific transcription factors. As an example, the transcription of the arsenic resistance system of T. thermophilus HB27 is regulated by TtSmtB, a protein belonging to the ArsR/SmtB family which acts as the As(V) and As(III) intracellular sensor [30]. In the absence of metal ions, the protein binds to regulatory regions upstream of TTC1502, encoding TtArsC, and TTC0354, encoding the efflux membrane protein TtArsX, (a P 1B -type ATPase, see above).…”