Arsenic Sensing and Resistance System in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Abstract

Arsenic is one of the most important global environmental pollutants. Here we show that the cyanobacterium Synechocystis sp. strain PCC 6803 contains an arsenic and antimony resistance operon consisting of three genes: arsB, encoding a putative arsenite and antimonite carrier, arsH, encoding a protein of unknown function, and arsC, encoding a putative arsenate reductase. While arsB mutant strains were sensitive to arsenite, arsenate, and antimonite, arsC mutants were sensitive only to arsenate. The arsH mutant strain showed no obvious phenotype under the conditions tested. In vivo the arsBHC operon was derepressed by oxyanions of arsenic and antimony (oxidation state, ؉3) and, to a lesser extent, by bismuth (oxidation state, ؉3) and arsenate (oxidation state, ؉5). In the absence of these effectors, the operon was repressed by a transcription repressor of the ArsR/SmtB family, encoded by an unlinked gene termed arsR. Thus, arsR null mutants showed constitutive derepression of the arsBHC operon. Expression of the arsR gene was not altered by the presence of arsenic or antimony compounds. Purified recombinant ArsR protein binds to the arsBHC promoter-operator region in the absence of metals and dissociates from the DNA in the presence of Sb(III) or As(III) but not in the presence of As(V), suggesting that trivalent metalloids are the true inducers of the system. DNase I footprinting experiments indicate that ArsR binds to two 17-bp direct repeats, with each one consisting of two inverted repeats, in the region from nucleotides ؊34 to ؉ 17 of the arsBHC promoter-operator.Arsenicals generated from natural and man-made sources are widely distributed contaminants of freshwater, ground water, and seawater (2,11,24). Probably because of this, many organisms contain either chromosomal or plasmid-encoded genes involved in arsenical resistance (ars genes). Most of the knowledge about ars genes and their regulation in prokaryotes comes from studies of ars operons in Escherichia coli and Staphylococcus species (recently reviewed in references 21, 26, and 27). The chromosomal ars operon of E. coli and the ars operons from staphylococcal plasmids pI258 and pSX267 are constituted by three genes, arsR, -B, and -C. arsR encodes a trans-acting repressor of the ArsR/SmtB family involved in transcriptional regulation, arsB encodes a membrane-bound arsenite carrier that exports arsenite but not arsenate, and arsC encodes a reductase that converts arsenate to arsenite. In contrast, the ars operons of E. coli plasmids R773 and R46 encode two additional proteins: ArsA, an arsenite-stimulated ATPase, and ArsD, another metalloid-responsive transcriptional repressor.Three different families of arsenate reductases have been described (for revisions, see references 18 and 26). The first family to be described was the product of the arsC gene from the E. coli plasmid R773 (6). This enzyme uses glutaredoxin as a source of reducing equivalents, and it is present in several gram-negative bacteria. The Staphylococcus aureus pI258 and the Bacillus ...

Cyanobacteria-Mediated Arsenic Redox Dynamics Is Regulated by Phosphate in Aquatic Environments

“…The expression of acr3 (encoding the As(III) efflux pump) could be similar to arsC, since it is in the same operon in Synechocystis. 22,23 Phosphate levels did not influence arsC expression ( Figure 5), which had also been demonstrated in Agrobacterium tumefaciens str. 5B and Shewanella sp.…”

Coregulated Genes Link Sulfide:Quinone Oxidoreductase and Arsenic Metabolism in Synechocystis sp. Strain PCC6803

“…ArsB, the membrane-localized protein in the well-documented arsenical resistance operon of Synechocystis (25), functions as an arsenite transporter. We aimed to test whether the protein encoded by the suoT gene has similar function, since SuoT is also a membrane protein and its gene is induced by arsenic.…”

“…Intracellular arsenic content was determined in the wild-type (WT) Synechocystis and ⌬suoT and ⌬arsB mutant strains (25). The cell cultures were treated with NaAsO 2 for 3 h in light, collected by centrifugation, washed once with BG-11 solution, and freeze-dried.…”

“…Synechocystis is rather tolerant, as it is able to grow in up to 3 mM arsenite and as high as 100 mM arsenate. This raised the question whether the arsenic transport system encoded by the arsBHC operon (25) under the regulation of the ArsR protein is exceptionally effective or perhaps some other biochemical process contributes to the detoxification.…”

Ecological Genomics of Marine Picocyanobacteria

“…Uptake of arsenate may occur through the PstSCAB system. Previous work with a freshwater cyanobacterium suggests that the canonical arsenate resistance mechanism consists of an arsenate reductase (ArsC) reducing arsenate to arsenite and an arsenite efflux pump (ArsB), with this system being regulated by the repressor ArsR (158). The absence of arsR from the coastal/mesotrophic Synechococcus sp.…”