Arsenic

Like silver, arsenic is predominantly toxic and devoid of any function as a trace element. Different forms of arsenic appear in any environmental sample, of which a fraction may be available to the biological system and are likely to be variable depending on the physicochemical status of that environmental niche. However, toxicity, solubility, and mobility can vary depending upon which species of arsenic is present, thus affecting the bioavailability of the arsenic contamination. Arsenate enters the bacterial cell via the rapid, nonspecific, and constitutive uptake systems for phosphate.

Mutation of this system leads to tolerance to arsenate. Both anions, phosphate and arsenate, may still be accumulated by the inducible, specific transport system Pst, but this system discriminates 100-fold better between both anions than Pit does [13,20,116,117]. However, tolerant mutant strains are always impaired in their phosphate metabolism and tend to revert readily.

In Gram-negative bacteria, the arsenic-resistance gene remains inactive with the absence of As (III) in the cell due to the binding of the ars operon repressor protein to the promoter region of the gene. The system is activated by As (III) binding to the repressor protein and freeing the promoter region for transcription. The freed promoter region is transcribed to produce various components of the mechanism such as ArsB, an arsenite-translocating protein that serves as a transmembrane efflux channel. This protein functions chemiosomotically, without any energy source, or by ATP hydrolysis when coupled with ArsA, an arsenite-specific ATPase [61,118-121].

The best studied example is the plasmid-encoded arsenic resistance from E. coli [122]. The ArsB protein in these systems is able to function alone [123]; therefore, arsenite efflux by the ArsAB complex is energized chemiosomotically and by ATP [124]. ArsA acts as a dimer with four ATP-binding sites, and homologs to this protein have been found in eubacteria, archebacteria, fungi, plants, and animals [121,125,126]. ArsC, the enzyme arsenate reductase, is also transcribed to reduce As(V) to As(III) because As(V) cannot pass through the ArsB/ArsA pump [127,128]. For the resistance determinant in E. coli, arsenate reduction by the ArsC protein is coupled to glutathione [129] via glutaredoxin [130,131]. For ArsC from Staphylococus aureus, the electron donor is thioredoxin [127].

ArsD is a regulatory protein for additional control over the expression of the system and ArsR is a transcriptional repressor. The mechanism varies slightly in Gram-positive bacteria, which lack ArsA and ArsD. Arsenical resistance is regulated mainly by the ArsR repressor, the first example of a "metal-fist" repressor in which an inducing metal — in this case As (III) — binds to the dimeric protein, thereby preventing binding to the operator region [117,132-137]. Related regulatory proteins regulate a Cd-P-type ATPase in Staphyloccoccus aureus and metallothionein synthesis in cyanobacteria.

In other arsenic resistance determinants, which are more expensive to run, an additional regulator exists, ArsD [138]. Although ArsR controls the basal level of expression of the ars operon, ArsD controls maximal expression, and both regulators compete for the same DNA-binding site [139]. Two copies of an arsRBCH operon are found in the chromosome of P. putida [9]. The arsH, originally identified in the ars cluster of a Tn2502 transposon, appears to be necessary for arsenic resistance in Yersinia enterocolitica [140]. It has been suggested that ArsH might be a transcriptional activator because a plasmid containing the genes arsRBC without arsH in Y. enterocolitica did not cause an increase in arsenic resistance [140].

The protozoon Leishmania, in addition to having a P-glycoprotein-related transporter, an ABCtransporter responsible for arsenite resistance [141], also gets rid of the toxicity by conjugating As (III) with glutathione or trypathione let out by the glutathione conjugate transporter [142-146]. Homologous arsenite transporters related to ArsB were found in S. cerevisiae, the ARS3p protein [61,147], and also in man [148,149]. Thus, metabolism of arsenic resistance seems to follow the same pattern in all organisms again: uptake by the phosphate system; reduction; efflux by ArsB-related proteins or ABC-transporters; and maybe even additional energizing by ArsA-related ATPases.

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