Article written and emailed by Dr. Neeta Bhagat

Arsenic is a known toxic metalloid, whose trivalent and pentavalent ions can inhibit many biochemical processes. Arsenic is a metalloid found in the environment, and it exists commonly in nature .The most often encountered arsenic forms are inorganic As(III) (arsenite) and As(V) , Methylated species, monomethyl arsonic acid (MMAA), dimethyl arsinic acid (DMAA) and trimethyl arsine oxide (TMAO), dominate in biomass, but have also been detected in soils. Also, As(V) and As(III) can be volatilized to arsine (AsH3); MMAA to monomethylarsine (CH3AsH2; MMA); DMAA to dimethylarsine (CH3)2AsH; DMA ;and TMAO to trimethylarsine (CH3)3As), TMA.Toxicity of As(III) is due to its binding to protein sulfhydryl groups. As(III) inhibits enzyme reactions requiring free sulfhydryl groups, leading to membrane degradation and cell death. As(V), is a toxic analog for inorganic phosphate in phosphorylating metabolism . As(V) competes with phosphate and therefore acts as an uncoupler of oxidative phosphorylation, resulting in inadequate supply of energy.

Today arsenic is posing serious threat to environmental pollution due to anthropogenic activities like use of arsenic containing insecticides, herbicides, fungicides, pesticides and wood preservatives and through mining and burning of coal. Long-term exposure to inorganic forms of arsenic has serious impact because these compounds have been recognized as skin and lung carcinogens in humans.

Due to its toxic properties it has been exploited in the production of antimicrobial agents, such as the first specific antibiotic (Salvorsan 606) and the African sleeping sickness drug Melarsen, in addition to the commonly used wood preservative chromated copper arsenate.

In the environment microorganism play important role in cyclization of arsenic with a multiple mechanisms like transformations between soluble and insoluble arsenic forms and toxic and nontoxic arsenic forms. Inorganic arsenic forms, As(V) and As(III), are subjected to microbiologically mediated oxidation-reduction reactions. Metabolism involve As(III) oxidation and As(V) dissimilatory reduction.

Some microorganism that play important role in transformation of arsenic are Sulfurospirillum barnesii, S. arsenophilum, Desulfotomaculum auripigmentum, Bacillus arsenicoselenatis, B. selenitireducens, Crysiogenes arsenatis, Sphingomonas spp., Pseudomonas spp. and Wolinella spp.

In addition there are some arsenic resistant bacteria that have different Aresnic reduction mechanism. E.coli, Staphylococcus aureus reduces As(V) to As(III) so that it is rapidly flushed out of cell.

Arsenic resistant found in microorganism is conferred by plasmid located gene found both in gram positive and gram negative bacterial species. This arsenic resistant gene (ars) consists of either three or five genes that are organized in single transcriptional unit. In Plasmid (R773) ,E.coli consist of five genes (arsRDABC) of ars Operon which are controlled by single promoter located on upstream of first cistron (ars R). Cistron arsRDABC encode an arsenic-inducible repressor(arsR). It is a negative regulatory protein that controls the upper level of transcription(arsD), an ATPase plus membrane located arsenite efflux pump (arsA and arsB) and an arsenate reductase (arsC). In Staphylococcus species plasmids pI258 and pSX267 are isolated, which contained conserved the arsR, arsB, and arsC cistrons , while the arsD and arsA cistrons are absent.

ARS operon is responsible for detoxification of arsenate , arsenite and antimonite. Arsenite and antimonite are transported out of the cell. The pump is composed of two polypeptides which are product of arsA and arsB gene Arsenate first must be reduced to arsenite before it is extruded. The third gene ars C is responsible for an arsenate reductase that uses reduced glutathione reduced glutathione (GSH) to convert arsenate to arsenite . arsC forms an active quaternary complex with GSH, arsenate and glutaredoxin 1.

Structure of operon

Ars A and Ars B
ArsA and ArsB form an anion-translocating ATPase. Ars B has hydrophobic character and forms memebrane associated channel.

Ars C
Ars C is member of thioredoxin superfamily which has betasheet core surrounded by alpha helices. It has active cystein residue. The arsC family also comprises the Spx proteins which are gram negative and gram positive transcription factors that regulate the transcription of multiple genes in response to disulphide stress.

ArsD and ArsR
ArsD and ArsR together regulate the ars operon. ArsD conferes resistance to arsenicals and antimonials in E. coli. It is a Trans acting repressor of arsRDABC operon. It has two pairs of vicinal cysteine residues Cys (12), (13), (112), (113). Metalloids bind to this site and triggers dissociation of arsD from the operon.

ArsR is a a trans-acting regulatory protein which acts as a repressor on the arsRDABC operon when no arsenic is present in the cell. In the presence of arsenic in the cell ArsR loses affinity for the operator and RNA polymerase can transcribe the arsDCAB genes.

ARS operon have been found in multicopy plasmids from both gram-positive and gram-negative bacteria.

About Author / Additional Info:
Dr. Neeta Bhagat, Assistant Professor in AIB, Amity University,NOIDA |