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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.

The authors review the publications on biological methylation. The topics of interest include bioalkylation, metalloids' methylation, arsenic methylation, antimony biomethylation and enzymology of biomethylation.

The Staphylococcus aureus plasmid pI258 confers resistance to arsenic and antimony. The plasmid DNA fragment conferring the resistance properties was cloned and characterized. Sequence analysis showed that the fragment contains three open reading frames corresponding to the three genes arsR, arsB and arsC. The genes showed significant homologies to the corresponding genes in S. xylosus. Deletion analysis showed that interactions between arsB and arsC confer resistance to arsenic and antimony. The results also showed that arsR codes for a repressor protein and is located upstream of the other ars genes.