Multiple sets of arsenic resistance genes are present within highly arsenic-resistant industrial strains of the biomining bacterium, Acidithiobacillus caldus

Abstract

The moderately thermophilic, sulfur-oxidizing bacterium Acidithiobacillus caldusis an important role player in continuous-flow biooxidation processes associated with the biooxidation of arsenopyrite concentrates. Certain isolates of these bacteria have been exposed to high concentrations of arsenic for many years and have become highly arsenic-resistant. We investigated the arsenic resistance genes of the highly resistant strains of At. caldus and compared these with those of less arsenic-resistant isolates. Up to three sets of arsenic resistance genes were found in the most resistant strains. One set of genes was located on a 12-kb Tn 21-like transposon flanked by Tn 21-like 40-bp inverted repeats (IR) and the transposon was active in the bacterium, Escherichia coli. This transposon was present only in highly arsenic-resistant strains and contained an unusual ars operon with arsR, arsC, two arsD-like, two arsA-like and arsB genes. Situated between the arsA and arsB genes are two open reading frames encoding for NADH oxidase-like enzyme and a protein with a CBS domain that is also present in IMP dehydrogenases. Deletion of one copy of the arsDA genes occurred readily but there were no marked differences in arsenic resistance in the deleted clones. The second set of arsenic resistance genes appeared to be related to the first set although some of the genes were missing. The third set was distantly related and appeared to be present on the chromosomes of all At. caldus strains irrespective of whether they had been exposed to elevated levels of arsenic.