By proteomic analysis we found a 21-kDa protein (P21) from Acidithiobacillus ferrooxidans ATCC 19859 whose synthesis was greatly increased by growth of the bacteria in pyrite, thiosulfate, elemental sulfur, CuS, and ZnS and was almost completely repressed by growth in ferrous iron. After we determined the N-terminal amino acid sequence of P21, we used the available preliminary genomic sequence of A. ferrooxidans ATCC 23270 to isolate the DNA region containing the p21 gene. The nucleotide sequence of this DNA fragment contained a putative open reading frame (ORF) coding for a 23-kDa protein. This difference in size was due to the presence of a putative signal peptide in the ORF coding for P21. When p21 was cloned and overexpressed in Escherichia coli, the signal peptide was removed, resulting in a mature protein with a molecular mass of 21 kDa and a calculated isoelectric point of 9.18. P21 exhibited 27% identity and 42% similarity to the Deinococcus radiodurans thiosulfate-sulfur transferase (rhodanese; EC 184.108.40.206) and similar values in relation to other rhodaneses, conserving structural domains and an active site with a cysteine, both characteristic of this family of proteins. However, the purified recombinant P21 protein did not show rhodanese activity. Unlike cytoplasmic rhodaneses, P21 was located in the periphery of A. ferrooxidans cells, as determined by immunocytochemical analysis, and was regulated depending on the oxidizable substrate. The genomic context around gene p21 contained other ORFs corresponding to proteins such as thioredoxins and sulfate-thiosulfate binding proteins, clearly suggesting the involvement of P21 in inorganic sulfur metabolism in A. ferrooxidans.