The acidophilic and strictly chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans oxidizes ferrous (Fe(II)) to ferric (Fe(III)) iron and reduced inorganic sulfur compounds (RISC) to sulfuric acid, in oxic conditions. The redox proteins involved in the electron transfer between Fe(II) and oxygen are encoded in the same transcriptional unit, the rus operon. The expression of this operon is induced in the presence of Fe(II), but not Fe(III), and is not repressed in the presence of sulfur (S0). A number of genes differentially expressed in iron or sulfur conditions have been identified by microarrays transcript profiling. We show here that the presence of Fe(II) induced the expression of the genes involved in iron oxidation and repressed the expression of the genes involved in RISC oxidation. Identification of the regulator(s) involved in this transcriptional regulation is underway. Two genes encoding putative regulators belonging to two transcriptional units located downstream from the rus operon have been cloned. One regulator with a putative ironsulfur cluster belongs to the IscR family and the other belongs to the two component sensor/regulator family. Expression of both genes is induced in the presence of Fe(II) and is not repressed by S0. The recombinant proteins have been purified and gel shift assays with the target regulatory regions are in progress.