Escherichia coli growing under anaerobic conditions produce H(2) and CO(2) by the enzymatic cleavage of formate that is produced from pyruvate at the end of glycolysis. Selenium is an integral part of formate dehydrogenase H (FDH H), which catalyses the first step in the formate hydrogen lyase (FHL) system. The genes of FHL system are transcribed only under anaerobic conditions, in the presence of a sigma 54-dependent transcriptional activator FhlA that binds formate as an effector molecule. Although the formate addition to the nutrient media has been an established procedure for inducing high FDH H activity, we have identified a low-salt nutrient medium containing <0.1% NaCl enabled constitutive, high expression of FDH H even without formate and d-glucose added to the medium. The novel conditions allowed us to study the effects of disrupting genes like trxB (thioredoxin reductase) or gor (glutathione reductase) on the production of FDH H activity and also reductive assimilation of selenite ( SeO 3(2-)) into the selenoprotein. Despite the widely accepted hypothesis that selenite is reduced by glutathione reductase-dependent system, it was demonstrated that trxB gene was essential for FDH H production and for labelling the FDH H polypeptide with 75Se-selenite. Our present study reports for the first time the physiological involvement of thioredoxin reductase in the reductive assimilation of selenite in E. coli.