Selenite assimilation into formate dehydrogenase H depends on thioredoxin reductase in Escherichia coli

Escherichia coli growing under anaerobic conditions produce H₂ and CO₂ 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 σ⁵⁴-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 (SeO3 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.