Functional differentiation of the catalytic subunits YnfE and YnfF involved in selenate and tellurate reduction in Escherichia coli

Selenate and tellurate are toxic chalcogen oxyanions that can be reduced by several bacteria to the less-toxic elemental forms of selenium and tellurium. In Escherichia coli, the ynfEFGH gene cluster encodes two complex iron-sulfur molybdoenzymes, YnfEGH and YnfFGH, involved in this reduction. This study investigated the functional relationship between the two catalytic subunits, YnfE and YnfF. Phylogenetic analysis revealed that their homologs were divided into three major clades, with YnfE and YnfF belonging to different subclades within the same clade. Reductase assays using gene-disruption strains revealed that selenate was mainly reduced by YnfE, whereas tellurate was reduced by both YnfE and YnfF. Analysis of the reaction product indicated that YnfE and YnfF catalyzed the two-electron reduction of selenate/tellurate to selenite/tellurite. These findings suggest that selenate and tellurate are initially reduced to selenite and tellurite, which are subsequently converted to elemental selenium and tellurium by intracellular thiols in E. coli.