A Michaelis-Menten kinetic analysis of the transport of sulfate, selenate, and selenite into Escherichia coli K-12 showed that the three dianions were transported by the same carrier. Km values, used as a measure of the affinity of each ligand for the carrier, showed that sulfate was bound 5 times more tightly than selenate and 37 times more tightly than selenite. The specificity ratio, V/,,3X., also indicated that sulfate was the preferred ligand. There was little difference in the ratios for selenate and selenite.Several studies suggest that microorganisms transport selenate and sulfate by the same carrier system (2-4, 9, 14). Once within the cell, selenate is assimilated by the enzyme systems responsible for sulfate metabolism. As a result, sulfur is randomly replaced by selenium in proteins and other biomolecules (1, 10). In contrast, the assimilation of selenite by several microbial systems, as well as by one mammalian enzyme, leads to the synthesis of several microbial selenoenzymes and of glutathione peroxidase, the selenoenzyme related to the role of selenium as an essential micronutrient for mammals (11,12).Despite the identification of these enzymes as selenoproteins, there remains the unresolved question of whether the mediated transport of selenite, the obligate step preceding its assimilation, is also independent of the transport of sulfate and selenate or whether the same transport system operates for all three dianions. We undertook a kinetic examination of the transport of selenite, selenate, and sulfate by wild-type Escherichia coli K-12. These kinetic analyses indicated that the three inorganic molecules were taken up by a common transport system that obeyed Michaelis-Menten kinetics. Both the specificity and the affinity were greater for sulfate than for either of the two selenium compounds; for the two compounds, a greater affinity was seen for selenate, but the transporter showed little difference in specificity toward either dianion.E. coli K-12 (ATCC 14948) was grown in a defined medium modified from one described by Davis and Mingioli (6) and used in our earlier experiments with this organism (4). The medium contained (grams per liter of distilled water) 7.0 of K2HPO4, 3.0 of KH2PO4, 0.5 of sodium citrate, 0.12 of