Metabolism of selenium by Escherichia coli: biosynthesis of selenomethionine.

Selenium, an element long known to be toxic to living organisms (1, 2), has only recently been implicated as an essential trace element for rats (3), chicks (4,5) and several other animals. The work of Franke (6), Horn et cd. (7), Painter (8, 9), and McConnell (10, 11) established that if toxic amounts of selenium were supplied in various chemical forms to a number of plants, animals, and microorganisms, the selenium is “bound” to the proteins of these organisms. Although it has been frequently suggested that selenium can replace sulfur in protein cyst(e)ine or methionine or both, the exact manner in which selenium is incorporated into protein has never been clarified. Two recent reviews dealing with the metabolism of selenium are available (2, 12). In 1957, Cowie and Cohen demonstrated that selenomethionine can completely replace methionine for the normal exponential growth of a methionine-requiring mutant of Escherichia coli, and that active enzymes are formed under their conditions. They also found that selenite is taken up by E. coli grown in the presence of methionine, and that selenium is incorporated into cellular protein, possibly as selenocysteine (13). The present results demonstrate that E. coli is capable of metabolizing inorganic selenium to amphoteric organic compounds which are incorporated into protein. Selenium was found to replace sulfur partially in methionine and possibly cyst(e)ine of bacterial protein.