Selenocysteine is a naturally occurring analog of cysteine in which the sulfur atom of the latter is replaced with selenium. This seleno-amino acid occurs as a specific component of various selenoproteins and selenium-dependent enzymes. Incorporation of selenocysteine into these proteins occurs cotranslationally as directed by the UGA codon. For this process, a special tRNA having an anticodon complimentary to UGA, tRNA Sec , is utilized. In Escherichia coli and related bacteria, this tRNA first is amino acylated with serine, and the seryl-tRNA Sec is converted to selenocysteyl-tRNA Sec . The specific incorporation of selenocysteine into proteins directed by the UGA codon depends on the synthesis of selenocysteyl-tRNA Sec . Included in the selenium delivery protein category are rhodaneses that mobilize selenium from inorganic sources and NIFS-like proteins that liberate elemental selenium from selenocysteine. The NIFS protein from Azotobacter vinelandii was found to serve as an efficient catalyst in vitro for delivery of selenium from free selenocysteine to Escherichia coli selenophosphate synthetase for selenophosphate formation. The widespread distribution of selenocysteine lyase in numerous bacterial species was reported and the bacterial enzymes, like the pig liver enzyme, required pyridoxal phosphate as cofactor. Three NIFS-like genes were isolated from E. coli by Esaki and coworkers and the expressed gene products were isolated and characterized. One of these NIFS-like proteins also exhibited a high preference for selenocysteine over cysteine. M. vannielii , an anaerobic methane-producing organism, that grows in a mineral medium containing formate as sole organic carbon source, synthesizes several specific selenoenzymes required for growth and energy production under these conditions.