Identification of a novel translation factor necessary for the incorporation of selenocysteine into protein

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Abstract
During the biosynthesis of selenoproteins in both prokaryotes and eukaryotes, selenocysteine is cotranslationally incorporated into the nascent polypeptide chain through a process directed by a UGA codon that normally functions as a stop codon. Recently, four genes have been identified whose products are required for selenocysteine incorporation in Escherichia coli. One of these genes, selC, codes for a novel transfer RNA species (tRNAUCA) that accepts serine and cotranslationally inserts selenocysteine by recognizing the specific UGA codon. The serine residue attached to this tRNA is converted to selenocysteine in a reaction dependent on functional selA and selD gene products. By contrast, the selB gene product (SELB) is not required until after selenocysteyl-tRNA biosynthesis. Here we present evidence indicating that SELB is a novel translation factor. The deduced amino-acid sequence of SELB exhibits extensive homology with the sequences of the translation initiation factor-2 (IF-2) and elongation factor Tu (EF-Tu). Furthermore, purified SELB protein binds guanine nucleotides in a 1:1 molar ratio and specifically complexes selenocysteyl-tRNAUCA, but does not interact with seryl-tRNAUCA. Thus, SELB could be an amino acid-specific elongation factor, replacing EF-Tu in a special translational step.