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

DURING the biosynthesis of selenoproteins in both prokaryotes and eukaryotes, selenocysteine is cotranslationally incorporated into the nascent polypeptide chain1, 2 through a process directed by a UGA codon that normally functions as a stop codon3–5. Recently, four genes have been identified whose products are required for selenocysteine incorporation in Escherichia coli6. 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 codon7. The serine residue attached to this tRNA is converted to selenocysteine in a reaction dependent on functional selA and selD gene products8. By contrast, the selB gene product (SELB) is not required until after selenocysteyl-tRNA biosynthesis8. 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.