Abstract
In eucaryotes, selenocysteine (SeCys) was found in some selenoproteins, but SeCys-tRNA was not recognized by EF-1α. A different translational elongation factor for SeCys-tRNA must therefore supply SeCys-tRNA to the machinery of selenoprotein translation. I found this factor in bovine liver extracts with a UGA-programmed ribosome binding assay. The activity of binding of [ 75Se]SeCys-tRNA to the UGA-programmed ribosomes was eluted in fractions 57–65 using a CM-Sephadex C-25 column, and separated from EF-1α (the activity of binding of [ 14C]Phe-tRNA to the UUU-programmed ribosomes) in fractions 25–37. EF-1α in fraction 25 could discriminate (UUU) 10 for [ 14C]Phe-tRNA. A factor in fraction 57 could discriminate (UGA) 10 for [ 75Se]SeCys-tRNA. The elution pattern of activity of binding of [ 75Se]SeCys-tRNA to the UGA-programmed ribosomes was almost identical to that of activity of protecting [ 75Se]SeCys-tRNA against alkaline hydrolysis (SePF activity) [FEBS Lett. 347 (1994) 137–1421]. These two activities might depend on the same factor. The activity of binding of [ 75Se]SeCys-tRNA to the UGA-programmed ribosomes directly indicates that a factor in fraction 57 is a new translational elongation factor for SeCys-tRNA in eucaryotes.
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