A new translational elongation factor for selenocysteyl‐tRNA in eucaryotes

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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.