A new translational elongation factor for selenocysteyl-tRNA in eucaryotes.

In eucaryotes, selenocysteine (SeCys) was found in some selenoproteins, but SeCys-tRNA was not recognized by EF-1 alpha. 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-1 alpha (the activity of binding of [14C]Phe-tRNA to the UUU-programmed ribosomes) in fractions 25-37. EF-1 alpha 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) 137-142]. 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.