THE ELONGATION-FACTOR-3 UNIQUE IN HIGHER FUNGI AND ESSENTIAL FOR PROTEIN-BIOSYNTHESIS IS AN E-SITE FACTOR

Two elongation factors drive the ribosomal elongation cycle; elongatio n factor 1 alpha (EP-1 alpha) mediates the binding of an aminoacyl-tRN A to the ribosomal A site, whereas elongation factor 2 (EF-2) catalyze s the translocation reaction. Ribosomes from yeast and other higher fu ngi require a third elongation factor (EF-3) which is essential for th e elongation process, but the step affected by EF-3 has not yet been i dentified. Here we demonstrate that the first and the third tRNA bindi ng site (A and E sites, respectively) of yeast ribosomes are reciproca lly linked; if the A site is occupied the E site has lost its binding capability, and vice versa, if the E site is occupied the A site has a low affinity for tRNAs. EF-3 is essential for EF-1 alpha-dependent A site binding of amino acyl-tRNA only when the E site is occupied with a deacylated tRNA. The ATP-dependent activity of EF-3 is required for the release of deacylated tRNA from the E site during A site occupatio n.