The binding sites for tRNA on eukaryotic ribosomes.

We have studied the non-enzymic binding of phe-tRNA to ribosomes from rat liver using deacylated tRNA to inhibit binding to the P-site and puromycin (5 x 10-minus3M) to inhibit binding to the A-site. We conclude that at a low concentration of magnesium ions (10mM) phe-tRNA is bound only at the A-site of 80S irbosomes, whereas at a high concentration of magnesium ions (40mM) phe-tRNA is also bound at the P-site. Studies with edeine indicate that, during non-enzymic binding of phe-tRNA, eukaryotic ribosomes (in contrast to prokarotic ribosomes) have the A-site of the 60S subunit and the initiation site of the 40S subunit juxtaposed. This may account for the differences observed, in formation of diphenylalanyl-tRNA and phenylalanyl-puromycin, between phe-tRNA bound non-enzymically to the P-sites of eukaryotic and prokaryotic ribosomes.