Mechanisms in protein synthesis IV. Further evidence for two different ribosomal sites, one binding formylmethionyl-tRNA, the other methionyl- and other aminoacyl-tRNA's

Abstract

The binding of N-acetylphenylalanyl tRNA to ribosomes (70-S) and their subunits (30-S and 50-S) in the presence of poly U was compared with that of phenylalanyl tRNA under the nonenzymatic conditions. The binding capacity of ribosomes for N-acetyl phenylalanyl tRNA was approximately one-half of that of ribosomes for phenylalanyl tRNA at low Mg2+ concentrations. In addition, when phenylalanyl tRNA was added to the ribosome-poly U complex carrying N-acetylphenylalanyl tRNA, the amount of total bound tRNA became almost equal to the amount of bound tRNA observed using phenylalanyl tRNA alone. On the other hand, 30-S subunits bound N-acetylphenylalanyl tRNA to the same degree as phenylalanyl tRNA. Furthermore, when 50-S subunits were added under conditions where most of active 30-S subunits had N-acetylphenylalanyl tRNA, the additional binding of phenylalanyl tRNA to this complex took place, but 50-S subunits did not stimulate the additional binding of N-acetylphenylalanyl tRNA.