Association of eukaryotic aminoacyl-tRNA synthetases with polyribosomes

After separation of a mitochondria-free extract of rabbit reticulocytes into a fraction of a ribosome-free extract and a fraction of mono- and polyribosomes the major portion of the aminoacyl-tRNA synthetase activity was found in the fraction of mono- and polyribosomes. All 15 of the aminoacyl-tRNA synthetases were found, although in somewhat different proportions, in these two fractions of the mitochondria-free extract of the reticulocytes. The amino-acyl-tRNA synthetases of the ribosome-free extract are present in two forms: an RNA-binding form and a form without affinity for high-molecular-weight RNAs. Aminoacyl-tRNA synthetases dissociated from complexes with polyribosomes are found only in RNA-binding form. All the aminoacyl-tRNA synthetases can be removed from these complexes by the addition of 16S rRNA from E. coli, poly(U), or tRNA from rabbit reticulocytes, which indicates that the aminoacyl-tRNA synthetases are in labile association with the RNA-component of the polyribosomes and that their interaction is rather nonspecific. After EDTA-induced dissociation of the polyribosomes aminoacyl-tRNA synthetase activity is found in a complex with both ribosomal subunits