Role of tissue specific factors in the translation of brain messenger ribonucleic acids in vitro.

The activity of brain ribosomal subunits was examined by measuring (a) the saturation of ribosomes with nascent polypeptides and the rates of release of completed chains; (b) interaction of the subunits with brain messenger RNA to form polysomal aggregates; (c) formation of polyphenylalanine in the presence of polyuridylic acid at high magnesium concentration and (d) inhibition by aurine tricarboxylic acid. The results showed that a portion of the subunits were defective in forming initiation complexes with brain messenger RNA, but translated polyuridylate efficiently. The subunits that did form polysomes were more competent than the heterologous systems (derived from Krebs ascites cells, reticulocytes or wheat germ) in carrying out reinitiations of brain mRNA translation. Both the homologous and the heterologous systems translated brain mRNA and synthesized the two brain specific proteins S-100 and the neuron specific enolase, indicating that each of the systems had all the necessary factors. However, homologous initiation factors, aminoacyl-tRNA synthetases and transfer RNAs were more effective, particularly at suboptimal concentrations. Our results suggest that discriminative translation of brain messenger RNA may take place based on relative proportions of required components in the reaction milieu rather than by the presence or absence of one or more special messenger RNA specific factors.