Eukaryotic translation initiation factor 2 (eIF-2) is a heterotrimer of three subunits designated alpha, beta, and gamma. Since they are found in equimolar amounts in the cell and are never detected as isolated subunits, we examined the basis of their balanced synthesis. Northern analysis of the K562 mRNA revealed the message for beta was five times more abundant than the message for alpha. Further analysis of total RNA from other human tissues showed that this difference in the pool sizes for the two messages was present in a variety of tissues. However, the magnitude of the difference varied between the tissues. The difference in pool sizes ranged from 3.6:1 in heart and brain to 15:1 in pancreas. Immunoprecipitation of eIF-2 alpha and beta protein from K562 cells labeled for times as short as 10 min revealed equimolar synthesis of alpha and beta despite a five-fold difference in the size of the mRNA pools. Comparison of the polysome profile for alpha and beta in K562 cells showed that the average alpha message had over 50% more ribosomes associated with it than the average beta message. Substitution of either the initiation codon context or the entire beta-mRNA leader for alpha did not significantly alter the translational efficiency of beta. Comparison of the rate of ribosomal elongation, however, showed that ribosomes on beta-mRNA elongated at a rate less than 4-fold that of eIF-2 alpha. Therefore the balanced translation of eIF-2 alpha and beta mRNA is the direct result of their different rates of ribosomal elongation. In addition, we have obtained the complete genomic clone of eIF-2 beta. The promoter region of this TATA-less gene also contains a high affinity binding site for the transcription factor alpha-PAL first identified in the promoter of eIF-2 alpha.
