Aminoacylation of initiator methionyl-tRNA(i) under conditions inhibitory to initiation of protein synthesis

Inhibition of protein synthesis in perfused rat liver deprived of either methionine or tryptophan results from a defect in peptide-chain initiation. Similarly, the decreased rate of protein synthesis in liver from rats deprived of food for 24 h and in skeletal muscle after 2 days of diabetes results from a defect in initiation. In the present study, the tissue content of tRNA(i)/(Met) and its level of aminoacylation were measured in these conditions to determine whether methionyl-tRNA(i)/(Met) formation is a mechanism involved in the regulation of initiation. The extent of aminoacylation of tRNA(i)/(Met) in livers perfused with supplemented medium or medium deficient in either methionine or tryptophan was 64 ± 2, 61 ± 3, and 66 ± 2% of the total accepting activity, respectively. The total tissue content of tRNA(i)/(Met), expressed as a percentage of total RNA, was 1.7 ± 0.1, 1.6 ± 0.1, and 1.6 ± 0.1 for the three conditions, respectively. In livers from starved rats, the extent of aminoacylation of tRNA(i)/(Met) was 80 ± 7% and the total tissue content of tRNA(i)/(Met) was 1.9 ± 0.1% compared with control values of 82 ± 6 and 2.0 ± 0.1%, respectively. In skeletal muscle from diabetic rats, the extent of aminoacylation of tRNA(i)/(Met) was 79 ± 4% and the total tissue content of tRNA(i)/(Met) was 2.0 ± 0.3% compared with values of 79 ± 5 and 2.0 ± 0.2% for control animals. The total quantity of amino acids bound to all species of tRNA in skeletal muscle from diabetic rats was reduced to 80% of control values but was similar to the reduction in total RNA content, suggesting that a decrease in tRNA content and not deacylation per se had occurred. Thus these data suggest that neither the extent of aminoacylation nor the total tissue content of tRNA(i)/(Met) plays a regulatory role in the inhibition of peptide-chain initiation under the conditions examined.