Partial Reaction of Peptide Initiation Inhibited by Phosphorylation of either Initiation Factor eIF-2 or 40S Ribosomal Proteins

Preparations of the hemin-controlled repressor (HCR) from rabbit reticulocytes contain 3′:5′-cyclic-AMP-independ ent protein kinase activity for the smallest subunit of the peptide initiation factor eIF-2 and for proteins of reticulocyte 40S ribosomal subunits. Binding of the ternary complex formed between Met-tRNAf, GTP, and eIF-2 to 40S ribosomal subunits is shown to be inhibited by phosphorylation of either the ribosomal subunits or eIF-2. The protein kinase activity responsible for phosphorylation of eIF-2 has been separated from the activity for phosphorylation of 40S ribosomal subunits and shown to independently block the same partial reaction of peptide initiation. It appears that different enzymes are involved, each capable of regulating peptide initiation at the same step but by a different mechanism.