Initiation of rabbit hemoglobin synthesis: methionine and formylmethionine at the N-terminal.

Ribosome-bound peptides were prepared from rabbit reticulocytes incubated in a reaction mixture that contained all essential amino acids except tryptophan. The peptides were fractionated by Sephadex gel filtration and the N-termini of these peptides were examined. Longer uncompleted hemoglobin chains (larger than 30 amino acids) had unblocked valine at the N-terminal position. In contrast, shorter initial parts of chains (smaller than 16 amino acids) did not have valine at the N-terminal position. These small peptides had methionine at the N-terminal, and 8% of the terminal methionine was presumably formylated. These findings indicate that hemoglobin chains are initiated from methionine or N-formylmethionine (or from both methionine and N-formylmethionine), and that the methionyl residue is hydrolyzed at an early stage of chain elongation.