1. The incorporation of labelled valine by rabbit reticulocytes into the N-terminal position of nascent haemoglobin was investigated by deaminating the nascent peptides with nitrous acid and isolating labelled α-hydroxyisovaleric acid and valine after acid hydrolysis. 2. The amount of radioactivity in α-hydroxyisovaleric acid relative to that in valine indicated the presence of 12·3% N-terminal valine having a free amino group. This high value suggests that most if not all nascent peptides contain valine in the N-terminal position. 3. Cell-free preparations containing reticulocyte ribosomes and pH5 enzymes incorporated α-hydroxy-[14C]isovaleryl-tRNA (where tRNA refers to transfer RNA), which was obtained by deamination of [14C]valyl-tRNA from yeast or liver with nitrous acid, into both soluble and nascent protein. 4. When the soluble protein was chromatographed on CM-cellulose, radioactivity was found to be associated with both the α-and β-globin chains. 5. The kinetics of hydrolysis of [14C]valine, was also investigated. Most of the material was hydrolysed rapidly at pH10, but a minor component that was relatively stable appeared to be present to the extent of about 10% of the total valyl-tRNA. Valine was, however, the only hydrolysis product detected by paper chromatography. 6. It is concluded that chain initiation in haemoglobin synthesis involves valine as the N-terminal amino acid and that the amino group of nascent protein is probably not substituted.

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