Interaction between the Synthesis of α and β Globin

We have examined the relationship between α and β globin chain syntheses by utilizing the distribution of isoleucyl residues in rabbit hemoglobin. The α globin chain contains three isoleucyl residues while the β chain of certain rabbits contains no isoleucine. O‐Methyl‐L‐threonine, an isoleucine isostere, inhibits incorporation of radiolabeled amino acids into α chains in rabbit reticulocytes. When α chain synthesis is inhibited by 50–85%, β synthesis is stimulated by 15–50%. The excess labeled β chains are not distinguishable from authentic β chains by any of the following criteria: (a) carboxymethyl cellulose chromatography in sodium phosphate‐urea buffers, (b) electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate, and (c) electrophoresis of methionine‐containing tryptic peptides. The stimulation of β synthesis continues after the pool of excess α chains has been exhausted by preincubation with O‐methyl‐L‐threonine. The stimulation does not occur, however, when 1 mM 2‐mercaptoethanol is added to the incubation medium or when the cells are excessively diluted in the incubation mixture. The rates of β chain initiation and elongation during stimulation have been compared to the rates during normal synthesis. Although both rates are increased, the rate of elongation increases more than initiation, suggesting that initiation is the rate‐limiting step in increased β chain production. The stimulation of β synthesis when α synthesis is inhibited is interpreted as resulting from relief of competition between α and β mRNAs for limiting components of the protein synthetic apparatus.