CONTROL OF HAEMOGLOBIN SYNTHESIS - DISTRIBUTION OF RIBOSOMES ON MESSENGER RNA FOR ALPHA AND BETA CHAINS

The nascent peptides attached to the polysomes of rabbit reticulocytes were labelled with [3H]lysine, and the specific activity of the tryptic peptides derived from them measured. The data show that ribosomes are uniformly distributed on mRNA when the cells are incubated under standard in vitro conditions. When cells were incubated in the absence of tryptophan, or when they were exposed briefly to sodium fluoride after the nascent peptides had been labelled, the ribosomes were found at one or other end of the mRNA. From the uniform distribution of the nascent chains in normally incubated cells we conclude that the rate of movement of the ribosomes over all regions of the mRNA is uniform. We are able to deduce the relative number of ribosomes carrying nascent α and β chains. The number of complete chains isolated with the ribosomes varied from 1 7 to 1 60 complete chain/nascent chains. In no case was an excess of complete α chains over β chains found attached to the ribosomes. These results are discussed in relation to the rate limiting step in the assembly of globin. © 1968.