Control of Haemoglobin Synthesis: a Difference in the Size of the Polysomes making α and β Chains

Abstract

HAEMOGLOBIN is composed of equimolar amounts of α and β chains. Although it is not essential to synthesize the same number of molecules of each chain, this was originally thought to be the case¹. The finding of a pool of α chains in rabbit reticulocytes implies that either at some stage there has been excess synthesis of α chains, or that the α chains are continually produced in excess of β chains^2–4. An understanding of the mechanisms which control the balance of the synthesis of the two chains requires elucidation of the rate limiting steps in the assembly of each chain and the factors which affect those steps. We have already shown that the ribosomes which make each chain are evenly distributed on their respective mRNAs, thus demonstrating that there are no single rate limiting steps in the translation of the two chains by the ribosomes, other than in the process of initiation⁵. In this work we discovered that polysomes containing relatively few ribosomes carried more nascent α chains than nascent β chains, and that large polysomes carried more β chains than α chains. Here we report experiments which describe this phenomenon.