Translational Control of Antibody Synthesis

Mouse plasma cytoma cells (5563) contain 2.5 times more L‐chain mRNA than H‐chain mRNA. The ratio of L‐ to H‐chain protein produced however appears related to the growth rate of the cells. During exponential growth, up to 2.5 times more L‐chain than H‐chain protein is produced, but as cells approach stationary growth phase, L‐ and H‐chain protein approach equal synthesis. The apparent decrease in L‐chain synthesis upon shifting from exponential to stationary growth appears not to be due to a relative reduction in peptide elongation but rather reflects differences in ribosome loading. During exponential growth, L‐chain polyribosomes contain 6–8 ribosomes/L‐chain mRNA whereas in stationary growth the figure drops to less than 3–4 ribosomes/L‐chain mRNA. A similar shift of polyribosome size is noted with total cellular polyribosome. Polyribosomes engaged in H‐chain synthesis remain fully loaded with ribosomes during conditions of L‐chain polyribosome size reduction. The size of polyribosomes synthesizing H‐chain protein can be reduced however under conditions of an increase in the intracellular level of H 2 L 2 .