Quantitative studies of the synthesis of the ribosomal proteins of the 50S ribosomal subunit have been made with growing versus valine-deprived HeLa cells. The synthesis of total cell protein and 50S subunits was also compared between the growing and nongrowing cells. It was found that between 12 and 20 hr of valine deprivation the net synthesis of 50S subunits drops to approximately 8% of that in control cells while the over-all synthesis of 50S subunit ribosomal proteins declines to approximately 12% of that in the controls. However, the synthesis rates for each of two particular 50S subunit proteins decline to approximately 8% of the rates in the growing cells, indicating that the synthesis of one of these proteins may be rate limiting for 50S subunit biosynthesis in valine-deprived HeLa cells. Other evidence indicates that the regulation of synthesis of the ribosomal proteins in valine deprivation depends on control at the level of transcription or translation rather than being a function of the relative valine content of these proteins.

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