The Effects of Haem on Translational Control of Protein Synthesis in Cell‐Free Extracts from Fed and Lysine‐Deprived Ehrlich Ascites Tumour Cells

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Abstract
Addition of heme to cell-free extracts from Ehrlich ascites tumor cells stimulates protein synthesis only in extracts from cells previously incubated in nutritionally complete conditions. Extracts from amino-acid-deprived cells do not respond to heme. The stimulation of protein synthesis in fed cell extracts is due to increased initiation on endogenous mRNA mediated by an increase in the levels of 40-S-subunit .cntdot. Met-tRNA initiation complexes. Extracts from starved cells exhibit a defect in 40-S initiation complex formation which cannot be overcome by heme. Experiments to test for the presence of an inhibitor of initiation in Ehrlich cell extracts by monitoring effects on translation in heme-supplemented reticulocyte lysates have revealed that extracts from both fed and starved cells contain 1 or more inhibitory activities which shut off protein synthesis, dissagregate polysomes and reduce the level of 40-S initiation complexes in the lysate. Extracts from starved cells are more inhibitory for protein synthesis than those from fed cells. Initiation factor eIF-2 is phosphorylated by an endogenous Ehrlich cell protein kinase in vitro, but this occurs to the same extent in extracts from fed and starved cells. A possible model is proposed for the role of eIF-2 in the control of protein synthesis by amino acid supply in Ehrlich cells.