The effects of amino acid starvation on regulation of polypeptide chain initiation in Ehrlich ascites tumor cells.

The ratio of initiation of protein synthesis in Ehrlich ascites tumor cells in culture is reduced by over 60% in the absence of a single essential amino acid. Cell-free extracts prepared from control and amino acid-starved cells retain some of the translational characteristics of these cells and are able to form [40 S.Met-tRNAfMet] initiation complexes. Studies with inhibitors show that up to 63% of the translation directed by endogenous mRNAs in vitro depends on reinitiation of polypeptide chains. Amino acid starvation inhibits this activity, as well as protein synthesis due to in vitro polysomal run-off, by up to 75%. Analysis of [40 S.Met-tRNAfMet]initiation complexes formed in vitro on native 40 S subunits shows that amino acid starvation causes up to a 77% decrease in the concentration of these complexes relative to the corresponding fed controls. This difference is eliminated by the addition of highly purified eukaryotic initiation factor eIF-2. Factor eIF-3 also stimulates [40 S.Met-tRNAfMet] formation in the cell extracts but does not abolish the difference between fed and starved preparations. Mixing experiments have not so far revealed any inhibitor of initiation complex formation in the starved cell extracts.