Activity of protein synthesis initiation factors in cytosol from rat liver and muscle and ascites cells.

The decrease induced by appropriate initiation factors in [Mg] required for optimal polyphenylalanine synthesis primed by poly(U) has been used as an assay to test for the presence of these factors in cytosol of rat liver and muscle and of ascites cells. By calculation and from changes in [Mg] optima on addition of phosphoenolpyruvate and phosphocreatine, the [Mg2+] as opposed to total [Mg] required for polyphenylalanine synthesis has been determined. Contrary to a previous report, factors necessary for polyphenylalanine synthesis at low [Mg] appear to be present in cytosol of muscle and liver as well as that from ascites cells. The significance of these finding in relation to distribution of specific initiation factors is discussed.