Defective initiation on natural messenger RNA by cell free systems from Krebs ascites cells incubated at elevated...

1. Cell-free systems prepared from Krebs II ascites cells incubated at 45 degrees C have a much lower endogenous activity than those from cells incubated at 37 degrees C. The endogenous activity is mainly due to completion of polypeptide chains initiated in the intact cell. The low activity of the 45 degrees C system is due to a lesion in initiation in cells incubated at 45 degrees C. 2. Cell-free systems from cells incubated at 45 degrees C can translate efficiently poly (U) at 8 mM Mg2+. However, they initiate poorly on globin mRNA, indicating that these systems reflect the situation in the intact cell. 3. The lesion in globin mRNA translation in 45 degrees C systems can be overcome by addition of reticulocyte initiation factors. At saturation concentrations of factors, the response of a 45 degrees C system is restored to almost normal. 4. 45 degrees C systems from 40-S initiation complexes with methionyl tRNAfmet almost as efficiently as normal, but their ability for form 80-S complexes with globin mRNA is impaired, unless they are supplied with exogenous initiation factors.