The control of protein synthesis during the stimulation of lymphocytes by phytohaemagglutinin. III. Poly(U) translation and the rate of polypeptide chain elongation.

1. Cell-free systems from phytohaemagglutinin-stimulated pig lymphocytes are much more active in both endogenous protein synthesis and the translation of poly(U) than those from unstimulated lymphocytes. 2. Addition of tRNA stimulates the translation of poly(U) and greatly reduces the difference between systems from stimulated and unstimulated lymphocytes. Endogenous protein synthesis is not increased by added tRNA. 3. Systems from stimulated lymphocytes have an increased capacity to form aminoacyl-tRNA with several different amino acids. This reaction is limited by the amount of tRNA present in the cell-free system. 4. The rate of polypeptide elongation is not affected by lymphocyte stimulation. The increased rate of protein synthesis must therefore be due to an increase in the frequency of initiation of the synthesis of protein molecules.