Post-microsomal fractions from rat brain which stimulate ribosomal amino acid incorporation

Post-microsomal fractions from rat brain were prepared which stimulated the synthesis of protein when endogenous mRNA was available and enhanced the synthesis of polyphenylalanine in the presence of poly-U and monomeric ribosomes. The amino-acid-activating (or transfer) enzymes did not appear to be responsible for the observed effect since the reaction mixture contained sufficient amounts of these enzymes to permit an optimum rate of amino acid incorporation. The effect was not solely due to the presence of messenger or amino acid transfer RNAs since the activity of the fractions was only partly reduced by prior treatment with ribonuclease. Heating led to complete inactivation of the fractions. Results from amino-acid-incorporation experiments and from sucrose density-gradient centrifugations indicate that the effect might be produced at two levels, one at the level of formation or preservation of polyribosomal structure and the other possibly at the level of peptide bond formation.