Protein synthesis by liver ribosomes from aged rats

The age-related decrease in protein synthesis by cell-free systems has been traced to a factor which can be obtained by high salt extraction of young polysomes. Such extracts, when added to old ribosomes in young post-ribosomal supernate, stimulate the level of Poly(U)-directed protein synthesis. Extracts of old polysomes have essentially no effect. The deficient factor is not EF-2 and is highly unlikely to be EF-1, as this component resides almost entirely in the post-ribosomal supernates used in the reaction mixture. Since initiation factors are not necessary for Poly(U)-directed protein synthesis and EF-1 and EF-2 do not appear to be involved, the nature of the soluble factor which is deficient in old ribosomes appears to lie outside of proteins which are commonly implicated in the age-related slowing of protein synthesis.