1. Peptide-elongation factors were purified from rat liver and hurnan tonsils and the contents of cholesteryl 14-methylhexadecanoate were determined in fractions obtained during enzyme purification. The relative contents of this compound in purified enzyme preparations was several times higher than that in the crude starting material. Elongation factors from human tonsils contained a significantly larger quantity of the cholesteryl ester than enzytne from rat liver. 2. Transfer enzymes extracted with various organic solvents showed variable decreased activities in both binding and peptidization assay. The decrease of enzymic activity was proportional to the amount of cholesteryl 14-methylhexadecanoate extracted from a given enzymic preparation. In systems containing both extracted elongation factors the polyphenylalanine synthesis was limited by the residual activity of the less active transfer factor. 3. The original enzymic activity of extracted transferases was fully recovered by the addition of pure cholesteryl 14-methylhexadecanoate in quantities corresponding to those extracted. 4. Increase of the relative contents of this cholesteryl ester during enzyme purification, decrease of the enzymic activity after the extraction and its recovery by the addition of this compound indicates that the presence of this ester in elongation factors is essential for the normal function of these enzymes. Cholesteryl 14-methylhexadecanoate seems to play an important role in protein synthesis. This compound stimulates the incorporation of labelled amino acids into tRNA of rat liver in vitro (Hradec & Dolejg, 1968). Purified aminoacyl-tRNA synthetases from mammalian tissues lost all or most of their activity after extraction with organic solvents (Hradec & Duiek, 1969). Only about 50% of the normal charging of tRNA was obtained with extracted pH 5 enzymes from rat liver (Hradec & Dugek, 1968b). In both these instances, a complete reactivation of extracted enzymes could be induced by the addition of cholesteryl 14-methylhexadecanoate into incubation mixtures. These results indicated that the presence of this ester in the molecule of aminoacyl-tRNA synthetases is apparently essential for their normal function. However, in contrast with only partially deactivated pH 5 enzyrnes no incorporation of labelled amino acids into ribosomal proteins was found in reaction mixtures containing extracted cell sap (Hradec & Duiek, 1968b) although normal incorporation was again obtained after the addition of cholesteryl 14-methylhexadecanoate. This apparent discrepancy indicated that the ester affects not only the function of aminoacyl-tRNA ligases but also that of some other soluble enzymes involved in protein synthesis.Enzymes participating in protein synthesis that are present in the soluble fraction of the cell may be divided into three main groups: those required for peptide-chain initiation, elongation and termination (Matthaei et al. 1968). Relatively little is known about the nature and formation of the initiation complex in mamma...

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