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Biochemistry 5 ) in promoting the formation of a D-amino acid intermediate. The pH 4.8 precipitate, which contained the s-RNA, did not respond to the assay for this postulated intermediate. The control experiment with L-leucine ( Fig. 7) supports our view that the factor in the pH 4.8 supernatant fraction is concerned with D-, but not with L-amino acid utilization. Additional research is proceeding, with the aim of isolating and characterizing the carrier of activated D-amino acids.The experiments in Table IV show that soluble components in addition to s R N A were necessary for polypeptide synthesis. These components may have included the postulated factor in D-amino acid transfer, and quite possibly messenger RNA. It is conceivable that a special form of m-RNA was the agent which controlled the specificity of peptide synthesis (Tables V and VI). Much of our future effort will be centered about the study of this problem.As in protein biosynthesis, the exact role of the ribosomes in polypeptide formation requires additional investigation. One particularly intriguing question is whether there exist in B. brevis cells unique ribosomes concerned with only gramicidin or tyrocidine (and not protein) synthesis. In view of the great diversity of polypeptides in nature, it will be of much interest to investigate intensively the mode of origin of this class of substances in various organisms, and to compare the biosynthetic mechanisms with those of proteins. REFERENCES

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of the guinea pig. Mention may also be made of tissue slice experiments on the biosynthesis of the low molecular weight protein insulin (Humbel, 1963;Mallory et al., 1964).The bovine adrenocorticotropin (ACTH)1 molecule, with 39 amino acid residues (Li et al., 1961), was selected for the present investigation. 123456789 10 11Seryr-Ser-Met-Glu-His-Phe-Arg-T ry-Gly-Lys-12 13 14