Incorporation of C14-labeled Amino Acids into Actinomycin and Protein by Streptomyces antibioticus

SUMMARY Radioisotope experiments have revealed that L- rather than n-valine is the precursor of the n-valine residues in the actino- mycin peptide. Since n-valine was taken up by the organism almost as well as L-valine, permeability does not appear to be a factor in the preferential utilization of the L isomer. L-Valine was also utilized for the synthesis of N-methyl-L-valine. From studies with glycine-l- and -2-C14, it was concluded that glycine is the precursor of sarcosine. The radioactivities of sarcosine- l-Cl4 and sarcosine-N-C14H3 were incorporated to an equal extent into the actinomycin peptide, which suggests that free glycine in Z&JO is converted to free sarcosine, which is then incorporated into the actinomycin peptide. L-Methionine-C14H3 and, to a limited extent, C-2 of glycine, were utilized as a source of the N-methyl group of sarcosine and N-methyl+valine. Formate was completely inactive as a precursor of methyl group. The development rapid extraction procedure for the assay of the antibiotic has made possible the study of amino acid incorporation into actinomycin in short term experiments. Under conditions that inhibit pro- tein synthesis, amino incorporation into antibiotic was greatly stimulated. The results indicate that antibiotic synthe- sis proceeds by a mechanism different from protein synthesis, although both processes compete for the available amino acids in the cell. AcknowZedgment.s-The authors wish to thank Dr. H. Fales, National Heart Institute, Institutes of Health, for his advice and suggestions in the preparation and degradation of the N-tosylsarcosine derivative, and Dr. S. Udenfriend for his con- tinued encouragement. REFERENCES