Nonribosomal Polypeptide Synthesis: The Biosynthesis of a Cyclic Peptide Antibiotic,

Peptide antibiotics are produced by a variety of microorganisms in special physiological cir-cumstances ( ee Gottlieb and Shaw, 1967). Certain strucgural features of these substances indicate that the biosynthetic mechanisms must differ from those of proteins. For example, residues other than the 20 or so L-:c-amino acids found in proteins are frequently present (including D-amino acids, hydroxy acids, and fatty acids), the molecules are often cyclic, and analog-type substitutions of individual residues are very common. Much evidence (well summarized by Katz, 1968) has been accum-ulated to date proving that ribosomes, transfer RNA (tRNA) and messenger RNA (mRNA) are not required for the formation of the antibiotic polypeptides. We have studied the biosynthesis of the cyclic decapeptide gramicidin S (Fig. 1) in order to gain an insight into the enzymatic mechanisms involved, and to make possible a detailed comparison with protein synthesis a carried out by ribosomes. AMINO ACID ACTIVATION Crude extracts, prepared from washed B. brevis cells by rupture in a French press followed by high-speed centrifugation, catalyze the ATP- and mag