BIOSYNTHESIS OF THE MODIFIED PEPTIDE ANTIBIOTIC NOSIHEPTIDE IN STREPTOMYCES-ACTUOSUS

The biosynthesis of the highly modified thiopeptide antibiotic, nosiheptide (1) was studied by feeding radioactive and stable-isotope-labeled precursors to cultures of the producing organism, Streptomyces actuosus. The stable isotope enrichments and/or coupling patterns in the isolated 1 were analyzed by NMR spectroscopy using various 1D and 2D NMR techniques. The results complete the confirmation of the amino acid components of the antibiotic and shed light on a number of mechanistic and stereochemical aspects of its assembly from these basic building blocks. The dehydroalanine and butyrine moieties are formed by an anti elimination of water from serine and threonine, respectively, the thiazole rings from cysteine residues with loss of the pro-3R hydrogen in the oxidation step, and the hydroxypyridine moiety from two intact serine residues, situated nine amino acids apart in the peptide chain, and the carboxyl group of an adjacent cysteine. According to N-15 studies which included the complete assignment of the N-15 NMR resonances of 1, the carboxy-terminal amide nitrogen of 1 originates from the amino group of serine, suggesting that the precursor peptide giving rise to 1 carries an additional carboxy-terminal serine residue which, except for its nitrogen, is removed during processing. The indolic acid moiety is elaborated separately by a direct rearrangement of tryptophan to 3-methylindole-2-carboxylic acid and methylation at C4; it seems to be attached to the peptide backbone prior to hydroxylation of the C4' methyl group. Possible mechanisms of some of the key reactions and the mode of assembly of the modified peptide structure are discussed.