Biosynthesis of ansatrienin (mycotrienin) and naphthomycin - Identification and analysis of two separate biosynthetic gene clusters in Streptomyces collinus Tu 1892

The polyketide chains of the two ansamycin antibiotics, ansatrienin (mycotr ienin) and naphthomycin produced by Streptomyces collinus are assembled usi ng 3-amino-5-hydroxybenzoic acid (AHBA) as a starter unit. The gene encodin g AHBA synthase, an enzyme which catalyzes the final step of AHBA biosynthe sis in the recently discovered aminoshikimate pathway, has been used to ide ntify two separate antibiotic biosynthetic gene clusters in S. collinus. In one of these clusters, analysis of approximately 20 kb of contiguous seque nce has revealed both a cluster of six genes presumed to play a role in the AHBA pathway and the beginning of a polyketide synthase (PKS) gene contain ing an acyl ACP ligase domain. This domain is likely responsible for loadin g AHBA onto the PKS. This gene cluster also contains chcA, encoding the enz yme 1-cyclohexenylcarbonyl CoA reductase, which is essential for the biosyn thesis of the cyclohexanecarboxylic acid moiety of ansatrienin from shikimi c acid, and a peptide synthetase. This gene cluster thus seems to control t he biosynthesis of ansatrienin, which contains a side chain of N-cyclohexan ecarbonyl-D-alanine esterified to the macrocyclic lactam backbone. In the p utative naphthomycin biosynthetic gene cluster approximately 13 kb of conti guous sequence has revealed a second set of the genes required for AHBA bio synthesis. In addition the end of a polyketide synthase and a gene putative ly involved in termination of the chain extension process, formation of an intramolecular amide bond between the AHBA nitrogen and the carboxyl group of the fully extended polyketide chain, have been identified. Thus, despite commonality in biosynthesis, the ansatrienin and naphthomycin biosynthetic gene clusters show clear organizational differences and carry separate set s of genes for AHBA biosynthesis.