RESUMO
The polyketide chains of the two ansamycin antibiotics, ansatrienin (mycotrienin) and naphthomycin produced by Streptomyces collinus are assembled using 3-amino-5-hydroxybenzoic acid (AHBA) as a starter unit. The gene encoding AHBA synthase, an enzyme which catalyzes the final step of AHBA biosynthesis in the recently discovered aminoshikimate pathway, has been used to identify two separate antibiotic biosynthetic gene clusters in S. collinus. In one of these clusters, analysis of approximately 20 kb of contiguous sequence 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 containing an acyl ACP ligase domain. This domain is likely responsible for loading AHBA onto the PKS. This gene cluster also contains chcA, encoding the enzyme 1-cyclohexenylcarbonyl CoA reductase, which is essential for the biosynthesis of the cyclohexanecarboxylic acid moiety of ansatrienin from shikimic acid, and a peptide synthetase. This gene cluster thus seems to control the biosynthesis of ansatrienin, which contains a side chain of N-cyclohexanecarbonyl-d-alanine esterified to the macrocyclic lactam backbone. In the putative naphthomycin biosynthetic gene cluster approximately 13 kb of contiguous sequence has revealed a second set of the genes required for AHBA biosynthesis. In addition the end of a polyketide synthase and a gene putatively 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 sets of genes for AHBA biosynthesis.
