Tailoring of Prematurely Released Polyketide Intermediates in Piericidin Biosynthesis

Piericidins make up a group of microbial-derived polyketide natural products with a wide variety of biological activities. Their biosynthetic gene clusters (BGCs) have been identified in several strains of Streptomyces. In Streptomyces pactum, the piericidin BGC splits into two loci consisting of genes that encode six modular (type I) polyketide synthases and five post polyketide synthase (PKS) modification enzymes. Here, we report that inactivation of the pieA5 gene of the piericidin PKS in a mutant strain of S. pactum, in which several other major biosynthetic pathways have been inactivated, led to the identification of four piericidin polyketide intermediates (1-4) that were offloaded prematurely from the PKS. Compounds 1-3 are derived from an isopropyl starter unit, whereas compound 4 is derived from an acetyl starter unit. Compounds 1 and 4 have a terminal carboxylic acid, whereas compounds 2 and 3 have a terminal amide and a pendant methyl group at C-2. Compound 3 also contains an epoxide ring in the polyketide chain. The results reveal the ability of the piericidin PKS to offload incomplete polyketide intermediates and uncover the highly promiscuous tailoring enzymes that can modify the prematurely released short chain polyketides.