Bacterial Diterpenoids and Thiocarboxylic Acid-Containing/Derived Natural Products

Of the more than 18,000 diterpenoids known to date, most are produced in plants and fungi with relatively few originating from bacteria. Platensimycin (PTM), platencin (PTN), and platensilin (PTL) are recently discovered diterpenoids of Streptomyces origin with unprecedented molecular scaffolds. Given the technical challenges of exploring natural product (NP) biosynthesis in higher organisms, the study of their biosynthesis in Streptomyces provides an outstanding opportunity to discover novel diterpenoid chemistry and enzymology. Furthermore, PTM, PTN, and PTL are potent inhibitors of bacterial and mammalian fatty acid biosynthesis and have shown promise as antibacterial and antidiabetic drug leads. However, their further development faces many obstacles, particularly due to their problematic pharmacokinetic profile. Engineering of the PTM/PTN/PTL biosynthetic machineries therefore holds promise to generate analogues with improved pharmacokinetics for these promising NP drug leads.

Thiocarboxylic acid-containing or derived NPs are extraordinarily rare, and their biosynthesis and biological significance are poorly understood. We recently discovered thioPTM, thioPTN, and thioPTL as the final products of the PTM/PTN/PTL biosynthetic machinery, dramatically increasing the number of known thiocarboxylic acid NPs and illuminating many aspects of their biosynthesis. Significantly, we identified a genetic cassette that encodes biosynthesis of the thiocarboxylic acid motif. This cassette is prevalent across bacterial genomes, implying that such NPs are underexplored. Consequently, thioPTM, thioPTN, and thioPTL biosynthesis represents an outstanding opportunity to study the construction of thiocarboxylic acid NPs and exploit this unusual functionality as an alternative pharmacophore in future drug discovery and development.

Since we initiated our program on PTM, PTN, and PTL as model systems for bacterial diterpenoid and/or thiocarboxylic acid biosynthesis in 2007, we have inspired the community to investigate bacteria as an underappreciated source of novel diterpenoids and made several seminal discoveries (Figure 1). Current efforts in our lab aim to exploit this knowledge towards (i) biochemical characterization of the pathways’ diterpenoid synthases and tailoring enzymes, (ii) mining actinomycetal genomes to discover novel diterpenoids and thiocarboxylic acid NPs, and (iii) engineering of native and heterologous producers to control their productivity and unlock new structural analogs.

Overview of PTM, PTN, and PTL biosynthesis, engineering, and discovery.

Figure 1: An overview of PTM, PTN, and PTL biosynthesis, engineering, and discovery. (A) The structures of PTM, PTN, PTL, and their thiocarboxylic acid counterparts. (B) The PTM, PTN, and PTL biosynthetic gene clusters. (C) Biosynthesis of the 3-amino-2,4-dihydroxybenzoic thioacid moiety in thioPTM, thioPTN, and thioPTL biosynthesis. (D) thioPTM, thioPTN, and thioPTL biosynthesis features novel diterpene synthases (PtmT3 and PtmT1) and late stage ADHBSH coupling machinery (PtmC).