A Nonfunctional Halogenase Masquerades as an Aromatizing Dehydratase in Biosynthesis of Pyrrolic Polyketides by Type I Polyketide Synthases

Gatekeeping Activity of Collinear Ketosynthase Domains Limits Product Diversity for Engineered Type I Polyketide Synthases

Engineered type I polyketide synthases (type I PKSs) can enable access to diverse polyketide pharmacophores and generate non-natural natural products. However, the promise of type I PKS engineering remains modestly realized at best. Here, we report that ketosynthase (KS) domains, the key carbon-carbon bond-forming catalysts, control which intermediates are allowed to progress along the PKS assembly lines and which intermediates are excluded. Using bimodular PKSs, we demonstrate that KSs can be exquisitely selective for the upstream polyketide substrate while retaining promiscuity for the extender unit that they incorporate. It is then the downstream KS that acts as a gatekeeper to ensure the fidelity of the extender unit incorporation by the upstream KS. We also demonstrate that these findings are not universally applicable; substrate-tolerant KSs do allow engineered polyketide intermediates to be extended. Our results demonstrate the utility for evaluating the KS-induced bottlenecks to gauge the feasibility of engineering PKS assembly lines.

Pyoluteorin regulates the biosynthesis of 2,4-DAPG through the TetR family transcription factor PhlH in Pseudomonas protegens Pf-5

Soil and rhizosphere bacteria act as a rich source of secondary metabolites, effectively fighting against a diverse array of pathogens. Certain Pseudomonas species harbor biosynthetic gene clusters for producing both pyoluteorin and 2,4-diacetylphloroglucinol (2,4-DAPG), which are polyketides that exhibit highly similar antimicrobial spectrum against bacteria and fungi or oomycete. A complex cross talk exists between pyoluteorin and 2,4-DAPG biosynthesis, and production of 2,4-DAPG was strongly repressed by pyoluteorin, yet the underlying mechanism is still elusive. In this study, we find that the TetR family transcription factor PhlH is involved in the cross talk between pyoluteorin and 2,4-DAPG biosynthesis. PhlH binds to a palindromic sequence within the promoter of phlG (PphlG), which encodes a C-C bond hydrolase responsible for degrading 2,4-DAPG. As a signaling molecule, pyoluteorin disrupts the PhlH-PphlG complex by binding to PhlH, leading to decreased levels of 2,4-DAPG. Proteomics data suggest that pyoluteorin regulates multiple physiological processes including fatty acid biosynthesis and transportation of taurine, siderophore, and amino acids. Our work not only reveals a novel mechanism of cross talk between pyoluteorin and 2,4-DAPG biosynthesis, but also highlights pyoluteorin's role as a messenger in the complex communication network of Pseudomonas.IMPORTANCEAntibiosis serves as a crucial defense mechanism for microbes against invasive bacteria and resource competition. These bacteria typically orchestrate the production of multiple antibiotics in a coordinated fashion, wherein the synthesis of one antibiotic inhibits the generation of another. This strategic coordination allows the bacterium to focus its resources on producing the most advantageous antibiotic under specific circumstances. However, the underlying mechanisms of distinct antibiotic production in bacterial cells remain largely elusive. In this study, we reveal that the TetR family transcription factor PhlH detects the secondary metabolite pyoluteorin and mediates the cross talk between pyoluteorin and 2,4-DAPG biosynthesis in the biocontrol strain Pseudomonas protegens Pf-5. These findings hold promise for future research, potentially informing the manipulation of these systems to enhance the effectiveness of biocontrol agents.

One-Pot Synthesis of Unprotected 2-Acylpyrroles from 1,2,3-Triazoles and 2-Hydroxymethylallyl Carbonates

An efficient, tandem one-pot approach to synthesize multisubstituted 2-acylpyrroles from readily prepared N-tosyl triazoles and 2-hydroxymethylallyl carbonates is reported. The reaction proceeds via Rh(II)-catalyzed O-H insertion, [3,3]-sigmatropic rearrangement, Pd(0)-catalyzed oxidative addition, intramolecular cyclization, DBU-promoted E1cB elimination, double bond isomerization, and aromatization, enabling the disconnection and formation of multiple bonds in one reactor. The approach represents a highly regioselective way to access di-, tri-, and tetra-substituted NH pyrroles with high efficiency.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as hyperispirone A from Hypericum beanii.