Two Pathway-Specific Transcriptional Regulators, PltR and PltZ, Coordinate Autoinduction of Pyoluteorin in Pseudomonas protegens Pf-5

Fructose promotes pyoluteorin biosynthesis via the CbrAB-CrcZ-Hfq/Crc pathway in the biocontrol strain Pseudomonas PA1201

Biocontrol strain Pseudomonas PA1201 produces pyoluteorin (Plt), which is an antimicrobial secondary metabolite. Plt represents a promising candidate pesticide due to its broad-spectrum antifungal and antibacterial activity. Although PA1201 contains a complete genetic cluster for Plt biosynthesis, it fails to produce detectable level of Plt when grown in media typically used for Pseudomonas strains. In this study, minimum medium (MM) was found to favor Plt biosynthesis. Using the medium M, which contains all the salts of MM medium except for mannitol, as a basal medium, we compared 10 carbon sources for their ability to promote Plt biosynthesis. Fructose, mannitol, and glycerol promoted Plt biosynthesis, with fructose being the most effective carbon source. Glucose or succinic acid had no significant effect on Plt biosynthesis, but effectively antagonized fructose-dependent synthesis of Plt. Promoter-lacZ fusion reporter strains demonstrated that fructose acted through activation of the pltLABCDEFG (pltL) operon but had no effect on other genes of plt gene cluster; glucose or succinic acid antagonized fructose-dependent pltL induction. Mechanistically, fructose-mediated Plt synthesis involved carbon catabolism repression. The two-component system CbrA/CbrB and small RNA catabolite repression control Z (crcZ) were essential for fructose-induced Plt synthesis. The small RNA binding protein Hfq and Crc negatively regulated fructose-induced Plt. Taken together, this study provides a new model of fructose-dependent Plt production in PA1201 that can help improve Plt yield by biosynthetic approaches.

Metal-Free Catalyzed Oxidation/Decarboxylative [3+2] Cycloaddition Sequences of 3-Formylchromones to Access Pyrroles with Anti-Cancer Activity

An efficient and direct approach to pyrroles was successfully developed by employing 3-formylchromones as decarboxylative coupling partners, and facilitated by microwave irradiation. The protocol utilizes easily accessible feedstocks, a catalytic amount of DBU without any metals, resulting in high efficiency and regioselectivity. Notably, all synthesized products were evaluated against five different cancer cell lines and compound 3l selectively inhibited the proliferation of HCT116 cells with an IC50 value of 10.65 μM.

TetR and OmpR family regulators in natural product biosynthesis and resistance

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

Plant-Associated Pseudomonads

Bacteria belonging to the genus Pseudomonas (the pseudomonads) are a group of Gammaproteobacteria that are characterized by a high metabolic versatility and adaption to different ecological niches [...].

Identification and Characterization of Bacteria-Derived Antibiotics for the Biological Control of Pea Aphanomyces Root Rot

Antibiosis has been proposed to contribute to the beneficial bacteria-mediated biocontrol against pea Aphanomyces root rot caused by the oomycete pathogen Aphanomyces euteiches. However, the antibiotics required for disease suppression remain unknown. In this study, we found that the wild type strains of Pseudomonas protegens Pf-5 and Pseudomonas fluorescens 2P24, but not their mutants that lack 2,4-diacetylphloroglucinol, strongly inhibited A. euteiches on culture plates. Purified 2,4-diacetylphloroglucinol compound caused extensive hyphal branching and stunted hyphal growth of A. euteiches. Using a GFP-based transcriptional reporter assay, we found that expression of the 2,4-diacetylphloroglucinol biosynthesis gene phlA_Pf-5 is activated by germinating pea seeds. The 2,4-diacetylphloroglucinol producing Pf-5 derivative, but not its 2,4-diacetylphloroglucinol non-producing mutant, reduced disease severity caused by A. euteiches on pea plants in greenhouse conditions. This is the first report that 2,4-diacetylphloroglucinol produced by strains of Pseudomonas species plays an important role in the biocontrol of pea Aphanomyces root rot.

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

The bacterial modular type I polyketide synthases (PKSs) typically furnish nonaromatic lactone and lactam natural products. Here, by the complete in vitro enzymatic production of the polyketide antibiotic pyoluteorin, we describe the biosynthetic mechanism for the construction of an aromatic resorcylic ring by a type I PKS. We find that the pyoluteorin type I PKS does not produce an aromatic product, rather furnishing an alicyclic dihydrophloroglucinol that is later enzymatically dehydrated and aromatized. The aromatizing dehydratase is encoded in the pyoluteorin biosynthetic gene cluster (BGC), and its presence is conserved in other BGCs encoding production of pyrrolic polyketides. Sequence similarity and mutational analysis demonstrates that the overall structure and position of the active site for the aromatizing dehydratase is shared with flavin-dependent halogenases albeit with a loss in ability to perform redox catalysis. We demonstrate that the post-PKS dehydrative aromatization is critical for the antibiotic activity of pyoluteorin.