Understanding the biosynthesis, metabolic regulation, and anti-phytopathogen activity of 3,7-dihydroxytropolone in
Pseudomonas
spp.
Authors: Moffat Alaster D. , Höing Lars , Santos-Aberturas Javier , Markwalder Tim , Malone Jacob G. , Teufel Robin , Truman Andrew W. 
Published: August 2024

The genus
Pseudomonas
is a prolific source of specialized metabolites with significant biological activities, including siderophores, antibiotics, and plant hormones. These molecules play pivotal roles in environmental interactions, influencing pathogenicity, inhibiting microorganisms, responding to nutrient limitation and abiotic challenges, and regulating plant growth. These properties mean that pseudomonads are suitable candidates as biological control agents against plant pathogens. Multiple transposon-based screens have identified a
Pseudomonas
biosynthetic gene cluster (BGC) associated with potent antibacterial and antifungal activities, which produces 7-hydroxytropolone (7-HT). In this study, we show that this BGC also makes 3,7-dihydroxytropolone (3,7-dHT), which has strong antimicrobial activity toward
Streptomyces scabies
, a potato pathogen. Through metabolomics and reporter assays, we unveil the involvement of cluster-situated genes in generating phenylacetyl-coenzyme A, a key precursor for tropolone biosynthesis via the phenylacetic acid catabolon. The clustering of these phenylacetic acid genes within tropolone BGCs is unusual in other Gram-negative bacteria. Our findings support the interception of phenylacetic acid catabolism via an enoyl-CoA dehydratase encoded in the BGC, as well as highlighting an essential role for a conserved thioesterase in biosynthesis. Biochemical assays were used to show that this thioesterase functions after a dehydrogenation–epoxidation step catalyzed by a flavoprotein. We use this information to identify diverse uncharacterized BGCs that encode proteins with homology to flavoproteins and thioesterases involved in tropolone biosynthesis. This study provides insights into tropolone biosynthesis in
Pseudomonas
, laying the foundation for further investigations into the ecological role of tropolone production.

IMPORTANCE

Pseudomonas
bacteria produce various potent chemicals that influence interactions in nature, such as metal-binding molecules, antibiotics, or plant hormones. This ability to synthesize bioactive molecules means that
Pseudomonas
bacteria may be useful as biological control agents to protect plants from agricultural pathogens, as well as a source of antibiotic candidates. We have identified a plant-associated
Pseudomonas
strain that can produce 3,7-dihydroxytropolone, which has broad biological activity and can inhibit the growth of
Streptomyces scabies
, a bacterium that causes potato scab. Following the identification of this molecule, we used a combination of genetic, chemical, and biochemical experiments to identify key steps in the production of tropolones in
Pseudomonas
species. Understanding this biosynthetic process led to the discovery of an array of diverse pathways that we predict will produce new tropolone-like molecules. This work should also help us shed light on the natural function of antibiotics in nature.