One sentence summary: Microbial secondary metabolites represent a significant source of potential drug leads; however, the majority of the corresponding biosynthetic genes are not expressed under normal laboratory conditions. In this review, we assess the capacity of exogenous small molecules, especially antibiotics, to activate these silent gene clusters. Editor: Aimee Shen ABSTRACTNatural products have traditionally served as a dominant source of therapeutic agents. They are produced by dedicated biosynthetic gene clusters that assemble complex, bioactive molecules from simple precursors. Recent genome sequencing efforts coupled with advances in bioinformatics indicate that the majority of biosynthetic gene clusters are not expressed under normal laboratory conditions. Termed 'silent' or 'cryptic', these gene clusters represent a treasure trove for discovery of novel small molecules, their regulatory circuits and their biosynthetic pathways. In this review, we assess the capacity of exogenous small molecules in activating silent secondary metabolite gene clusters. Several approaches that have been developed are presented, including coculture techniques, ribosome engineering, chromatin remodeling and high-throughput elicitor screens. The rationale, applications and mechanisms attendant to each are discussed. Some general conclusions can be drawn from our analysis: exogenous small molecules comprise a productive avenue for the discovery of cryptic metabolites. Specifically, growth-inhibitory molecules, in some cases clinically used antibiotics, serve as effective inducers of silent biosynthetic gene clusters, suggesting that old antibiotics may be used to find new ones. The involvement of natural antibiotics in modulating secondary metabolism at subinhibitory concentrations suggests that they represent part of the microbial vocabulary through which inter-and intraspecies interactions are mediated.

While bacterial genomes typically contain numerous secondary metabolite biosynthetic gene clusters, only a small fraction of these are expressed at any given time. The remaining majority is inactive or silent, and methods that awaken them would greatly expand our repertoire of bioactive molecules. We recently devised a new approach for identifying inducers of silent gene clusters and proposed that the clinical antibiotic trimethoprim acted as a global activator of secondary metabolism in Burkholderia thailandensis. Herein, we report that trimethoprim triggers the production of over 100 compounds that are not observed under standard growth conditions, thus drastically modulating the secondary metabolic output of B. thailandensis. Using MS/MS networking and NMR, we assign structures to ∼40 compounds, including a group of new molecules, which we call acybolins. With methods at hand for activation of silent gene clusters and rapid identification of small molecules, the hidden secondary metabolomes of bacteria can be interrogated.

The explosion of microbial genome sequences has shown that bacteria harbor an immense, largely untapped potential for the biosynthesis of diverse natural products, which have traditionally served as an important source of pharmaceutical compounds. Most of the biosynthetic genes that can be detected bioinformatically are only weakly expressed, or not at all, under standard laboratory growth conditions. Herein we review three recent approaches that have been developed for inducing these so-called silent biosynthetic gene clusters: insertion of constitutively active promoters using CRISPR-Cas9, high-throughput elicitor screening for identification of small molecule inducers, and reporter-guided mutant selection for creation of overproducing strains. Together with strategies implemented previously, these approaches promise to unleash the products of silent gene clusters for years to come.

Sarcophyton glaucum is one of the most abundant and chemically studied soft corals with over 100 natural products reported in the literature, primarily cembrane diterpenoids. Yet, wide variation in the chemistry observed from S. glaucum over the past 50 years has led to its reputation as a capricious producer of bioactive metabolites. Recent molecular phylogenetic analysis revealed that S. glaucum is not a single species but a complex of at least seven genetically distinct species not distinguishable using traditional taxonomic criteria. We hypothesized that perceived intraspecific chemical variation observed in S. glaucum was actually due to differences between cryptic species (interspecific variation). To test this hypothesis, we collected Sarcophyton samples in Palau, performed molecular phylogenetic analysis, and prepared chemical profiles of sample extracts using gas chromatography-flame ionization detection. Both unsupervised (principal component analysis) and supervised (linear discriminant analysis) statistical analyses of these profiles revealed a strong relationship between cryptic species membership and chemical profiles. Liquid chromatography with tandem mass spectrometry-based analysis using feature-based molecular networking permitted identification of the chemical drivers of this difference between clades, including cembranoid diterpenes (2R,11R,12R)-isosarcophytoxide (5), (2S,11R,12R)-isosarcophytoxide (6), and isosarcophine (7). Our results suggest that early chemical studies of Sarcophyton may have unknowingly conflated different cryptic species of S. glaucum, leading to apparently idiosyncratic chemical variation.

Natural products have been an important source of therapeutic agents and chemical tools. The recent realization that many natural product biosynthetic genes are silent or sparingly expressed during standard laboratory growth has prompted efforts to investigate their regulation and develop methods to induce their expression. Because it is difficult to intuit signals that induce a given biosynthetic locus, we recently implemented a forward chemical-genetic approach to identify such inducers. In the current work, we applied this approach to nine silent biosynthetic loci in the model bacterium Burkholderia thailandensis to systematically screen for elicitors from a library of Food and Drug Administration–approved drugs. We find that β-lactams, fluoroquinolones, antifungals, and, surprisingly, calcimimetics, phenothiazine antipsychotics, and polyaromatic antidepressants are the most effective global inducers of biosynthetic genes. Investigations into the mechanism of stimulation of the silent virulence factor malleicyprol by the β-lactam piperacillin allowed us to elucidate the underlying regulatory circuits. Low-dose piperacillin causes oxidative stress, thereby inducing redox-sensing transcriptional regulators, which activate malR, a pathway-specific positive regulator of the malleicyprol gene cluster. Malleicyprol is thus part of the OxyR and SoxR regulons in B. thailandensis, allowing the bacterium to initiate virulence in response to oxidative stress. Our work catalogs a diverse array of elicitors and a previously unknown regulatory input for secondary metabolism in B. thailandensis.