Shedding light on sunscreen biosynthesis in zebrafish

Zebrafish can synthesize a sunscreen compound called gadusol, which was previously thought to be acquired only through the diet

LuxR homolog-linked biosynthetic gene clusters in Proteobacteria

Microbes are a major source of antibiotics, pharmaceuticals, and other bioactive compounds. The production of many specialized microbial metabolites is encoded in biosynthetic gene clusters (BGCs). A challenge associated with natural product discovery is that many BGCs are not expressed under laboratory growth conditions. Here we report a genome-mining approach to discover BGCs with luxRtype quorum sensing (QS) genes, which code for regulatory proteins that control gene expression. Our results show that BGCs linked to genes coding for LuxR-like proteins are widespread in Proteobacteria. In addition, we show that associations between luxR homolog genes and BGCs have evolved independently many times, with functionally diverse gene clusters. Overall, these clusters may provide a source of new natural products for which there is some understanding about how to elicit production. IMPORTANCE Bacteria biosynthesize specialized metabolites with a variety of ecological functions, including defense against other microbes. Genes that code for specialized metabolite biosynthetic enzymes are frequently clustered together. These BGCs are often regulated by a transcription factor encoded within the cluster itself. These pathway-specific regulators respond to a signal or indirectly through other means of environmental sensing. Many specialized metabolites are not produced under laboratory growth conditions, and one reason for this issue is that laboratory growth media lack environmental cues necessary for BGC expression. Here, we report a bioinformatics study that reveals that BGCs are frequently linked to genes coding for LuxR family QS-responsive transcription factors in the phylum Proteobacteria. The products of these luxR homolog-associated gene clusters may serve as a practical source of bioactive metabolites

A Prodrug Resistance Mechanism Is Involved in Colibactin Biosynthesis and Cytotoxicity

Commensal Escherichia coli residing in the human gut produce colibactin, a small-molecule genotoxin of unknown structure that has been implicated in the development of colon cancer. Colibactin biosynthesis is hypothesized to involve a prodrug resistance strategy that entails initiation of biosynthesis via construction of an N-terminal prodrug scaffold and late-stage cleavage of this structural motif during product export. Here we describe the biochemical characterization of the prodrug synthesis, elongation, and cleavage enzymes from the colibactin biosynthetic pathway. We show that nonribosomal peptide synthetases ClbN and ClbB assemble and process an <i>N</i>-acyl-d-asparagine prodrug scaffold that serves as a substrate for the periplasmic d-amino peptidase ClbP. In addition to affording information about structural features of colibactin, this work reveals the biosynthetic logic underlying the prodrug resistance strategy and suggests that cytotoxicity requires amide bond cleavage

Correction to Isolation of a Metabolite from the <i>pks</i> Island Provides Insights into Colibactin Biosynthesis and Activity

Correction to Isolation of a Metabolite from the <i>pks</i> Island Provides Insights into Colibactin Biosynthesis and Activit

Isolation of a Metabolite from the <i>pks</i> Island Provides Insights into Colibactin Biosynthesis and Activity

Colibactin is a structurally uncharacterized, genotoxic natural product produced by commensal and pathogenic strains of <i>E. coli</i> that harbor the <i>pks</i> island. A new metabolite has been isolated from a <i>pks</i>⁺ <i>E. coli</i> mutant missing an essential biosynthetic enzyme. The unusual azaspiro[2.4] bicyclic ring system of this molecule provides new insights into colibactin biosynthesis and suggests a mechanism through which colibactin and other <i>pks</i>-derived metabolites may exert genotoxicity

Characterization of Polyketide Synthase Machinery from the <i>pks</i> Island Facilitates Isolation of a Candidate Precolibactin

Colibactin is a human gut bacterial genotoxin of unknown structure that has been linked to colon cancer. The biosynthesis of this elusive metabolite is directed by the <i>pks</i> gene cluster, which encodes a hybrid nonribosomal peptide synthetase-polyketide synthase (NRPS-PKS) assembly line that is hypothesized to use the unusual polyketide building block aminomalonate. This biosynthetic pathway is thought to initially produce an inactive intermediate (precolibactin) that is processed to the active toxin. Here, we report the first <i>in vitro</i> biochemical characterization of the PKS components of the <i>pks</i> enzymatic assembly line. We evaluate PKS extender unit utilization and show that ClbG, a freestanding acyltransferase (AT) from the <i>pks</i> gene cluster, recognizes aminomalonyl-acyl carrier protein (AM-ACP) and transfers this building block to multiple PKS modules, including a <i>cis</i>-AT PKS ClbI. We also use genetics to explore the <i>in vivo</i> role of ClbG in colibactin and precolibactin biosynthesis. Unexpectedly, production of previously identified <i>pks</i>-associated metabolites is dramatically increased in a Δ<i>clbP</i>/Δ<i>clbG</i> mutant strain, enabling the first structure elucidation of a bithiazole-containing candidate precolibactin. This work provides new insights into the unusual biosynthetic capabilities of the <i>pks</i> gene cluster, offers further support for the hypothesis that colibactin directly damages DNA, and suggests that additional, uncharacterized <i>pks</i>-derived metabolites containing aminomalonate play critical roles in genotoxicity

Enantioselective Thiourea-Catalyzed Intramolecular Cope-Type Hydroamination

Catalysis of Cope-type rearrangements of bis-homoallylic hydroxylamines is demonstrated using chiral thiourea derivatives. This formal intramolecular hydroamination reaction provides access to highly enantioenriched α-substituted pyrrolidine products and represents a complementary approach to metal-catalyzed methods