Chemical Stability of Petrichorins

The structure of petrichorin C1 (4) converted from petrichorin C (3) was determined using NMR spectroscopy and X-ray crystallography. The chemical stability of petrichorins A and C (1 and 3) was investigated by NMR spectroscopy, X-ray crystallography, and calculations.

Multifunctional P450 Monooxygenase CftA Diversifies the Clifednamide Pool through Tandem C-H Bond Activations

Polycyclic tetramate macrolactams (PTMs) are a class of structurally complex hybrid polyketide-nonribosomal peptide (PK-NRP) natural products produced by diverse bacteria. Several PTMs display pharmaceutically interesting bioactivities, and the early stages of PTM biosynthesis involving polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) enzymology are well studied. However, the timing and mechanisms of post PKS-NRPS oxidations by P450 monooxygenases encoded in PTM biosynthetic gene clusters (BGCs) remain poorly characterized. Here we demonstrate that CftA, encoded in clifednamide-type PTM BGCs, is a multifunctional P450 monooxygenase capable of converting the C29-C30 ethyl side chain of ikarugamycin to either a C29-C30 methyl ketone or a C29-C30 hydroxymethyl ketone through C-H bond activation, resulting in the formation of clifednamide A or clifednamide C, respectively. We also report the complete structure of clifednamide C solved via multidimensional NMR (COSY, HSQC, HMBC, NOESY, and TOCSY) using material purified from an engineered Streptomyces strain optimized for production. Finally, the in vitro reconstitution of recombinant CftA catalytic activity revealed the oxidation cascade for sequential conversion of ikarugamycin to clifednamide A and clifednamide C. Our findings confirm prior genetics-based predictions on the origins of clifednamide complexity via P450s encoded in PTM BGCs and place CftA into a growing group of multifunctional P450s that tailor PTM natural products through late-stage regioselective C-H bond activation.

Genomic discovery of an evolutionarily programmed modality for small-molecule targeting of an intractable protein surface

The vast majority of intracellular protein targets are refractory toward small-molecule therapeutic engagement, and additional therapeutic modalities are needed to overcome this deficiency. Here, the identification and characterization of a natural product, WDB002, reveals a therapeutic modality that dramatically expands the currently accepted limits of druggability. WDB002, in complex with the FK506-binding protein (FKBP12), potently and selectively binds the human centrosomal protein 250 (CEP250), resulting in disruption of CEP250 function in cells. The recognition mode is unprecedented in that the targeted domain of CEP250 is a coiled coil and is topologically featureless, embodying both a structural motif and surface topology previously considered on the extreme limits of "undruggability" for an intracellular target. Structural studies reveal extensive protein-WDB002 and protein-protein contacts, with the latter being distinct from those seen in FKBP12 ternary complexes formed by FK506 and rapamycin. Outward-facing structural changes in a bound small molecule can thus reprogram FKBP12 to engage diverse, otherwise "undruggable" targets. The flat-targeting modality demonstrated here has the potential to expand the druggable target range of small-molecule therapeutics. As CEP250 was recently found to be an interaction partner with the Nsp13 protein of the SARS-CoV-2 virus that causes COVID-19 disease, it is possible that WDB002 or an analog may exert useful antiviral activity through its ability to form high-affinity ternary complexes containing CEP250 and FKBP12.

Bioinformatic and Functional Evaluation of Actinobacterial Piperazate Metabolism

Piperazate (Piz) is a nonproteinogenic amino acid noted for its unusual N-N bond motif. Piz is a proline mimic that imparts conformational rigidity to peptides. Consequently, piperazyl molecules are often bioactive and desirable for therapeutic exploration. The in vitro characterization of Kutzneria enzymes KtzI and KtzT recently led to a biosynthetic pathway for Piz. However, Piz anabolism in vivo has remained completely uncharacterized. Herein, we describe the systematic interrogation of actinobacterial Piz metabolism using a combination of bioinformatics, genetics, and select biochemistry. Following studies in Streptomyces flaveolus, Streptomyces lividans, and several environmental Streptomyces isolates, our data suggest that KtzI-type enzymes are conditionally dispensable for Piz production. We also demonstrate the feasibility of Piz monomer production using engineered actinobacteria for the first time. Finally, we show that some actinobacteria employ fused KtzI-KtzT chimeric enzymes to produce Piz. Our findings have implications for future piperazyl drug discovery, pathway engineering, and fine chemical bioproduction.

Native and Engineered Clifednamide Biosynthesis in Multiple Streptomyces spp

Polycyclic tetramate macrolactam (PTM) natural products are produced by actinomycetes and other bacteria. PTMs are often bioactive, and the simplicity of their biosynthetic clusters make them attractive for bioengineering. Clifednamide-type PTMs from Streptomyces sp. strain JV178 contain a distinctive ketone group, suggesting the existence of a novel PTM oxidizing enzyme. Here, we report the new cytochrome P450 enzyme (CftA) is required for clifednamide production. Genome mining was used to identify several new clifednamide producers, some having improved clifednamide yields. Using a parallel synthetic biology approach, CftA isozymes were used to engineer the ikarugamycin pathway of Streptomyces sp. strain NRRL F-2890 to yield clifednamides. Further, we observed that strong CftA expression leads to the production of a new PTM, clifednamide C. We demonstrate the utility of both genome mining and synthetic biology to rapidly increase clifednamide production.

Chemical and Biological Aspects of Nutritional Immunity-Perspectives for New Anti-Infectives that Target Iron Uptake Systems

Upon bacterial infection, one of the defense mechanisms of the host is the withdrawal of essential metal ions, in particular iron, which leads to "nutritional immunity". However, bacteria have evolved strategies to overcome iron starvation, for example, by stealing iron from the host or other bacteria through specific iron chelators with high binding affinity. Fortunately, these complex interactions between the host and pathogen that lead to metal homeostasis provide several opportunities for interception and, thus, allow the development of novel antibacterial compounds. This Review focuses on iron, discusses recent highlights, and gives some future perspectives which are relevant in the fight against antibiotic resistance.

Targeted discovery of polycyclic tetramate macrolactams from an environmental Streptomyces strain

A targeted polymerase chain reaction (PCR)-based screening approach was used to identify candidate polycyclic tetramate macrolactam (PTM) biosynthetic gene clusters in environmental Streptomyces isolates. Isolation and characterization of the small molecules produced by one of the strains confirmed the production of two new PTMs (clifednamides A, 4, and B, 5) and, more generally, the utility of using a targeted approach for the discovery of new members of this interesting class.

Quorum-sensing-regulated bactobolin production by Burkholderia thailandensis E264

Bacterial acyl-homoserine lactones upregulated an uncharacterized gene cluster (bta) in Burkholderia thailandensis E264 to produce an uncharacterized polar antibiotic. The antibiotic is identified as a mixture of four bactobolins. Annotation of the bta cluster allows us to propose a biosynthetic scheme for bactobolin and reveals unusual enzymatic reactions for further study.

Biosynthesis of 2-hydroxyethylphosphonate, an unexpected intermediate common to multiple phosphonate biosynthetic pathways

Phosphonic acids encompass a common yet chemically diverse class of natural products that often possess potent biological activities. Here we report that, despite the significant structural differences among many of these compounds, their biosynthetic routes contain an unexpected common intermediate, 2-hydroxyethyl-phosphonate, which is synthesized from phosphonoacetaldehyde by a distinct family of metal-dependent alcohol dehydrogenases (ADHs). Although the sequence identity of the ADH family members is relatively low (34-37%), in vitro biochemical characterization of the homologs involved in biosynthesis of the antibiotics fosfomycin, phosphinothricin tripeptide, and dehydrophos (formerly A53868) unequivocally confirms their enzymatic activities. These unique ADHs have exquisite substrate specificity, unusual metal requirements, and an unprecedented monomeric quaternary structure. Further, sequence analysis shows that these ADHs form a monophyletic group along with additional family members encoded by putative phosphonate biosynthetic gene clusters. Thus, the reduction of phosphonoacetaldehyde to hydroxyethyl-phosphonate may represent a common step in the biosynthesis of many phosphonate natural products, a finding that lends insight into the evolution of phosphonate biosynthetic pathways and the chemical structures of new C-P containing secondary metabolites.

Unusual transformations in the biosynthesis of the antibiotic phosphinothricin tripeptide

Phosphinothricin tripeptide (PTT, phosphinothricylalanylalanine) is a natural-product antibiotic and potent herbicide that is produced by Streptomyces hygroscopicus ATCC 21705 (ref. 1) and Streptomyces viridochromogenes DSM 40736 (ref. 2). PTT has attracted widespread interest because of its commercial applications and unique phosphinic acid functional group. Despite intensive study since its discovery in 1972 (see ref. 3 for a comprehensive review), a number of steps early in the PTT biosynthetic pathway remain uncharacterized. Here we report a series of interdisciplinary experiments involving the construction of defined S. viridochromogenes mutants, chemical characterization of accumulated intermediates, and in vitro assay of selected enzymes to examine these critical steps in PTT biosynthesis. Our results indicate that early PTT biosynthesis involves a series of catalytic steps that to our knowledge has not been described so far, including a highly unusual reaction for carbon bond cleavage. In sum, we define a pathway for early PTT biosynthesis that is more complex than previously appreciated.

Heterologous production of fosfomycin and identification of the minimal biosynthetic gene cluster

Fosfomycin is a clinically utilized, highly effective antibiotic, which is active against methicillin- and vancomycin-resistant pathogens. Here we report the cloning and characterization of a complete fosfomycin biosynthetic cluster from Streptomyces fradiae and heterologous production of fosfomycin in S. lividans. Sequence analysis coupled with gene deletion and disruption revealed that the minimal cluster consists of fom1-4, fomA-D. A LuxR-type activator that was apparently required for heterologous fosfomycin production was also discovered approximately 13 kb away from the cluster and was named fomR. The genes fomE and fomF, previously thought to be involved in fosfomycin biosynthesis, were shown not to be essential by gene disruption. This work provides new insights into fosfomycin biosynthesis and opens the door for fosfomycin overproduction and creation of new analogs via biomolecular pathway engineering.

Molecular cloning, sequence analysis, and heterologous expression of the phosphinothricin tripeptide biosynthetic gene cluster from Streptomyces viridochromogenes DSM 40736

A fosmid library from genomic DNA of Streptomyces viridochromogenes DSM 40736 was constructed and screened for the presence of genes known to be involved in the biosynthesis of phosphinothricin tripeptide (PTT). Eight positives were identified, one of which was able to confer PTT biosynthetic capability upon Streptomyces lividans after integration of the fosmid into the chromosome of this heterologous host. Sequence analysis of the 40,241-bp fosmid insert revealed 29 complete open reading frames (ORFs). Deletion analysis demonstrated that a minimum set of 24 ORFs were required for PTT production in the heterologous host. Sequence analysis revealed that most of these PTT genes have been previously identified in either S. viridochromogenes or S. hygroscopicus (or both), although only 11 out of 24 of these ORFs have experimentally defined functions. Three previously unknown genes within the cluster were identified and are likely to have roles in the stepwise production of phosphonoformate from phosphonoacetaldehyde. This is the first report detailing the entire PTT gene cluster from any producing streptomycete.