Jeffrey E. Janso

Biosynthetic Potential of Phylogenetically Unique Endophytic Actinomycetes from Tropical Plants

ABSTRACT The culturable diversity of endophytic actinomycetes associated with tropical, native plants is essentially unexplored. In this study, 123 endophytic actinomycetes were isolated from tropical plants collected from several locations in Papua New Guinea and Mborokua Island, Solomon Islands. Isolates were found to be prevalent in roots but uncommon in leaves. Initially, isolates were dereplicated to the strain level by ribotyping. Subsequent characterization of 105 unique strains by 16S rRNA gene sequence analysis revealed that 17 different genera were represented, and rare genera, such as Sphaerisporangium and Planotetraspora, which have never been previously reported to be endophytic, were quite prevalent. Phylogenetic analyses grouped many of the strains into clades distinct from known genera within Thermomonosporaceae and Micromonosporaceae, indicating that they may be unique genera. Bioactivity testing and liquid chromatography-mass spectrometry (LC-MS) profiling of crude fermentation extracts were performed on 91 strains. About 60% of the extracts exhibited bioactivity or displayed LC-MS profiles with spectra indicative of secondary metabolites. The biosynthetic potential of 29 nonproductive strains was further investigated by the detection of putative polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) genes. Despite their lack of detectable secondary metabolite production in fermentation, most were positive for type I (66%) and type II (79%) PKS genes, and all were positive for NRPS genes. These results suggest that tropical plants from New Guinea and the adjacent archipelago are hosts to unique endophytic actinomycetes that possess significant biosynthetic potential.

Biosynthesis of Diazepinomicin/ECO-4601, a Micromonospora Secondary Metabolite with a Novel Ring System

The novel microbial metabolite diazepinomicin/ECO-4601 (1) has a unique tricyclic dibenzodiazepinone core, which was unprecedented among microbial metabolites. Labeled feeding experiments indicated that the carbocyclic ring and the ring nitrogen of tryptophan could be incorporated via degradation to the 3-hydroxyanthranilic acid, forming ring A and the nonamide nitrogen of 1. Genomic analysis of the biosynthetic locus indicated that the farnesyl side chain was mevalonate derived, the 3-hydroxyanthranilic acid moiety could be formed directly from chorismate, and the third ring was constructed via 3-amino-5-hydroxybenzoic acid. Successful incorporation of 4,6-D2-3-hydroxyanthranilic acid into ring A of 1 via feeding experiments supports the genetic analysis and the allocation of the locus to this biosynthesis. These studies highlight the enzymatic complexity needed to produce this structural type, which is rare in nature.

Discovery and Assembly Line Biosynthesis of the Lymphostin Pyrroloquinoline Alkaloid Family of mTOR Inhibitors in Salinispora Bacteria

The pyrroloquinoline alkaloid family of natural products, which includes the immunosuppressant lymphostin, has long been postulated to arise from tryptophan. We now report the molecular basis of lymphostin biosynthesis in three marine Salinispora species that maintain conserved biosynthetic gene clusters harboring a hybrid nonribosomal peptide synthetase-polyketide synthase that is central to lymphostin assembly. Through a series of experiments involving gene mutations, stable isotope profiling, and natural product discovery, we report the assembly-line biosynthesis of lymphostin and nine new analogues that exhibit potent mTOR inhibitory activity.

Cytotoxic Spliceostatins from Burkholderia sp. and Their Semisynthetic Analogues

The spliceostatin class of natural products was reported to be potent cytotoxic agents via inhibition of the spliceosome, a key protein complex in the biosynthesis of mature mRNA. As part of an effort to discover novel leads for cancer chemotherapy, we re-examined this class of compounds from several angles, including fermentation of the producing strains, isolation and structure determination of new analogues, and semisynthetic modification. Accordingly, a group of spliceostatins were isolated from a culture broth of Burkholderia sp. FERM BP-3421, and their structures identified by analysis of spectroscopic data. Semisynthesis was performed on the major components 4 and 5 to generate ester and amide derivatives with improved in vitro potency. With their potent activity against tumor cells and unique mode of action, spliceostatins can be considered potential leads for development of cancer drugs.

Evaluating Indole-Related Derivatives as Precursors in the Directed Biosynthesis of Diazepinomicin Analogues

The effectiveness of precursor-directed biosynthesis to generate diazepinomicin (1) analogues with varied ring-A substitutents was investigated by feeding commercially available, potential ring-A precursors such as fluorinated tryptophans, halogenated anthranilates, and various substituted indoles into growing actinomycete culture DPJ15 (genus Micromonospora). Two new monofluorinated diazepinomicin analogues (2 and 3) were identified and characterized by spectroscopic methods. Both derivatives showed modest antibacterial activity against the Gram-positive coccus Staphylococcus aureus with MIC values in the range 8-32 microg/mL.

A Cell Wall-Active Lipopeptide from the Fungus Pochonia bulbillosa

Bioassay-directed fractionation of a fermentation of Pochonia bulbinosa, culture 38G272, led to the isolation of a series of structurally novel, prospective cell wall-active lipopeptides. The main component of this suite is 1, a linear hexapeptide with a delta-hydroxymyristic acid amide substituted N-terminus. The structure was deduced using high-field microsample NMR, Fourier transform mass spectrometry, and microscale chemical degradation. The potent cell wall activity and synthetically accessible structure of 1 make it a potential lead for further investigation.

Probing natural product biosynthetic pathways using Fourier transform ion cyclotron resonance mass spectrometry.

Two natural products, diazepinomicin (1) and dioxapyrrolomycin (2), containing stable isotopic labels of (15)N or deuterium, were used to demonstrate the utility of Fourier transform ion cyclotron resonance mass spectrometry for probing natural product biosynthetic pathways. The isotopic fine structures of significant ions were resolved and subsequently assigned elemental compositions on the basis of highly accurate mass measurements. In most instances the mass measurement accuracy is less than one part per million (ppm), which typically makes the identification of stable-isotope labeling unambiguous. In the case of the mono-(15)N-labeled diazepinomicin (1) derived from labeled tryptophan, tandem mass spectrometry located this (15)N label at the non-amide nitrogen. Through the use of exceptionally high mass resolving power of over 125,000, the isotopic fine structure of the molecular ion cluster of 1 was revealed. Separation of the (15)N(2) peak from the isobaric (13)C(15)N peak, both having similar abundances, demonstrated the presence of a minor amount of doubly (15)N-labeled diazepinomicin (1). Tandem mass spectrometry amplified this isotopic fine structure (Deltam=6.32 mDa) from mDa to 1 Da scale thereby allowing more detailed scrutiny of labeling content and location. Tandem mass spectrometry was also used to assign the location of deuterium labeling in two deuterium-labeled diazepinomicin (1) samples. In one case three deuterium atoms were incorporated into the dibenzodiazepine core; while in the other a mono-D label was mainly incorporated into the farnesyl side chain. The specificity of (15)N-labeling in dioxapyrrolomycin (2) and the proportion of the (15)N-label contained in the nitro group were determined from the measurement of the relative abundance of the (14)NO(2)(1-) and (15)NO(2)(1-) fragment ions.