Steroids from the Meliaceae family and their biological activities

Steroids are farnesyl diphosphate-derived triterpene derivatives widely distributed in Meliaceae plants that can have several health benefits due to their biological activities. This literature survey on chemical and pharmacological studies of steroids from the Meliaceae plants indicates that 157 distinct steroids classified into six subclasses including (in decreasing number): pregnane-, stigmastane-, ergostane-, cholestane-, androstane- and ecdysterone-type steroids have been reported from a total of 49 plant species. This review aims to provide a reference document compiling information about the occurrence, chemistry and biological activities of meliaceous steroids for the period from 1988 to July 2023. In particular, generalities about the chemistry of steroids with unusual skeletons and underlying biosynthetic pathways are highlighted. In addition, some structural relationships between different compound types and their biological activities are presented. The information used during the writing of this paper was collected from the online libraries PubMed, Google Scholar and Scifinder using the keywords steroids and Meliaceae with no language restriction. This review points out new avenues for further investigations of steroids from plants of the Meliaceae family.

Mass Spectrometric Analysis of Cucurbitacins and Dihydrocucurbitacins from the Tuber of Citrullus naudinianus

The vast pool of structurally and functionally distinct secondary metabolites (i.e., natural products (NPs)) is constantly being expanded, a process also driven by the rapid progress in the development of analytical techniques. Such NPs often show potent biological activities and are therefore prime candidates for drug development and medical applications. The ethyl acetate extract of the tuber of Citrullus naudinianus (C. naudinianus), an African melon with edible fruits and seeds, shows in vitro immunomodulatory activity presumably elicited by cucurbitacins that are known major constituents of this plant. Further potentially immunomodulatory cucurbitacins or cucurbitacin derivatives were assumed to be in the tuber. Given the typically high content of cucurbitacins with similar physicochemical features but often distinct bioactivities, an efficient and reliable separation process is a prerequisite for their detailed characterization and assessment in terms of bioactivity. We therefore developed a detection method to screen and differentiate cucurbitacins via high-performance liquid chromatography/quadrupole-time-of-flight tandem mass spectrometry (HPLC-QTOF-MS/MS). In order to confirm the identification, the fragmentation patterns of two cucurbitacins and one 23,24-dihydrocucurbitacin were also investigated. Six characteristic fragments were identified and three of them were employed for the identification of cucurbitacins and 23,24-dihydrocucurbitacins in the extract. As a result, in addition to eight previously reported cucurbitacins from this plant four distinct 23,24-dihydrocucurbitacins (B, D, E, and I) were putatively identified and newly found in the ethyl acetate extract of the tuber of C. naudinianus. The established methodology enables rapid and efficient LC-MS-based analysis and identification of cucurbitacins and 23,24-dihydrocucurbitacins in plant extracts.

Bacterial Dehydrogenases Facilitate Oxidative Inactivation and Bioremediation of Chloramphenicol

Antimicrobial resistance represents a major threat to human health and knowledge of the underlying mechanisms is therefore vital. Here, we report the discovery and characterization of oxidoreductases that inactivate the broad-spectrum antibiotic chloramphenicol via dual oxidation of the C3-hydroxyl group. Accordingly, chloramphenicol oxidation either depends on standalone glucose-methanol-choline (GMC)-type flavoenzymes, or on additional aldehyde dehydrogenases that boost overall turnover. These enzymes also enable the inactivation of the chloramphenicol analogues thiamphenicol and azidamfenicol, but not of the C3-fluorinated florfenicol. Notably, distinct isofunctional enzymes can be found in Gram-positive (e. g., Streptomyces sp.) and Gram-negative (e. g., Sphingobium sp.) bacteria, which presumably evolved their selectivity for chloramphenicol independently based on phylogenetic analyses. Mechanistic and structural studies provide further insights into the catalytic mechanisms of these biotechnologically interesting enzymes, which, in sum, are both a curse and a blessing by contributing to the spread of antibiotic resistance as well as to the bioremediation of chloramphenicol.

Antibacterial Synnepyrroles from Human-Associated Nocardiopsis sp. Show Protonophore Activity and Disrupt the Bacterial Cytoplasmic Membrane

Actinobacteria have traditionally been an important source of bioactive natural products, although many genera remain poorly explored. Here, we report a group of distinctive pyrrole-containing natural products, named synnepyrroles, from Nocardiopsis synnemataformans. Detailed structural characterization by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy combined with isotope-labeling experiments revealed their molecular structures and biosynthetic precursors acetate, propionate, aspartate, and (for branched analogues) valine. The biosynthetic data points toward an unusual pathway for pyrrole formation via condensation of aspartate with diverse fatty acids that give rise to a unique pyrrole-3,4-dicarboxylate core and variable linear or terminally branched alkyl side chains. In addition, the bioactivity and mode of action of synnepyrrole A were characterized in Bacillus subtilis. Orienting assessment of the phenotype of synnepyrrole A-treated bacteria by high-resolution microscopy suggested the cytoplasmic membrane as the target structure. Further characterization of the membrane effects demonstrated dissipation of the membrane potential and intracellular acidification indicative of protonophore activity. At slightly higher concentrations, synnepyrrole A compromised the barrier function of the cytoplasmic membrane, allowing the passage of otherwise membrane-impermeable dye molecules.

An acetyltransferase controls the metabolic flux in rubromycin polyketide biosynthesis by direct modulation of redox tailoring enzymes

The often complex control of bacterial natural product biosynthesis typically involves global and pathway-specific transcriptional regulators of gene expression, which often limits the yield of bioactive compounds under laboratory conditions. However, little is known about regulation mechanisms on the enzymatic level. Here, we report a novel regulatory principle for natural products involving a dedicated acetyltransferase, which modifies a redox-tailoring enzyme and thereby enables pathway furcation and alternating pharmacophore assembly in rubromycin polyketide biosynthesis. The rubromycins such as griseorhodin (grh) A are complex bioactive aromatic polyketides from Actinobacteria with a hallmark bisbenzannulated [5,6]-spiroketal pharmacophore that is mainly installed by two flavoprotein monooxygenases. First, GrhO5 converts the advanced precursor collinone into the [6,6]-spiroketal containing dihydrolenticulone, before GrhO6 effectuates a ring contraction to afford the [5,6]-spiroketal. Our results show that pharmacophore assembly in addition involves the acetyl-CoA-dependent acetyltransferase GrhJ that activates GrhO6 to allow the rapid generation and release of its labile product, which is subsequently sequestered by ketoreductase GrhO10 and converted into a stable intermediate. Consequently, the biosynthesis is directed to the generation of canonical rubromycins, while the alternative spontaneous [5,6]-spiroketal hydrolysis to a ring-opened pathway product is thwarted. Presumably, this allows the bacteria to rapidly adjust the biosynthesis of functionally distinct secondary metabolites depending on nutrient and precursor (i.e. acetyl-CoA) availability. Our study thus illustrates how natural product biosynthesis can be enzymatically regulated and provides new perspectives for the improvement of in vitro enzyme activities and natural product titers via biotechnological approaches.

Characterization of the acetyltransferase GrhJ reveals the surprising acetylation of flavoenzyme GrhO6 in rubromycin polyketide biosynthesis, showcasing a novel principle for the enzymatic regulation of secondary metabolic pathways.

Three Rings to Rule Them All: How Versatile Flavoenzymes Orchestrate the Structural Diversification of Natural Products

The structural diversification of natural products is instrumental to their versatile bioactivities. In this context, redox tailoring enzymes are commonly involved in the modification and functionalization of advanced pathway intermediates en route to the mature natural products. In recent years, flavoprotein monooxygenases have been shown to mediate numerous redox tailoring reactions that include not only (aromatic) hydroxylation, Baeyer–Villiger oxidation, or epoxidation reactions but also oxygenations that are coupled to extensive remodeling of the carbon backbone, which are often central to the installment of the respective pharmacophores. In this Perspective, we will highlight recent developments and discoveries in the field of flavoenzyme catalysis in bacterial natural product biosynthesis and illustrate how the flavin cofactor can be fine-tuned to enable chemo-, regio-, and stereospecific oxygenations via distinct flavin-C4a-peroxide and flavin-N5-(per)oxide species. Open questions remain, e.g., regarding the breadth of chemical reactions enabled particularly by the newly discovered flavin-N5-oxygen adducts and the role of the protein environment in steering such cascade-like reactions. Outstanding cases involving different flavin oxygenating species will be exemplified by the tailoring of bacterial aromatic polyketides, including enterocin, rubromycins, rishirilides, mithramycin, anthracyclins, chartreusin, jadomycin, and xantholipin. In addition, the biosynthesis of tropone natural products, including tropolone and tropodithietic acid, will be presented, which features a recently described prototypical flavoprotein dioxygenase that may combine flavin-N5-peroxide and flavin-N5-oxide chemistry. Finally, structural and mechanistic features of selected enzymes will be discussed as well as hurdles for their application in the formation of natural product derivatives via bioengineering.

Catalytic Control of Spiroketal Formation in Rubromycin Polyketide Biosynthesis

The medically important bacterial aromatic polyketide natural products typically feature a planar, polycyclic core structure. An exception is found for the rubromycins, whose backbones are disrupted by a bisbenzannulated [5,6]‐spiroketal pharmacophore that was recently shown to be assembled by flavin‐dependent enzymes. In particular, a flavoprotein monooxygenase proved critical for the drastic oxidative rearrangement of a pentangular precursor and the installment of an intermediate [6,6]‐spiroketal moiety. Here we provide structural and mechanistic insights into the control of catalysis by this spiroketal synthase, which fulfills several important functions as reductase, monooxygenase, and presumably oxidase. The enzyme hereby tightly controls the redox state of the substrate to counteract shunt product formation, while also steering the cleavage of three carbon‐carbon bonds. Our work illustrates an exceptional strategy for the biosynthesis of stable chroman spiroketals.

Bacterial flavoprotein monooxygenase YxeK salvages toxic S-(2-succino)-adducts via oxygenolytic C-S bond cleavage

Thiol-containing nucleophiles such as cysteine react spontaneously with the citric acid cycle intermediate fumarate to form S-(2-succino)-adducts. In Bacillus subtilis, a salvaging pathway encoded by the yxe operon has recently been identified for the detoxification and exploitation of these compounds as sulfur sources. This route involves acetylation of S-(2-succino)cysteine to N-acetyl-2-succinocysteine, which is presumably converted to oxaloacetate and N-acetylcysteine, before a final deacetylation step affords cysteine. The critical oxidative cleavage of the C-S bond of N-acetyl-S-(2-succino)cysteine was proposed to depend on the predicted flavoprotein monooxygenase YxeK. Here, we characterize YxeK and verify its role in S-(2-succino)-adduct detoxification and sulfur metabolism. Detailed biochemical and mechanistic investigation of YxeK including ¹⁸ O-isotope-labeling experiments, homology modeling, substrate specificity tests, site-directed mutagenesis, and (pre-)steady-state kinetics provides insight into the enzyme's mechanism of action, which may involve a noncanonical flavin-N5-peroxide species for C-S bond oxygenolysis.

A Flavoprotein Dioxygenase Steers Bacterial Tropone Biosynthesis via Coenzyme A-Ester Oxygenolysis and Ring Epoxidation

Bacterial tropone natural products such as tropolone, tropodithietic acid, or the roseobacticides play crucial roles in various terrestrial and marine symbiotic interactions as virulence factors, antibiotics, algaecides, or quorum sensing signals. We now show that their poorly understood biosynthesis depends on a shunt product from aerobic CoA-dependent phenylacetic acid catabolism that is salvaged by the dedicated acyl-CoA dehydrogenase-like flavoenzyme TdaE. Further characterization of TdaE revealed an unanticipated complex catalysis, comprising substrate dehydrogenation, noncanonical CoA-ester oxygenolysis, and final ring epoxidation. The enzyme thereby functions as an archetypal flavoprotein dioxygenase that incorporates both oxygen atoms from O₂ into the substrate, most likely involving flavin-N5-peroxide and flavin-N5-oxide species for consecutive CoA-ester cleavage and epoxidation, respectively. The subsequent spontaneous decarboxylation of the reactive enzyme product yields tropolone, which serves as a key virulence factor in rice panicle blight caused by pathogenic edaphic Burkholderia plantarii. Alternatively, the TdaE product is most likely converted to more complex sulfur-containing secondary metabolites such as tropodithietic acid from predominant marine Rhodobacteraceae (e.g., Phaeobacter inhibens).

Enzymatic spiroketal formation via oxidative rearrangement of pentangular polyketides

The structural complexity and bioactivity of natural products often depend on enzymatic redox tailoring steps. This is exemplified by the generation of the bisbenzannulated [5,6]-spiroketal pharmacophore in the bacterial rubromycin family of aromatic polyketides, which exhibit a wide array of bioactivities such as the inhibition of HIV reverse transcriptase or DNA helicase. Here we elucidate the complex flavoenzyme-driven formation of the rubromycin pharmacophore that is markedly distinct from conventional (bio)synthetic strategies for spiroketal formation. Accordingly, a polycyclic aromatic precursor undergoes extensive enzymatic oxidative rearrangement catalyzed by two flavoprotein monooxygenases and a flavoprotein oxidase that ultimately results in a drastic distortion of the carbon skeleton. The one-pot in vitro reconstitution of the key enzymatic steps as well as the comprehensive characterization of reactive intermediates allow to unravel the intricate underlying reactions, during which four carbon-carbon bonds are broken and two CO₂ become eliminated. This work provides detailed insight into perplexing redox tailoring enzymology that sets the stage for the (chemo)enzymatic production and bioengineering of bioactive spiroketal-containing polyketides.

Rubromycin family of natural products belongs to aromatic polyketides with diverse bioactivities, but details of their biosynthesis are limited. Here, the authors report the complete in vitro reconstitution of enzymatic formation of the spiroketal moiety of rubromycin polyketides, driven by flavin-dependent enzymes, and characterize reaction intermediates.

The devil is in the details: The chemical basis and mechanistic versatility of flavoprotein monooxygenases

The ubiquitous flavoenzymes commonly catalyze redox chemistry such as the monooxygenation of organic substrates and are both widely utilized in nature (e.g., in primary and secondary metabolism) and of significant industrial interest. In this work, we highlight the structural and mechanistic characteristics of the distinct types of flavoprotein monooxygenases (FPMOs). We thereby illustrate the chemical basis of FPMO catalysis, which enables reactions such as (aromatic) hydroxylation, epoxidation, (de)halogenation, heteroatom oxygenation, Baeyer-Villiger oxidation, α-hydroxylation of ketones, or non-oxidative carbon-hetero bond cleavage. This seemingly unmatched versatility in oxygenation chemistry results from extensive fine-tuning and regiospecific functionalization of the flavin cofactor that is tightly controlled by the surrounding protein matrix. Accordingly, FPMOs steer the formation of covalent flavin-oxygen adducts for oxygen transfer in the form of the classical flavin-C4a-(hydro)peroxide or the recently discovered N5-functionalized flavins (i.e. the flavin-N5-oxide and the flavin-N5-peroxide), while in rare cases covalent oxygen adduct formation may be foregone entirely. Finally, we speculate about hitherto undiscovered flavin-mediated oxygenation reactions and compare FPMOs to cytochrome P450 monooxygenases, before addressing open questions and challenges for the future investigation of FPMOs.

Bacterial Tropone Natural Products and Derivatives: Overview of their Biosynthesis, Bioactivities, Ecological Role and Biotechnological Potential

Tropone natural products are non-benzene aromatic compounds of significant ecological and pharmaceutical interest. Herein, we highlight current knowledge on bacterial tropones and their derivatives such as tropolones, tropodithietic acid, and roseobacticides. Their unusual biosynthesis depends on a universal CoA-bound precursor featuring a seven-membered carbon ring as backbone, which is generated by a side reaction of the phenylacetic acid catabolic pathway. Enzymes encoded by separate gene clusters then further modify this key intermediate by oxidation, CoA-release, or incorporation of sulfur among other reactions. Tropones play important roles in the terrestrial and marine environment where they act as antibiotics, algaecides, or quorum sensing signals, while their bacterial producers are often involved in symbiotic interactions with plants and marine invertebrates (e. g., algae, corals, sponges, or mollusks). Because of their potent bioactivities and of slowly developing bacterial resistance, tropones and their derivatives hold great promise for biomedical or biotechnological applications, for instance as antibiotics in (shell)fish aquaculture.

Oxidative Carbon Backbone Rearrangement in Rishirilide Biosynthesis

The structural diversity of type II polyketides is largely generated by tailoring enzymes. In rishirilide biosynthesis by Streptomyces bottropensis, ¹³C-labeling studies previously implied extraordinary carbon backbone and side-chain rearrangements. In this work, we employ gene deletion experiments and in vitro enzyme studies to identify key biosynthetic intermediates and expose intricate redox tailoring steps for the formation of rishirilides A, B, and D and lupinacidin A. First, the flavin-dependent RslO5 reductively ring-opens the epoxide moiety of an advanced polycyclic intermediate to form an alcohol. Flavin monooxygenase RslO9 then oxidatively rearranges the carbon backbone, presumably via lactone-forming Baeyer-Villiger oxidation and subsequent intramolecular aldol condensation. While RslO9 can further convert the rearranged intermediate to rishirilide D and lupinacidin A, an additional ketoreductase RslO8 is required for formation of the main products rishirilide A and rishirilide B. This work provides insight into the structural diversification of aromatic polyketide natural products via unusual redox tailoring reactions that appear to defy biosynthetic logic.

Structural and Mechanistic Basis of an Oxepin-CoA Forming Isomerase in Bacterial Primary and Secondary Metabolism

Numerous aromatic compounds are aerobically degraded in bacteria via the central intermediate phenylacetic acid (paa). In one of the key steps of this widespread catabolic pathway, 1,2-epoxyphenylacetyl-CoA is converted by PaaG into the heterocyclic oxepin-CoA. PaaG thereby elegantly generates an α,β-unsaturated CoA ester that is predisposed to undergo β-oxidation subsequent to hydrolytic ring-cleavage. Moreover, oxepin-CoA serves as a precursor for secondary metabolites (e.g., tropodithietic acid) that act as antibiotics and quorum-sensing signals. Here we verify that PaaG adopts a second role in aromatic catabolism by converting cis-3,4-didehydroadipoyl-CoA into trans-2,3-didehydroadipoyl-CoA and corroborate a Δ³,Δ²-enoyl-CoA isomerase-like proton shuttling mechanism for both distinct substrates. Biochemical and structural investigations of PaaG reveal active site adaptations to the structurally different substrates and provide detailed insight into catalysis and control of stereospecificity. This work elucidates the mechanism of action of unusual isomerase PaaG and sheds new light on the ubiquitous enoyl-CoA isomerases of the crotonase superfamily.

Unusual flavoenzyme catalysis in marine bacteria

Ever since the discovery of the flavin cofactor more than 80 years ago, flavin-dependent enzymes have emerged as ubiquitous and versatile redox catalysts in primary metabolism. Yet, the recent advances in the discovery and characterization of secondary metabolic pathways exposed new roles for flavin-mediated catalysis in the generation of structurally complex natural products. Here, we review a selection of key biosynthetic flavoenzymes from marine bacterial secondary metabolism and illustrate how their functional and mechanistic investigation expanded our view of the cofactor's chemical repertoire and led to the discovery of a previously unknown flavin redox state.

Biochemical Establishment and Characterization of EncM's Flavin-N5-oxide Cofactor

The ubiquitous flavin-dependent monooxygenases commonly catalyze oxygenation reactions by means of a transient C4a-peroxyflavin. A recent study, however, suggested an unprecedented flavin-oxygenating species, proposed as the flavin-N5-oxide (Fl(N5[O])), as key to an oxidative Favorskii-type rearrangement in the biosynthesis of the bacterial polyketide antibiotic enterocin. This stable superoxidized flavin is covalently tethered to the enzyme EncM and converted into FADH2 (Fl(red)) during substrate turnover. Subsequent reaction of Fl(red) with molecular oxygen restores the postulated Fl(N5[O]) via an unknown pathway. Here, we provide direct evidence for the Fl(N5[O]) species via isotope labeling, proteolytic digestion, and high-resolution tandem mass spectrometry of EncM. We propose that formation of this species occurs by hydrogen-transfer from Fl(red) to molecular oxygen, allowing radical coupling of the formed protonated superoxide and anionic flavin semiquinone at N5, before elimination of water affords the Fl(N5[O]) cofactor. Further biochemical and spectroscopic investigations reveal important features of the Fl(N5[O]) species and the EncM catalytic mechanism. We speculate that flavin-N5-oxides may be intermediates or catalytically active species in other flavoproteins that form the anionic semiquinone and promote access of oxygen to N5.

Direct capture and heterologous expression of Salinispora natural product genes for the biosynthesis of enterocin

Heterologous expression of secondary metabolic pathways is a promising approach for the discovery and characterization of bioactive natural products. Herein we report the first heterologous expression of a natural product from the model marine actinomycete genus Salinispora. Using the recently developed method of yeast-mediated transformation-associated recombination for natural product gene clusters, we captured a type II polyketide synthase pathway from Salinispora pacifica with high homology to the enterocin pathway from Streptomyces maritimus and successfully produced enterocin in two different Streptomyces host strains. This result paves the way for the systematic interrogation of Salinispora's promising secondary metabolome.

A multitasking vanadium-dependent chloroperoxidase as an inspiration for the chemical synthesis of the merochlorins

The vanadium-dependent chloroperoxidase Mcl24 was discovered to mediate a complex series of unprecedented transformations in the biosynthesis of the merochlorin meroterpenoid antibiotics. In particular, a site-selective naphthol chlorination is followed by an oxidative dearomatization/terpene cyclization sequence to build up the stereochemically complex carbon framework of the merochlorins in one step. Inspired by the enzyme reactivity, a chemical chlorination protocol paralleling the biocatalytic process was developed. These chemical studies led to the identification of previously overlooked merochlorin natural products.

One-pot enzymatic synthesis of merochlorin A and B

The polycycles merochlorin A and B are complex halogenated meroterpenoid natural products with significant antibacterial activities and are produced by the marine bacterium Streptomyces sp. strain CNH-189. Heterologously produced enzymes and chemical synthesis are employed herein to fully reconstitute the merochlorin biosynthesis in vitro. The interplay of a dedicated type III polyketide synthase, a prenyl diphosphate synthase, and an aromatic prenyltransferase allow formation of a highly unusual aromatic polyketide-terpene hybrid intermediate which features an unprecedented branched sesquiterpene moiety from isosesquilavandulyl diphosphate. As supported by in vivo experiments, this precursor is furthermore chlorinated and cyclized to merochlorin A and isomeric merochlorin B by a single vanadium-dependent haloperoxidase, thus completing the remarkably efficient pathway.

Flavin-mediated dual oxidation controls an enzymatic Favorskii-type rearrangement

Flavoproteins catalyze a diversity of fundamental redox reactions and are one of the most studied enzyme families^1,2. As monooxygenases, they are universally thought to control oxygenation by means of a peroxyflavin species that transfers a single atom of molecular oxygen to an organic substrate^1,3,4. Here we report that the bacterial flavoenzyme EncM^5,6 catalyzes the peroxyflavin-independent oxygenation-dehydrogenation dual oxidation of a highly reactive poly(β-carbonyl). The crystal structure of EncM with bound substrate mimics coupled with isotope labeling studies reveal previously unknown flavin redox biochemistry. We show that EncM maintains an unanticipated stable flavin oxygenating species, proposed to be a flavin-N5-oxide, to promote substrate oxidation and trigger a rare Favorskii-type rearrangement that is central to the biosynthesis of the antibiotic enterocin. This work provides new insight into the fine-tuning of the flavin cofactor in offsetting the innate reactivity of a polyketide substrate to direct its efficient electrocyclization.

Studies on the mechanism of ring hydrolysis in phenylacetate degradation: a metabolic branching point

The widespread, long sought-after bacterial aerobic phenylalanine/phenylacetate catabolic pathway has recently been elucidated. It proceeds via coenzyme A (CoA) thioesters and involves the epoxidation of the aromatic ring of phenylacetyl-CoA, subsequent isomerization to an uncommon seven-membered C-O-heterocycle (oxepin-CoA), and non-oxygenolytic ring cleavage. Here we characterize the hydrolytic oxepin-CoA ring cleavage catalyzed by the bifunctional fusion protein PaaZ. The enzyme consists of a C-terminal (R)-specific enoyl-CoA hydratase domain (formerly MaoC) that cleaves the ring and produces a highly reactive aldehyde and an N-terminal NADP(+)-dependent aldehyde dehydrogenase domain that oxidizes the aldehyde to 3-oxo-5,6-dehydrosuberyl-CoA. In many phenylacetate-utilizing bacteria, the genes for the pathway exist in a cluster that contains an NAD(+)-dependent aldehyde dehydrogenase in place of PaaZ, whereas the aldehyde-producing hydratase is encoded outside of the cluster. If not oxidized immediately, the reactive aldehyde condenses intramolecularly to a stable cyclic derivative that is largely prevented by PaaZ fusion in vivo. Interestingly, the derivative likely serves as the starting material for the synthesis of antibiotics (e.g. tropodithietic acid) and other tropone/tropolone related compounds as well as for ω-cycloheptyl fatty acids. Apparently, bacteria made a virtue out of the necessity of disposing the dead-end product with ring hydrolysis as a metabolic branching point.

Human peripheral blood monuclear cells transfected with messenger RNA stimulate antigen-specific cytotoxic T-lymphocytes in vitro

The efficiency of test vaccines needs to be evaluated by quantification of the triggered cellular immune response. Usually, for these assays, autologous target cells expressing the vaccine antigen are required. In the context of messenger RNA (mRNA)-based vaccinations, the target cells used for the read-out are mRNA-transfected monocyte-derived dendritic cells (Mo-DCs). Their production typically requires samples of 100 ml blood from the patients, and limits the number of assays that can be performed. We show here that fresh peripheral blood mononuclear cells (PBMCs) can be transfected with mRNA by electroporation. Such cells are as efficient as mRNA-transfected Mo-DCs for their ability to activate memory T cells in vitro. Thus, mRNA-transfected PBMCs are a convenient replacement of mRNA-transfected Mo-DCs for the in vitro monitoring of natural or vaccine-induced immune responses.

Polarization of immunity induced by direct injection of naked sequence-stabilized mRNA vaccines

In the context of developing a safe genetic vaccination strategy we tested and studied globin-stabilized mRNA-based vaccination in mice. This vaccination strategy has the advantages of genetic vaccination (easy production, adaptability to any disease and inexpensive storage when lyophilized), but not the drawbacks of DNA vaccination (long-term uncontrolled expression of a transgene, possibility of integration into the host genome and possible induction of anti-DNA antibodies). We report here that injection of naked β-globin untranslated region (UTR)-stabilized mRNA coding for β-galactosidase is followed by detectable translation in vivo. In addition, we show that such a vaccination strategy primes a T helper 2 (Th2) type of response which can be enhanced and shifted to a Th1-type immune response by application of recombinant granulocyte/macrophage colony-stimulating factor 1 day after mRNA injection. Our data demonstrate that the administration of globin UTR-stabilized mRNA is a versatile vaccination strategy that can be manipulated to fit the requirement of antiviral, antibacterial or antitumor immunity.

Reducing the concentration of selected marker improves efficiency of cotransfer of unselected DNA into SV40-transformed human fibroblasts

A substantial increase in transfer of unselected DNA to two human SV40-transformed fibroblast cell lines was obtained by reducing the concentration of the cotransferred selected marker DNA. The average amount of unselected DNA transferred, even under favorable conditions, was still low compared to that reported for some rodent cell lines. Our results suggest that in human fibroblasts there is strong competition between exogenous DNA molecules for integration and maintenance, and that more unselected DNA is retained in the presence of only one copy of the selected marker.