Selvamicin, an atypical antifungal polyene from two alternative genomic contexts

The bacteria harbored by fungus-growing ants produce a variety of small molecules that help maintain a complex multilateral symbiosis. In a survey of antifungal compounds from these bacteria, we discovered selvamicin, an unusual antifungal polyene macrolide, in bacterial isolates from two neighboring ant nests. Selvamicin resembles the clinically important antifungals nystatin A₁ and amphotericin B, but it has several distinctive structural features: a noncationic 6-deoxymannose sugar at the canonical glycosylation site and a second sugar, an unusual 4-O-methyldigitoxose, at the opposite end of selvamicin's shortened polyene macrolide. It also lacks some of the pharmacokinetic liabilities of the clinical agents and appears to have a different target. Whole genome sequencing revealed the putative type I polyketide gene cluster responsible for selvamicin's biosynthesis including a subcluster of genes consistent with selvamicin's 4-O-methyldigitoxose sugar. Although the selvamicin biosynthetic cluster is virtually identical in both bacterial producers, in one it is on the chromosome, in the other it is on a plasmid. These alternative genomic contexts illustrate the biosynthetic gene cluster mobility that underlies the diversity and distribution of chemical defenses by the specialized bacteria in this multilateral symbiosis.

Recognition of Membrane Sterols by Polyene Antifungals Amphotericin B and Natamycin, A (13)C MAS NMR Study

The molecular action of polyene macrolides with antifungal activity, amphotericin B and natamycin, involves recognition of sterols in membranes. Physicochemical and functional studies have contributed details to understanding the interactions between amphotericin B and ergosterol and, to a lesser extent, with cholesterol. Fewer molecular details are available on interactions between natamycin with sterols. We use solid state (13)C MAS NMR to characterize the impact of amphotericin B and natamycin on mixed lipid membranes of DOPC/cholesterol or DOPC/ergosterol. In cholesterol-containing membranes, amphotericin B addition resulted in marked increase in both DOPC and cholesterol (13)C MAS NMR linewidth, reflecting membrane insertion and cooperative perturbation of the bilayer. By contrast, natamycin affects little either DOPC or cholesterol linewidth but attenuates cholesterol resonance intensity preferentially for sterol core with lesser impact on the chain. Ergosterol resonances, attenuated by amphotericin B, reveal specific interactions in the sterol core and chain base. Natamycin addition selectively augmented ergosterol resonances from sterol core ring one and, at the same time, from the end of the chain. This puts forward an interaction model similar to the head-to-tail model for amphotericin B/ergosterol pairing but with docking on opposite sterol faces. Low toxicity of natamycin is attributed to selective, non-cooperative sterol engagement compared to cooperative membrane perturbation by amphotericin B.

Stimuli-responsive polymers for antimicrobial therapy: drug targeting, contact-killing surfaces and competitive release

INTRODUCTION

Polymers can be designed to modify their features as a function of the level and nature of the surrounding microorganisms. Such responsive polymers can endow drug delivery systems and drug-medical device combination products with improved performance against intracellular infections and biofilms.

AREAS COVERED

Knowledge on microorganism growth environment outside and inside cells and formation of biofilm communities on biological and synthetic surfaces, together with advances in materials science and drug delivery are prompting strategies with improved efficacy and safety compared to traditional systemic administration of antimicrobial agents. This review deals with antimicrobial strategies that rely on: (i) polymers that disintegrate or undergo phase-transitions in response to changes in enzymes, pH and pO2 associated to microorganism growth; (ii) stimuli-responsive polymers that expose contact-killing groups when microorganisms try to adhere; and (iii) bioinspired polymers that recognize microorganisms for triggered (competitive/affinity-driven) drug release.

EXPERT OPINION

Prophylaxis and treatment of infections may benefit from polymers that are responsive to the unique changes that microbial growth causes in the surrounding environment or that even recognize the microorganism itself or its quorum sensing signals. These polymers may offer novel tools for the design of macrophage-, bacteria- and/or biofilm-targeted nanocarriers as well as of medical devices with switchable antibiofouling properties.

Visceral leishmaniasis: Revisiting current treatments and approaches for future discoveries

The current treatments for visceral leishmaniasis are old and toxic with limited routes of administration. The emergence of drug-resistant Leishmania threatens the efficacy of the existing reservoir of antileishmanials, leading to an urgent need to develop new treatments. It is particularly important to review and understand how the current treatments act against Leishmania in order to identify valid drug targets or essential pathways for next-generation antileishmanials. It is equally important to adapt newly emerging biotechnologies to facilitate the current research on the development of novel antileishmanials in an efficient fashion. This review covers the basic background of the current visceral leishmaniasis treatments with an emphasis on the modes of action. It briefly discusses the role of the immune system in aiding the chemotherapy of leishmaniasis, describes potential new antileishmanial drug targets and pathways, and introduces recent progress on the utilization of high-throughput phenotypic screening assays to identify novel antileishmanial compounds.

Application of isothermal titration calorimetry as a tool to study natural product interactions

The study of molecular interactions of natural products by isothermal titration calorimetry (ITC) is a potent tool to get new insights of the underpinning driving forces.

Biotechnological production and application of the antibiotic pimaricin: biosynthesis and its regulation

Pimaricin (natamycin) is a small polyene macrolide antibiotic used worldwide. This efficient antimycotic and antiprotozoal agent, produced by several soil bacterial species of the genus Streptomyces, has found application in human therapy, in the food and beverage industries and as pesticide. It displays a broad spectrum of activity, targeting ergosterol but bearing a particular mode of action different to other polyene macrolides. The biosynthesis of this only antifungal agent with a GRAS status has been thoroughly studied, which has permitted the manipulation of producers to engineer the biosynthetic gene clusters in order to generate several analogues. Regulation of its production has been largely unveiled, constituting a model for other polyenes and setting the leads for optimizing the production of these valuable compounds. This review describes and discusses the molecular genetics, uses, mode of action, analogue generation, regulation and strategies for increasing pimaricin production yields.

Functional analysis of filipin tailoring genes from Streptomyces filipinensis reveals alternative routes in filipin III biosynthesis and yields bioactive derivatives

Background

Streptomyces filipinensis is the industrial producer of filipin, a pentaene macrolide, archetype of non-glycosylated polyenes, and widely used for the detection and the quantitation of cholesterol in biological membranes and as a tool for the diagnosis of Niemann–Pick type C disease. Genetic manipulations of polyene biosynthetic pathways have proven useful for the discovery of products with improved properties. Here, we describe the late biosynthetic steps for filipin III biosynthesis and strategies for the generation of bioactive filipin III derivatives at high yield.

Results

A region of 13,778 base pairs of DNA from the S. filipinensis genome was isolated, sequenced, and characterized. Nine complete genes and two truncated ORFs were located. Disruption of genes proved that this genomic region is part of the biosynthetic cluster for the 28-membered ring of the polyene macrolide filipin. This set of genes includes two cytochrome P450 monooxygenase encoding genes, filC and filD, which are proposed to catalyse specific hydroxylations of the macrolide ring at C26 and C1′ respectively. Gene deletion and complementation experiments provided evidence for their role during filipin III biosynthesis. Filipin III derivatives were accumulated by the recombinant mutants at high yield. These have been characterized by mass spectrometry and nuclear magnetic resonance following high-performance liquid chromatography purification thus revealing the post-polyketide steps during polyene biosynthesis. Two alternative routes lead to the formation of filipin III from the initial product of polyketide synthase chain assembly and cyclization filipin I, one trough filipin II, and the other one trough 1′-hydroxyfilipin I, all filipin III intermediates being biologically active. Moreover, minimal inhibitory concentration values against Candida utilis and Saccharomyces cerevisiae were obtained for all filipin derivatives, finding that 1′-hydroxyfilipin and especially filipin II show remarkably enhanced antifungal bioactivity. Complete nuclear magnetic resonance assignments have been obtained for the first time for 1′-hydroxyfilipin I.

Conclusions

This report reveals the existence of two alternative routes for filipin III formation and opens new possibilities for the generation of biologically active filipin derivatives at high yield and with improved properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0307-4) contains supplementary material, which is available to authorized users.

Natamycin - antimycotic of polyene macrolides class with unusual properties

In the current literature review issues regarding physicochemical peculiarities, mechanism ot action and satety aspects ot polyene macrolides class compound natamycin are enlightened along with the extensive clinical data upon the use ot pharmaceuticals containing this active ingredient.

Formulation and antifungal performance of natamycin-loaded liposomal suspensions: the benefits of sterol-enrichment

The aim of this study is to develop and evaluate food-grade liposomal delivery systems for the antifungal compound natamycin. Liposomes made of various soybean lecithins are prepared by solvent injection, leading to small unilamellar vesicles (<130 nm) with controlled polydispersity, able to encapsulate natamycin without significant modification of their size characteristics. Presence of charged phospholipids and reduced content of phosphatidylcholine in the lecithin mixture are found to be beneficial for natamycin encapsulation, indicating electrostatic interactions of the preservative with the polar head of the phospholipids. The chemical instability of natamycin upon storage in these formulations is however significant and proves that uncontrolled leakage out of the liposomes occurs. Efficient prevention of natamycin degradation is obtained by incorporation of sterols (cholesterol, ergosterol) in the lipid mixture and is linked to higher entrapment levels and reduced permeability of the phospholipid membrane provided by the ordering effect of sterols. Comparable action of ergosterol is observed at concentrations 2.5-fold lower than cholesterol and attributed to a preferential interaction of natamycin-ergosterol as well as a higher control of membrane permeability. Fine-tuning of sterol concentration allows preparation of liposomal suspensions presenting modulated in vitro release kinetics rates and enhanced antifungal activity against the model yeast Saccharomyces cerevisiae.

Control of human and plant fungal pathogens using pentaene macrolide 32, 33-didehydroroflamycoin

AIMS

The aim of this study was to address the toxicity of recently described polyene macrolide 32, 33-didehydroroflamycoin (DDHR) on a wide range of fungal pathogens and its potential to control plant fungal diseases.

METHODS AND RESULTS

The antifungal activity of DDHR in vitro was examined against common human and plant pathogenic fungi using a broth microdilution assay and a disk diffusion assay. Minimum inhibitory concentrations ranged from 12·5 to 35 μg ml(-1) . A radial growth inhibition assay showed that DDHR inhibited mycelia growth, inducing mycelial necrosis and affecting sporulation. During the in vivo assay on apple fruits administration of DDHR 1 h before fungal inoculation inhibited spreading of the infection. Importantly, DDHR exhibited no phytotoxic effects on the model plant, Capsicum annum, verified by the plant growth rate and chlorophyll content.

CONCLUSIONS

DDHR inhibits growth of various plant pathogens in vitro with the strongest activity against Alternaria alternata, Colletotrichum acutatum and Penicillium expansum, and protects apple fruits from decay.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of the inhibitory effect of DDHR on important pathogenic fungal isolates. DDHR could be a good scaffold for developing new antifungal agents for fruit and vegetable protection.

Osmosensing and scaffolding functions of the oligomeric four-transmembrane domain osmosensor Sho1

The yeast high osmolarity glycerol (HOG) pathway activates the Hog1 MAP kinase, which coordinates adaptation to high osmolarity conditions. Here we demonstrate that the four-transmembrane (TM) domain protein Sho1 is an osmosensor in the HKR1 sub-branch of the HOG pathway. Crosslinking studies indicate that Sho1 forms planar oligomers of the dimers-of-trimers architecture by dimerizing at the TM1/TM4 interface and trimerizing at the TM2/TM3 interface. High external osmolarity induces structural changes in the Sho1 TM domains and Sho1 binding to the cytoplasmic adaptor protein Ste50, which leads to Hog1 activation. Besides its osmosensing function, the Sho1 oligomer serves as a scaffold. By binding to the TM proteins Opy2 and Hkr1 at the TM1/TM4 and TM2/TM3 interface, respectively, Sho1 forms a multi-component signalling complex that is essential for Hog1 activation. Our results illuminate how the four TM domains of Sho1 dictate the oligomer structure as well as its osmosensing and scaffolding functions.

Recent progress in the study of the interactions of amphotericin B with cholesterol and ergosterol in lipid environments

In the past decade substantial progress has been made in understanding the organization and biological activity of amphotericin B (AmB) in the presence of sterols in lipid environments. This review concentrates mainly on interactions of AmB with lipids and sterols, AmB channel formation in membranes, AmB aggregation, AmB modifications important for understanding its biological activity, and AmB models explaining its mechanism of action. Most of the reviewed studies concern monolayers at the water–gas interface, monolayers deposited on a solid substrate by use of the Langmuir–Blodgett technique, micelles, vesicles, and multi-bilayers. Liposomal AmB formulations and drug delivery are intentionally omitted, because several reviews dedicated to this subject are already available.

The negligible effects of the antifungal natamycin on cholesterol-dipalmitoyl phosphatidylcholine monolayers may explain its low oral and topical toxicity for mammals

Natamycin is an effective, broad spectrum antifungal with no reported resistance, in contrast to most antimicrobials. It also exhibits reduced (oral and topical) toxicity to humans, which is probably associated with the lack of effects on mammalian cell membranes. In this paper we employ Langmuir monolayers to mimic a cell membrane, whose properties are interrogated with various techniques. We found that natamycin has negligible effects on Langmuir monolayers of dipalmitoyl phosphatidylcholine (DPPC), but it strongly affects cholesterol monolayers. Natamycin causes the surface pressure isotherm of a cholesterol monolayer to expand even at high surface pressures since it penetrates into the hydrophobic chains. It also reduces the compressibility modulus, probably because natamycin disturbs the organization of the cholesterol molecules, as inferred with polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). In mixed cholesterol/DPPC monolayers, strong effects from natamycin were only observed when the cholesterol concentration was 50mol% or higher, well above its concentration in a mammalian cell membrane. For a sterol concentration that mimics a real cell membrane in mammals, i.e. with 25mol% of cholesterol, the effects were negligible, which may explain why natamycin has low toxicity when ingested and/or employed to treat superficial fungal infections.

New pharmacological properties of Medicago sativa and Saponaria officinalis saponin-rich fractions addressed to Candida albicans

The antifungal activity of the saponin-rich fractions (SFs) from Medicago sativa (aerial parts and roots) and Saponaria officinalis (used as a well-known source of plant saponins) against Candida albicans reference and clinical strains, their yeast-to-hyphal conversion, adhesion, and biofilm formation was investigated. Direct fungicidal/fungistatic properties of the tested phytochemicals used alone, as well as their synergy with azoles (probably resulting from yeast cell wall instability) were demonstrated. Here, to the best of our knowledge, we report for the first time the ability of saponin-rich extracts of M. sativa and S. officinalis to inhibit C. albicans germ tube formation, limit hyphal growth, reduce yeast adherence and biofilm formation, and eradicate mature (24 h) Candida biofilm. Moreover, M. sativa SFs (mainly obtained from aerial parts), in the range of concentrations which were active modulators of Candida virulence factors, exhibited low cytotoxicity against the mouse fibroblast line L929. These properties seem to be very promising in the context of using plant-derived SFs as potential novel antifungal therapeutics supporting classic drugs or as ingredients of disinfectants.

Clinical utility of caspofungin eye drops in fungal keratitis

Treatment of fungal keratitis remains challenging. To date, only the polyenes and azoles are commonly used topically in the management of fungal keratitis. Natamycin, a polyene, is the only antifungal eye drop that is commercially available; the remainder are prepared in-house and are used in an 'off-label' manner. Failure of medical treatment for fungal keratitis is common, hence there is a need for more effective topical antifungal therapy. To increase the antifungal eye drop armamentarium, it is important to investigate the utility of other classes of antifungal agents for topical use. Caspofungin, an echinocandin antifungal agent, could potentially be used to address the existing shortcomings. However, little is known about the usefulness of topically administered caspofungin. This review will briefly explore the incidence, epidemiology and antifungal treatment of fungal keratitis. It will focus primarily on evidence related to the efficacy, safety and practicality of using caspofungin eye drops in fungal keratitis.

Synthetic multivalent antifungal peptides effective against fungi

Taking advantage of the cluster effect observed in multivalent peptides, this work describes antifungal activity and possible mechanism of action of tetravalent peptide (B4010) which carries 4 copies of the sequence RGRKVVRR through a branched lysine core. B4010 displayed better antifungal properties than natamycin and amphotericin B. The peptide retained significant activity in the presence of monovalent/divalent cations, trypsin and serum and tear fluid. Moreover, B4010 is non-haemolytic and non-toxic to mice by intraperitoneal (200 mg/kg) or intravenous (100 mg/kg) routes. S. cerevisiae mutant strains with altered membrane sterol structures and composition showed hyper senstivity to B4010. The peptide had no affinity for cell wall polysaccharides and caused rapid dissipation of membrane potential and release of vital ions and ATP when treated with C. albicans. We demonstrate that additives which alter the membrane potential or membrane rigidity protect C. albicans from B4010-induced lethality. Calcein release assay and molecular dynamics simulations showed that the peptide preferentially binds to mixed bilayer containing ergosterol over phophotidylcholine-cholesterol bilayers. The studies further suggested that the first arginine is important for mediating peptide-bilayer interactions. Replacing the first arginine led to a 2-4 fold decrease in antifungal activities and reduced membrane disruption properties. The combined in silico and in vitro approach should facilitate rational design of new tetravalent antifungal peptides.

Contact lenses for antifungal ocular drug delivery: a review

INTRODUCTION

Fungal keratitis, a potentially blinding disease, has been difficult to treat due to the limited number of approved antifungal drugs and the taxing dosing regimen. Thus, the development of a contact lens (CL) as an antifungal drug delivery platform has the potential to improve the treatment of fungal keratitis. A CL can serve as a drug reservoir to continuously release drugs to the cornea, while limiting drug loss through tears, blinking, drainage and non-specific absorption.

AREAS COVERED

This review will provide a summary of currently available methods for delivering antifungal drugs from commercial and model CLs, including vitamin E coating, impregnated drug films, cyclodextrin-functionalized hydrogels, polyelectrolyte hydrogels and molecular imprinting. This review will also highlight some of the main factors that influence antifungal drug delivery with CLs.

EXPERT OPINION

Several novel CL materials have been developed, capable of extended drug release profiles with a wide range of antifungal drugs lasting from 8 h to as long as 21 days. However, there are factors, such as first-order release kinetics, effectiveness of continuous drug release, microbial resistance, ocular toxicity and potential complications from inserting a CL in an infected eye, that still need to be addressed before commercial applications can be realized.

Construction of a Streptomyces lydicus A01 transformant with a chit42 gene from Trichoderma harzianum P1 and evaluation of its biocontrol activity against Botrytis cinerea

Streptomyces lydicus A01 and Trichoderma harzianum P1 are potential biocontrol agents of fungal diseases in plants. S. lydicus A01 produces natamycin to bind the ergosterol of the fungal cell membrane and inhibits the growth of Botrytis cinerea. T. harzianum P1, on the other hand, features high chitinase activity and decomposes the chitin in the cell wall of B. cinerea. To obtain the synergistic biocontrol effects of chitinase and natamycin on Botrytis cinerea, this study transformed the chit42 gene from T. harzianum P1 to S. lydicus A01. The conjugal transformant (CT) of S. lydicus A01 with the chit42 gene was detected using polymerase chain reaction (PCR). Associated chitinase activity and natamycin production were examined using the 3, 5-dinitrosalicylic acid (DNS) method and ultraviolet spectrophotometry, respectively. The S. lydicus A01-chit42 CT showed substantially higher chitinase activity and natamycin production than its wild type strain (WT). Consequently, the biocontrol effects of S. lydicus A01-chit42 CT on B. cinerea, including inhibition to spore germination and mycelial growth, were highly improved compared with those of the WT. Our research indicates that the biocontrol effect of Streptomyces can be highly improved by transforming the exogenous resistance gene, i.e. chit42 from Trichoderma, which not only enhances the production of antibiotics, but also provides a supplementary function by degrading the cell walls of the pathogens.

A functional variomics tool for discovering drug-resistance genes and drug targets

Comprehensive discovery of genetic mechanisms of drug resistance and identification of in vivo drug targets represent significant challenges. Here we present a functional variomics technology in the model organism Saccharomyces cerevisiae. This tool analyzes numerous genetic variants and effectively tackles both problems simultaneously. Using this tool, we discovered almost all genes that, due to mutations or modest overexpression, confer resistance to rapamycin, cycloheximide, and amphotericin B. Most significant among the resistance genes were drug targets, including multiple targets of a given drug. With amphotericin B, we discovered the highly conserved membrane protein Pmp3 as a potent resistance factor and a possible target. Widespread application of this tool should allow rapid identification of conserved resistance mechanisms and targets of many more compounds. New genes and alleles that confer resistance to other stresses can also be discovered. Similar tools in other systems, such as human cell lines, will also be useful.

Mechanism of activity and toxicity of Nystatin-Intralipid

A novel lipid formulation of Nystatin (NYT), Nystatin-Intralipid (NYT-IL), which was found to be more active and less toxic in vitro and in vivo, was developed in our laboratory. The aim of the present study was to explore the possible mechanisms underlying its biological activity. To assess mechanisms affecting fungal cells we conducted the following experiments: killing kinetics, scanning and transmission electron microscopy (EM), measurements of potassium ion leakage and susceptibility in the presence of ergosterol. To study mechanisms affecting mammalian cells, we evaluated the effect of NYT-IL on a kidney cell line, with respect to viability, metabolic activity, potassium leakage and internalization of FITC-labeled human transferrin. NYT-IL exhibited killing kinetics patterns against Candida albicans similar to those of NYT and caused disruption of fungal cells and potassium ion leakage. Susceptibility tests showed that NYT-IL had lower antifungal activity in the presence of ergosterol. Thus, NYT-IL acts apparently by damaging fungal membrane, possibly through interaction with ergosterol, and maybe by additional modes of action. NYT-IL did not cause potassium leakage from mammalian kidney cells at any tested concentration and was not cytotoxic, whereas NYT, at high concentrations, led to K(+) leakage and was cytotoxic. Furthermore, the high NYT concentration interfered in the internalization process of human transferrin receptor (hTfnR) while NYT-IL did not. In summary, the Intralipid formulation of NYT diminishes the mechanisms responsible for toxicity to mammalian cells but preserves mechanisms of action against fungi, thereby suggesting superiority of NYT-IL as compared to NYT as an antifungal agent.

The effect of natamycin on the transcriptome of conidia of Aspergillus niger

The impact of natamycin on Aspergillus niger was analysed during the first 8 h of germination of conidia. Polarisation, germ tube formation, and mitosis were inhibited in the presence of 3 and 10 μM of the anti-fungal compound, while at 10 μM also isotropic growth was affected. Natamycin did not have an effect on the decrease of microviscosity during germination and the concomitant reduction in mannitol and trehalose levels. However, it did abolish the increase of intracellular levels of glycerol and glucose during the 8 h period of germination.

Natamycin hardly affected the changes that occur in the RNA profile during the first 2 h of germination. During this time period, genes related to transcription, protein synthesis, energy and cell cycle and DNA processing were particularly up-regulated. Differential expression of 280 and 2586 genes was observed when 8 h old germlings were compared with conidia that had been exposed to 3 μM and 10 μM natamycin, respectively. For instance, genes involved in ergosterol biosynthesis were down-regulated. On the other hand, genes involved in endocytosis and the metabolism of compatible solutes, and genes encoding protective proteins were up-regulated in natamycin treated conidia.

Polyene antibiotic that inhibits membrane transport proteins

The limited therapeutic arsenal and the increase in reports of fungal resistance to multiple antifungal agents have made fungal infections a major therapeutic challenge. The polyene antibiotics are the only group of antifungal antibiotics that directly target the plasma membrane via a specific interaction with the main fungal sterol, ergosterol, often resulting in membrane permeabilization. In contrast to other polyene antibiotics that form pores in the membrane, the mode of action of natamycin has remained obscure but is not related to membrane permeabilization. Here, we demonstrate that natamycin inhibits growth of yeasts and fungi via the immediate inhibition of amino acid and glucose transport across the plasma membrane. This is attributable to ergosterol-specific and reversible inhibition of membrane transport proteins. It is proposed that ergosterol-dependent inhibition of membrane proteins is a general mode of action of all the polyene antibiotics, of which some have been shown additionally to permeabilize the plasma membrane. Our results imply that sterol-protein interactions are fundamentally important for protein function even for those proteins that are not known to reside in sterol-rich domains.

Phylogenetic and preliminary phenotypic analysis of yeast PAQR receptors: potential antifungal targets

Proteins belonging to the Progestin and AdipoQ Receptor (PAQR) superfamily of membrane bound receptors are ubiquitously found in fungi. Nearly, all fungi possess two evolutionarily distinct paralogs of PAQR protein, which we have called the PQRA and PQRB subtypes. In the model fungus Saccharomyces cerevisiae, these subtypes are represented by the Izh2p and Izh3p proteins, respectively. S. cerevisiae also possesses two additional PQRA-type receptors called Izh1p and Izh4p that are restricted to other species within the “Saccharomyces complex”. Izh2p has been the subject of several recent investigations and is of particular interest because it regulates fungal growth in response to proteins produced by plants and, as such, represents a new paradigm for interspecies communication. We demonstrate that IZH2 and IZH3 gene dosage affects resistance to polyene antifungal drugs. Moreover, we provide additional evidence that Izh2p and Izh3p negatively regulate fungal filamentation. These data suggest that agonists of these receptors might make antifungal therapeutics, either by inhibiting fungal development or by sensitizing fungi to the toxic effects of current antifungal therapies. This is particularly relevant for pathogenic fungi such as Candida glabrata that are closely related to S. cerevisiae and contain the same complement of PAQR receptors.

Synthesis-enabled functional group deletions reveal key underpinnings of amphotericin B ion channel and antifungal activities

Amphotericin B is the archetype for small molecules that form transmembrane ion channels. However, despite extensive study for more than five decades, even the most basic features of this channel structure and its contributions to the antifungal activities of this natural product have remained unclear. We herein report that a powerful series of functional group-deficient probes have revealed many key underpinnings of the ion channel and antifungal activities of amphotericin B. Specifically, in stark contrast to two leading models, polar interactions between mycosamine and carboxylic acid appendages on neighboring amphotericin B molecules are not required for ion channel formation, nor are these functional groups required for binding to phospholipid bilayers. Alternatively, consistent with a previously unconfirmed third hypothesis, the mycosamine sugar is strictly required for promoting a direct binding interaction between amphotericin B and ergosterol. The same is true for cholesterol. Synthetically deleting this appendage also completely abolishes ion channel and antifungal activities. All of these results are consistent with the conclusion that a mycosamine-mediated direct binding interaction between amphotericin B and ergosterol is required for both forming ion channels and killing yeast cells. The enhanced understanding of amphotericin B function derived from these synthesis-enabled studies has helped set the stage for the more effective harnessing of the remarkable ion channel-forming capacity of this prototypical small molecule natural product.

An efficient method for the production of isotopically enriched cholesterol for NMR

(13)C-Cholesterol was produced with high efficiency by a genetically engineered yeast strain. The method produces ∼ 1 mg of cholesterol per gram of glucose using 100 ml of culture medium. Uniform 94% enrichment where the most abundant product is the fully enriched isotopomer (u-(13)C(27)) is obtained using (u-(13)C(6), 99%) glucose medium. High enrichment is very important for relaxation experiments, but for NMR applications where carbon-carbon couplings are measured, this is problematic. A good compromise between sensitivity and cost consists in diluting (u-(13)C(6), 25%) with natural-abundance glucose. With a 2:3 ratio, the maximal amount of singlets can be obtained in 1 dimensional (D) carbon and 2D heteronuclear single-quantum correlation (HSQC) spectra with 6× intensity increase relative to natural-abundance samples. The use of (1-(13)C(1)-glucose, 99%) or (2-(13)C(1)-glucose, 99%) as isotope sources allows the labeling of the cholesterol in multiple mostly nonvicinal positions and reach 45× intensity increase. As an alternative, the dilution of (u-(13)C(6), 99%) glucose can be used to simultaneously enrich eleven pairs of (13)C up to ∼ 1,000× natural-abundance probability, which should be very beneficial to double-quantum NMR experiments including the INADEQUATE and related pulse sequences. The flexibility of the method and the potential to adapt the culture protocol to specific needs should find many applications in chemistry and biology and in different fields of NMR and MS.

Molecular control of polyene macrolide biosynthesis: direct binding of the regulator PimM to eight promoters of pimaricin genes and identification of binding boxes

Control of polyene macrolide production in Streptomyces natalensis is mediated by the transcriptional activator PimM. This regulator, which combines an N-terminal PAS domain with a C-terminal helix-turn-helix motif, is highly conserved among polyene biosynthetic gene clusters. PimM, truncated forms of the protein without the PAS domain (PimM(ΔPAS)), and forms containing just the DNA-binding domain (DBD) (PimM(DBD)) were overexpressed in Escherichia coli as GST-fused proteins. GST-PimM binds directly to eight promoters of the pimaricin cluster, as demonstrated by electrophoretic mobility shift assays. Assays with truncated forms of the protein revealed that the PAS domain does not mediate specificity or the distinct recognition of target genes, which rely on the DBD domain, but significantly reduces binding affinity up to 500-fold. Transcription start points were identified by 5'-rapid amplification of cDNA ends, and the binding regions of PimM(DBD) were investigated by DNase I protection studies. In all cases, binding took place covering the -35 hexamer box of each promoter, suggesting an interaction of PimM and RNA polymerase to cause transcription activation. Information content analysis of the 16 sequences protected in target promoters was used to deduce the structure of the PimM-binding site. This site displays dyad symmetry, spans 14 nucleotides, and adjusts to the consensus TVGGGAWWTCCCBA. Experimental validation of this binding site was performed by using synthetic DNA duplexes. Binding of PimM to the promoter region of one of the polyketide synthase genes from the Streptomyces nodosus amphotericin cluster containing the consensus binding site was also observed, thus proving the applicability of the findings reported here to other antifungal polyketides.