Two Delta9-stearic acid desaturases are required for Aspergillus nidulans growth and development

Unsaturated fatty acid perturbation combats emerging triazole antifungal resistance in the human fungal pathogen Aspergillus fumigatus

Contemporary antifungal therapies utilized to treat filamentous fungal infections are inhibited by intrinsic and emerging drug resistance. Consequently, there is an urgent need to develop novel antifungal compounds that are effective against drug-resistant filamentous fungi. Here, we utilized an Aspergillus fumigatus cell-based high-throughput screen to identify small molecules with antifungal activity that also potentiated triazole activity. The screen identified 16 hits with promising activity against A. fumigatus. A nonspirocyclic piperidine, herein named MBX-7591, exhibited synergy with triazole antifungal drugs and activity against pan-azole-resistant A. fumigatus isolates. MBX-7591 has additional potent activity against Rhizopus species and CO2-dependent activity against Cryptococcus neoformans. Chemical, genetic, and biochemical mode of action analyses revealed that MBX-7591 increases cell membrane saturation by decreasing oleic acid content. MBX-7591 has low toxicity in vivo and shows good efficacy in decreasing fungal burden in a murine model of invasive pulmonary aspergillosis. Taken together, our results suggest MBX-7591 is a promising hit with a novel mode of action for further antifungal drug development to combat the rising incidence of triazole-resistant filamentous fungal infections.IMPORTANCEThe incidence of infections caused by fungi continues to increase with advances in medical therapies. Unfortunately, antifungal drug development has not kept pace with the incidence and importance of fungal infections, with only three major classes of antifungal drugs currently available for use in the clinic. Filamentous fungi, also called molds, are particularly recalcitrant to contemporary antifungal therapies. Here, a recently developed Aspergillus fumigatus cell reporter strain was utilized to conduct a high-throughput screen to identify small molecules with antifungal activity. An emphasis was placed on small molecules that potentiated the activity of contemporary triazole antifungals and led to the discovery of MBX-7591. MBX-7591 potentiates triazole activity against drug-resistant molds such as A. fumigatus and has activity against Mucorales fungi. MBX-7591's mode of action involves inhibiting the conversion of saturated to unsaturated fatty acids, thereby impacting fungal membrane integrity. MBX-7591 is a novel small molecule with antifungal activity poised for lead development.

Comprehensive analysis of the composition of the major phospholipids during the asexual life cycle of the filamentous fungus Aspergillus nidulans

Filamentous fungi undergo significant cellular morphological changes during their life cycle. It has recently been reported that deletions of genes that are involved in phospholipid synthesis led to abnormal hyphal morphology and differentiation in filamentous fungi. Although these results suggest the importance of phospholipid balance in their life cycle, comprehensive analyses of cellular phospholipids are limited. Here, we performed lipidomic analysis of A. nidulans during morphological changes in a liquid medium and of colonies on a solid medium. We observed that the phospholipid composition and transcription of the genes involved in phospholipid synthesis changed dynamically during the life cycle. Specifically, the levels of phosphatidylethanolamine, and highly unsaturated phospholipids increased during the establishment of polarity. Furthermore, we demonstrated that the phospholipid composition in the hyphae at colony margins is similar to that during conidial germination. Furthermore, we demonstrated that common and characteristic phospholipid changes occurred during germination in A. nidulans and A. oryzae, and that species-specific changes also occurred. These results suggest that the exquisite regulation of phospholipid composition is crucial for the growth and differentiation of filamentous fungi.

The Heat Shock Transcription Factor HsfA Plays a Role in Membrane Lipids Biosynthesis Connecting Thermotolerance and Unsaturated Fatty Acid Metabolism in Aspergillus fumigatus

Thermotolerance is a remarkable virulence attribute of Aspergillus fumigatus, but the consequences of heat shock (HS) to the cell membrane of this fungus are unknown, although this structure is one of the first to detect changes in ambient temperature that imposes on the cell a prompt adaptative response. Under high-temperature stress, fungi trigger the HS response controlled by heat shock transcription factors, such as HsfA, which regulates the expression of heat shock proteins. In yeast, smaller amounts of phospholipids with unsaturated fatty acid (FA) chains are synthesized in response to HS, directly affecting plasma membrane composition. The addition of double bonds in saturated FA is catalyzed by Δ9-fatty acid desaturases, whose expression is temperature-modulated. However, the relationship between HS and saturated/unsaturated FA balance in membrane lipids of A. fumigatus in response to HS has not been investigated. Here, we found that HsfA responds to plasma membrane stress and has a role in sphingolipid and phospholipid unsaturated biosynthesis. In addition, we studied the A. fumigatus Δ9-fatty acid desaturase sdeA and discovered that this gene is essential and required for unsaturated FA biosynthesis, although it did not directly affect the total levels of phospholipids and sphingolipids. sdeA depletion significantly sensitizes mature A. fumigatus biofilms to caspofungin. Also, we demonstrate that hsfA controls sdeA expression, while SdeA and Hsp90 physically interact. Our results suggest that HsfA is required for the adaptation of the fungal plasma membrane to HS and point out a sharp relationship between thermotolerance and FA metabolism in A. fumigatus. IMPORTANCE Aspergillus fumigatus causes invasive pulmonary aspergillosis, a life-threatening infection accounting for high mortality rates in immunocompromised patients. The ability of this organism to grow at elevated temperatures is long recognized as an essential attribute for this mold to cause disease. A. fumigatus responds to heat stress by activating heat shock transcription factors and chaperones to orchestrate cellular responses that protect the fungus against damage caused by heat. Concomitantly, the cell membrane must adapt to heat and maintain physical and chemical properties such as the balance between saturated/unsaturated fatty acids. However, how A. fumigatus connects these two physiological responses is unclear. Here, we explain that HsfA affects the synthesis of complex membrane lipids such as phospholipids and sphingolipids and controls the enzyme SdeA, which produces monounsaturated fatty acids, raw material for membrane lipids. These findings suggest that forced dysregulation of saturated/unsaturated fatty acid balance might represent novel strategies for antifungal therapy.

Regulators of the Asexual Life Cycle of Aspergillus nidulans

The genus Aspergillus, one of the most abundant airborne fungi, is classified into hundreds of species that affect humans, animals, and plants. Among these, Aspergillus nidulans, as a key model organism, has been extensively studied to understand the mechanisms governing growth and development, physiology, and gene regulation in fungi. A. nidulans primarily reproduces by forming millions of asexual spores known as conidia. The asexual life cycle of A. nidulans can be simply divided into growth and asexual development (conidiation). After a certain period of vegetative growth, some vegetative cells (hyphae) develop into specialized asexual structures called conidiophores. Each A. nidulans conidiophore is composed of a foot cell, stalk, vesicle, metulae, phialides, and 12,000 conidia. This vegetative-to-developmental transition requires the activity of various regulators including FLB proteins, BrlA, and AbaA. Asymmetric repetitive mitotic cell division of phialides results in the formation of immature conidia. Subsequent conidial maturation requires multiple regulators such as WetA, VosA, and VelB. Matured conidia maintain cellular integrity and long-term viability against various stresses and desiccation. Under appropriate conditions, the resting conidia germinate and form new colonies, and this process is governed by a myriad of regulators, such as CreA and SocA. To date, a plethora of regulators for each asexual developmental stage have been identified and investigated. This review summarizes our current understanding of the regulators of conidial formation, maturation, dormancy, and germination in A. nidulans.

Steryl Acetyl Hydrolase 1 (BbSay1) Links Lipid Homeostasis to Conidiogenesis and Virulence in the Entomopathogenic Fungus Beauveria bassiana

Beauveria bassiana, as a well-studied entomopathogenic fungus, has a great potential for the biological control of insect pests. Lipid metabolism has been linked to the life cycle of B. bassiana; however, the underlying mechanisms remain unknown. In this study, a homolog of yeast steryl acetyl hydrolase 1 (Say1) was functionally characterized. The loss of B. bassianaSAY1 (BbSAY1) impaired the lipid homeostasis in conidia, with a significant reduction in oleic acid content. The ΔBbsay1 mutant strain displayed anelevated accumulation of lipid bodies and aweakened membrane permeability. As for phenotypic aspects, gene loss resulted in significant defects in germination, conidiation, and virulence. Our findings highlight that Say1, involved in lipid homeostasis, contributes to the cytomembrane integrity, development, and virulence in B. bassiana.

Long-chain unsaturated fatty acids are involved in the viability and itraconazole susceptibility of Aspergillus fumigatus

The prevalence of invasive aspergillosis with azole resistance is increasing, but the mechanisms underlying the development of resistance and treatment strategies are still limited. The present work is focused on finding a relationship between long-chain unsaturated fatty acids (LCUFAs), Aspergillus fumigatus development, and antifungal resistance. The effects of LCUFAs on antifungal agents in vitro were determined, and the stearic acid desaturase gene (sdeA) of A. fumigatus was characterized. In in vitro antifungal tests, LCUFAs antagonized the antifungal activity of itraconazole by extracting it from media, thereby preventing it from entering cells. The OA auxotrophic phenotype caused by an sdeA deletion confirmed that SdeA was required for OA biosynthesis in A. fumigatus. Furthermore, several low-level sdeA-overexpressing mutants with impaired vegetative growth phenotypes were successfully constructed. Additionally, an sdeA-overexpressing mutant, OEsdeA-5, showed lowered sensitivity levels to itraconazole. Moreover, RNA sequencing of OEsdeA-5 revealed that the altered gene-expression pattern. Through targeted metabolomics, decreased palmitic acid and stearic acid contents, accompanied by higher palmitoleic acid, margaroleic acid, and OA production levels, were found in OEsdeA-5. This study provides a novel insight of understanding of azole resistance and a potential target for drug development.

Enhancement of the productivity of free dihomo-γ-linolenic acid via co-overexpression of elongase and two desaturase genes in Aspergillus oryzae

Free dihomo-γ-linolenic acid (DGLA), a polyunsaturated free fatty acid (FFA), is a precursor of the eicosanoid prostaglandin E1 and is expected to be a source material for artificial production. We previously constructed the Aspergillus oryzae mutant strain ARA1 that produced free DGLA from the disruptant of faaA, an acyl-CoA synthetase gene, where FFA productivity increased by 9.2-fold compared with that of the wild-type strain. Here, we aimed to achieve enhancement of free DGLA productivity. Because saturated FFAs, such as palmitic acid and stearic acid, accounted for about 45% and 25% of total FFAs produced by ARA1, respectively, we used a strategy to facilitate elongation and desaturation of these FFAs to oleic acid and linoleic acid by overexpressing genes encoding elongase, Δ9-desaturase, and Δ12-desaturase originally expressed in A. oryzae. Ten genes were predicted to encode desaturases, and their overexpression DNA constructs were introduced into ARA1. AO090001000224 and AO090011000488 facilitated Δ12-desaturation and Δ9-desaturation most, respectively, following overexpression. Next, ARA1 strain was modified to DGLA1cre strain for producing free DGLA as a final product, and co-overexpression of these two desaturase genes was then introduced to DGLA1cre. Moreover, single overexpression of two genes predicted to encode elongases was additionally introduced, and only AO090003000572 facilitated elongation. Consequently, in the co-overexpression mutant of AO090001000224, AO090011000488, and AO090003000572, free DGLA content ratio increased by 1.8-fold from ARA1 to 14.5%, and the productivity also increased by 1.8-fold to 0.086 mmol/g-dry-cell-weight. The yield was 284 mg/L. These findings provided insights into strategies for improving microbial production of polyunsaturated FFAs.

Overexpression of OLE1 Enhances Cytoplasmic Membrane Stability and Confers Resistance to Cadmium in Saccharomyces cerevisiae

The heavy metal cadmium is widely used and released into the environment, posing a severe threat to crops and humans. Saccharomyces cerevisiae is one of the most commonly used organisms in the investigation of environmental metal toxicity. We investigated cadmium stress and the adaptive mechanisms of yeast by screening a genome-wide essential gene overexpression library. A candidate gene, OLE1, encodes a delta-9 desaturase and was associated with high anti-cadmium-stress activity. The results demonstrated that the expression of OLE1 was positively correlated with cadmium stress tolerance and induction was independent of Mga2p and Spt23p (important regulatory factors for OLE1). Moreover, in response to cadmium stress, cellular levels of monounsaturated fatty acids were increased. The addition of exogenous unsaturated fatty acids simulated overexpression of OLE1, leading to cadmium resistance. Such regulation of OLE1 in the synthesis of unsaturated fatty acids may serve as a positive feedback mechanism to help cells counter the lipid peroxidation and cytoplasmic membrane damage caused by cadmium.

IMPORTANCE

A S. cerevisiae gene encoding a delta-9 desaturase, OLE1, was associated with high anti-cadmium-stress activity. The data suggest that the regulation of OLE1 in the synthesis of unsaturated fatty acids may serve as a positive feedback mechanism to help yeast cells counter the lipid peroxidation and cytoplasmic membrane damage caused by cadmium. The discovery of OLE1 involvement in membrane stability may indicate a novel defense strategy against cadmium stress.

Characterization of the Far Transcription Factor Family in Aspergillus flavus

Metabolism of fatty acids is a critical requirement for the pathogenesis of oil seed pathogens including the fungus Aspergillus flavus Previous studies have correlated decreased ability to grow on fatty acids with reduced virulence of this fungus on host seed. Two fatty acid metabolism regulatory transcription factors, FarA and FarB, have been described in other filamentous fungi. Unexpectedly, we find A. flavus possesses three Far homologs, FarA, FarB, and FarC, with FarA and FarC showing a greater protein similarity to each other than FarB. farA and farB are located in regions of colinearity in all Aspergillus spp. sequenced to date, whereas farC is limited to a subset of species where it is inserted in an otherwise colinear region in Aspergillus genomes. Deletion and overexpression (OE) of farA and farB, but not farC, yielded mutants with aberrant growth patterns on specific fatty acids as well as altered expression of genes involved in fatty acid metabolism. Marked differences included significant growth defects of both ∆farA and ∆farB on medium-chain fatty acids and decreased growth of OE::farA on unsaturated fatty acids. Loss of farA diminished expression of mitochondrial β-oxidation genes whereas OE::farA inhibited expression of genes involved in unsaturated fatty acid catabolism. FarA also positively regulated the desaturase genes required to generate polyunsaturated fatty acids. Aflatoxin production on toxin-inducing media was significantly decreased in the ∆farB mutant and increased in the OE::farB mutant, with gene expression data supporting a role for FarB in tying β-oxidation processes with aflatoxin accumulation.

The histone acetyltransferase GcnE (GCN5) plays a central role in the regulation of Aspergillus asexual development

Acetylation of histones is a key regulatory mechanism of gene expression in eukaryotes. GcnE is an acetyltransferase of Aspergillus nidulans involved in the acetylation of histone H3 at lysine 9 and lysine 14. Previous works have demonstrated that deletion of gcnE results in defects in primary and secondary metabolism. Here we unveil the role of GcnE in development and show that a ∆gcnE mutant strain has minor growth defects but is impaired in normal conidiophore development. No signs of conidiation were found after 3 days of incubation, and immature and aberrant conidiophores were found after 1 week of incubation. Centroid linkage clustering and principal component (PC) analysis of transcriptomic data suggest that GcnE occupies a central position in Aspergillus developmental regulation and that it is essential for inducing conidiation genes. GcnE function was found to be required for the acetylation of histone H3K9/K14 at the promoter of the master regulator of conidiation, brlA, as well as at the promoters of the upstream developmental regulators of conidiation flbA, flbB, flbC, and flbD (fluffy genes). However, analysis of the gene expression of brlA and the fluffy genes revealed that the lack of conidiation originated in a complete absence of brlA expression in the ∆gcnE strain. Ectopic induction of brlA from a heterologous alcA promoter did not remediate the conidiation defects in the ∆gcnE strain, suggesting that additional GcnE-mediated mechanisms must operate. Therefore, we conclude that GcnE is the only nonessential histone modifier with a strong role in fungal development found so far.

Characterization of three Δ9-fatty acid desaturases with distinct substrate specificity from an oleaginous fungus Cunninghamella echinulata

In oleaginous fungus Cunninghamella echinulata, Δ9-fatty acid desaturase introduces the first double bond into a saturated fatty acid. Three distinct genes, designated as d9dma, d9dmb and d9dmc, all encoding putative Δ9-fatty acid desaturases were isolated from this strain. The predicted proteins showed 79-87 % identity to other fungal Δ9-fatty acid desaturases. They all contain three conserved histidine boxes, C-terminal cytochrome b 5 fusion and four transmembrane domains characteristic of Δ9-desaturase. Each putative Δ9-desaturase gene from C. echinulata was able to complement the ole1 mutation in Saccharomyces cerevisiae L8-14C through heterologous expression. Analysis of the fatty acid composition of the transgenic yeast revealed that the conversion rates of 16:0 and 18:0 by D9DMA were obviously higher than those of D9DMB and D9DMC. In addition, D9DMA, D9DMB and D9DMC all had a substrate preference for 18:0 compared with 16:0. Of interest, D9DMA could saturate 12:0, 14:0, 16:0, 17:0, 18:0 and 20:0, while D9DMB saturated 14:0, 16:0, 17:0, 18:0 and 20:0. We also noticed that the transcriptional level of d9dma in C. echinulata was stimulated by cell growth but not by decline in temperature. In contrast, expression of d9dmb and d9dmc was regulated by neither cell growth nor decline in temperature in this strain.

Disparate proteome responses of pathogenic and nonpathogenic aspergilli to human serum measured by activity-based protein profiling (ABPP)

Aspergillus fumigatus is the primary pathogen causing the devastating pulmonary disease Invasive Aspergillosis in immunocompromised individuals. There is high genomic synteny between A. fumigatus and closely related rarely pathogenic Neosartorya fischeri and Aspergillus clavatus genomes. We applied activity-based protein profiling to compare unique or overexpressed activity-based probe-reactive proteins of all three fungi over time in minimal media growth and in response to human serum. We found 360 probe-reactive proteins exclusive to A. fumigatus, including known virulence associated proteins, and 13 proteins associated with stress response exclusive to A. fumigatus culture in serum. Though the fungi are highly orthologous, A. fumigatus has a significantly greater number of ABP-reactive proteins across varied biological process. Only 50% of expected orthologs of measured A. fumigatus reactive proteins were observed in N. fischeri and A. clavatus. Activity-based protein profiling identified a number of processes that were induced by human serum in A. fumigatus relative to N. fischeri and A. clavatus. These included actin organization and assembly, transport, and fatty acid, cell membrane, and cell wall synthesis. Additionally, signaling proteins regulating vegetative growth, conidiation, and cell wall integrity, required for appropriate cellular response to external stimuli, had higher activity-based probe-protein reaction over time in A. fumigatus and N. fisheri, but not in A. clavatus. Together, we show that measured proteins and physiological processes identified solely or significantly over-represented in A. fumigatus reveal a unique adaptive response to human protein not found in closely related, but rarely pathogenic aspergilli. These unique activity-based probe-protein responses to culture condition may reveal how A. fumigatus initiates pulmonary invasion leading to Invasive Aspergillosis.

Knockout of fatty acid desaturase genes in Pichia pastoris GS115 and its effect on the fatty acid biosynthesis and physiological consequences

Unsaturated fatty acids (UFAs), including oleic acid (OA, C18:1n-9), linoleic acid (LA, C18:2n-6) and α-linolenic acid (ALA, C18:3n-3), are major components of membrane lipids in Pichia pastoris GS115. In order to clarify the biosynthesis pathway of UFAs on the molecular level and investigate their possible roles in growth and development of this strain, we here report modified strains with disrupted desaturase gene by homologous recombination. Gas chromatography analysis of fatty acid composition in the corresponding mutants confirmed that ∆(12)-desaturase encoded by Fad12 was responsible for the formation of LA, and ALA was synthesized by ∆(15)-desaturase encoded by Fad15. Simultaneous deletion of Fad9A and Fad9B was lethal and supplementation of OA could restore growth, indicating that possibly both Fad9A and Fad9B encoded ∆(9)-desaturase that converted SA into OA. Phenotypic analysis demonstrated that wild type and Fad15 mutant grew at almost the same rate, Fad12 mutant grew much slower than these two strains. Moreover, OA was positively correlated to cold tolerance and ethanol tolerance of GS115, whereas LA and ALA did not affect cold tolerance and ethanol tolerance of it. In addition, we showed that tolerance of GS115 to high concentration of methanol was independent of these three UFAs.

Cheese can reduce indexes that estimate fatty acid desaturation. Results from the Oslo Health Study and from experiments with human hepatoma cells

Previously, cheese intake was shown to be inversely related to serum triglycerides, raising the possibility that cheese might inhibit triglyceride synthesis, which is governed by fatty acid desaturases. Therefore, analyses were done to study whether cheese intake was associated with indexes that reflect fatty acid desaturation in 121 healthy ethnic Norwegians aged 40-45 years, a subsample from the Oslo Health Study (N = 18 777). Experiments with human hepatoma cells (HepG2) were done to clarify whether cheese might have a causal effect on desaturases. Fatty acid distribution in lipids of human sera and HepG2 cells was determined by gas chromatography. Δ9-Desaturase was estimated by the (16:1,n-7)/(16:0) and (18:1,n-9)/(18:0) ratios, abbreviated ds9_1 and ds9_2, and Δ5-desaturase (ds5) by the (20:4,n-6)/(18:2,n-6) ratio. Correlation, ANOVA, and multiple linear regression models were used to study associations. Oslo Health Study: Subjects with cheese intake >4-6 times per week had 33% lower ds9_1 and 16% lower ds5 than subjects with intake ≤ 4-6 times per week. The cheese intake vs. ds5 association prevailed when adjusting for sex, time since last meal, fatty fish, vegetables, fruit-berries, fruit juice, cod liver oil, coffee, alcohol, body mass index, physical activity, length of education, and smoking. HepG2 cells: An ethanol extract of Jarlsberg cheese lowered the desaturase indexes. Inhibition of ds9_1 increased with increasing amount cheese extract added. Thus, cheese may contain inhibitors of desaturases, thereby providing an explanation for the previously reported negative association between cheese intake and triglycerides.

Dual transcriptional profiling of a bacterial/fungal confrontation: Collimonas fungivorans versus Aspergillus niger

Interactions between bacteria and fungi cover a wide range of incentives, mechanisms and outcomes. The genus Collimonas consists of soil bacteria that are known for their antifungal activity and ability to grow at the expense of living fungi. In non-contact confrontation assays with the fungus Aspergillus niger, Collimonas fungivorans showed accumulation of biomass concomitant with inhibition of hyphal spread. Through microarray analysis of bacterial and fungal mRNA from the confrontation arena, we gained new insights into the mechanisms underlying the fungistatic effect and mycophagous phenotype of collimonads. Collimonas responded to the fungus by activating genes for the utilization of fungal-derived compounds and for production of a putative antifungal compound. In A. niger, differentially expressed genes included those involved in lipid and cell wall metabolism and cell defense, which correlated well with the hyphal deformations that were observed microscopically. Transcriptional profiles revealed distress in both partners: downregulation of ribosomal proteins and upregulation of mobile genetic elements in the bacteria and expression of endoplasmic reticulum stress and conidia-related genes in the fungus. Both partners experienced nitrogen shortage in each other's presence. Overall, our results indicate that the Collimonas/Aspergillus interaction is a complex interplay between trophism, antibiosis and competition for nutrients.

Two conserved Z9-octadecanoic acid desaturases in the red flour beetle, Tribolium castaneum

Z9 Desaturases catalyse the formation of a cis-unsaturated bond in the Δ9 position of the saturated fatty acids stearate and palmitate. They are considered essential enzymes in eukaryotic organisms as their Z9 unsaturated fatty acid products are required for homeostatic roles such as maintenance of membrane fluidity. Two putative Z9 acyl Coenzyme-A desaturase genes were identified in the red flour beetle, Tribolium castaneum, genome (TcasZ9desA and B) based on their similarity to acyl CoA-desaturases of other insects. TcasZ9desA and B share 75% nucleic acid sequence identity and appear to be functionally conserved; the genes were cloned and expressed in the yeast strain Saccharomyces cerevisiae (ole1); both genes complemented the yeast requirement for Z9 fatty acids and produced substantial quantities of Z9 desaturated products with a stearate>palmitate chain length preference. Quantitative PCR analysis of transcripts in RNA obtained from adult, larval and pupal stages of the beetles show TcasZ9desA and B are expressed at similar levels in all stages, with the pupal stage having the lowest expression.

Filamentous fungi for production of food additives and processing aids

Filamentous fungi are metabolically versatile organisms with a very wide distribution in nature. They exist in association with other species, e.g. as lichens or mycorrhiza, as pathogens of animals and plants or as free-living species. Many are regarded as nature's primary degraders because they secrete a wide variety of hydrolytic enzymes that degrade waste organic materials. Many species produce secondary metabolites such as polyketides or peptides and an increasing range of fungal species is exploited commercially as sources of enzymes and metabolites for food or pharmaceutical applications. The recent availability of fungal genome sequences has provided a major opportunity to explore and further exploit fungi as sources of enzymes and metabolites. In this review chapter we focus on the use of fungi in the production of food additives but take a largely pre-genomic, albeit a mainly molecular, view of the topic.

The effect of temperature on Natural Antisense Transcript (NAT) expression in Aspergillus flavus

Naturally occurring Antisense Transcripts (NATs) compose an emerging group of regulatory RNAs. These regulatory elements appear in all organisms examined, but little is known about global expression of NATs in fungi. Analysis of currently available EST sequences suggests that 352 cis NATs are present in Aspergillus flavus. An Affymetrix GeneChip microarray containing probes for these cis NATs, as well as all predicted genes in A. flavus, allowed a whole genome expression analysis of these elements in response to two ecologically important temperatures for the fungus. RNA expression analysis showed that 32 NATs and 2,709 genes were differentially expressed between 37 degrees C, the optimum temperature for growth, and 28 degrees C, the conducive temperature for the biosynthesis of aflatoxin (AF) and many other secondary metabolites. These NATs correspond to sense genes with diverse functions including transcription initiation, carbohydrate processing and binding, temperature sensitive morphogenesis, and secondary metabolism. This is the first report of a whole genome transcriptional analysis of NAT expression in a fungus.

Spore germination in Mortierella alpina is associated with a transient depletion of arachidonic acid and induction of fatty acid desaturase gene expression

Mortierella alpina is an oleaginous filamentous fungus whose vegetative mycelium is known to accumulate triglyceride oil containing large amounts of arachidonic acid (ARA 20:4, n - 6). We report that the spores of Mortierella alpina also contain a large proportion of ARA, comprising 50% of total fatty acid. Fatty acid desaturase genes were not expressed in dormant spores but were induced during germination, following a significant drop in the level of ARA (down from 50% of total fatty acid to 12%) prior to germ-tube emergence. We propose that ARA serves as a reserve supply of carbon and energy that is utilised during the early stages of spore germination in Mortierella alpina.

Oxylipins as developmental and host-fungal communication signals

Pathogenic microbes and their hosts have acquired complex signalling mechanisms to appraise themselves of the environmental milieu in the ongoing battle for survival. Several recent studies have implicated oxylipins as a novel class of host-microbe signalling molecules. Oxylipins represent a vast and diverse family of secondary metabolites that originate from the oxidation or further conversion of polyunsaturated fatty acids. Among the microbial oxylipins, the fungal oxylipins are best characterized and function as hormone-like signals that modulate the timing and balance between asexual and sexual spore development in addition to toxin production. Coupled with other studies that implicate a role for fungal oxylipins in pathogenesis by Aspergillus and Candida spp., these results suggest that host and microbial oxylipins might interfere with the metabolism, perception or signalling processes of each other.

Differential gene expression by Metarhizium anisopliae growing in root exudate and host (Manduca sexta) cuticle or hemolymph reveals mechanisms of physiological adaptation

Like many other fungal pathogens Metarhizium anisopliae is a facultative saprophyte with both soil-dwelling and insect pathogenic life-stages. In addition, as M. anisopliae traverses the cuticle and enters the hemolymph it must adapt to several different host environments. In this study, we used expressed sequence tags and cDNA microarray analyses to demonstrate that physiological adaptation by M. anisopliae to insect cuticle, insect hemolymph, bean root exudate (a model for life in the soil), and nutrient rich Sabouraud dextrose broth (SDB) involves different subsets of genes. Overall, expression patterns in cuticle and hemolymph clustered separately from expression patterns in root exudates and SDB, indicative of critical differences in transcriptional control during pathogenic and saprophytic growth. However, there were differences in gene expression between hemolymph and cuticle and these mostly involved perception mechanisms, carbon metabolism, proteolysis, cell surface properties, and synthesis of toxic metabolites. These differences suggest previously unsuspected stratagems of fungal pathogenicity that can be tested experimentally. Examples include the switch-off of cuticle-degrading proteases and a dramatic cell wall reorganization during growth in hemolymph.

Endogenous lipogenic regulators of spore balance in Aspergillus nidulans

The ability of fungi to produce both meiospores and mitospores has provided adaptive advantages in survival and dispersal of these organisms. Here we provide evidence of an endogenous mechanism that balances meiospore and mitospore production in the model filamentous fungus Aspergillus nidulans. We have discovered a putative dioxygenase, PpoC, that functions in association with a previously characterized dioxygenase, PpoA, to integrate fatty acid derived oxylipin and spore production. In contrast to PpoA, deletion of ppoC significantly increased meiospore production and decreased mitospore development. Examination of the PpoA and PpoC mutants indicate that this ratio control is associated with two apparent feedback loops. The first loop shows ppoC and ppoA expression is dependent upon, and regulates the expression of, nsdD and brlA, genes encoding transcription factors required for meiospore or mitospore production, respectively. The second loop suggests Ppo oxylipin products antagonistically signal the generation of Ppo substrates. These data support a case for a fungal "oxylipin signature-profile" indicative of relative sexual and asexual spore differentiation.

The fatty acid profile of vegetative Azotobacter vinelandii ATCC 12837: growth phase-dependence

Fatty acids of Azotobacter vinelandii ATCC 12837 were determined at various times during aerobic vegetative growth at 30 degrees C to provide baseline data for studying the effects of chemical agents on the organism's survival and fatty acid biosynthesis. Palmitate (16:0) was the highest at 36.7+/-4.3 mol% (mean+/-SD) after the first 5 h in fresh culture, decreasing slightly to 33.4+/-2.6 mol% at 49 h. The other fatty acids were therefore each normalized as a ratio of 16:0. At 5 h, as a ratio of 16:0, myristate (14:0) was 0.14+/-0.06, palmitoleate (16:1cDelta9-10) 0.13+/-0.06, oleate (18:1cDelta9-10) 0.21+/-0.12, cis-vaccenate (18:1cDelta11-12) 0.30+/-0.17 and stearate (18:0) 0.68+/-0.02. As the growth phase advanced to 49 h, 14:0 and 16:1cDelta9-10 increased, 18:1cDelta9-10 decreased and cis-vaccenate reciprocally increased, whereas 18:0 decreased. These suggest that the saturated fatty acid biosynthesis pathway yielded 16:0 and 18:0 in the 5-h lag period. By desaturation, 18:0 formed the unsaturated fatty acid (UFA) 18:1cDelta9-10. As the culture aged, the anaerobic UFA biosynthesis pathway formed 16:1cDelta9-10, which was elongated to 18:1cDelta11-12. These fatty acid alterations represent a homeoviscous adaptation, modulating the microbe's membrane lipid viscosity for optimal cellular function.

Deletion of the Delta12-oleic acid desaturase gene of a nonaflatoxigenic Aspergillus parasiticus field isolate affects conidiation and sclerotial development

AIMS

To investigate how linoleic acid affects conidial production and sclerotial development in a strictly mitotic Aspergillus parasiticus field isolate as related to improving biocompetitivity of atoxigenic Aspergillus species.

METHODS AND RESULTS

We disrupted A. parasiticusDelta12-oleic acid desaturase gene (odeA) responsible for the conversion of oleic acid to linoleic acid. We examined conidiation and sclerotial development of SRRC 2043 and three isogenic mutant strains deleted for the odeA gene (DeltaodeA), either with or without supplementing linoleic acid, on one complex potato dextrose agar (PDA) medium and on two defined media: nitrate-containing Czapek agar (CZ) and Cove's ammonium medium (CVN). The DeltaodeA mutants produced less conidia than the parental strain on all media. Linoleic acid supplementation (as sodium linoleate at 0.3 and 1.2 mg ml(-1)) restored the DeltaodeA conidial production comparable to or exceeding the unsupplemented parental level, and the effect was medium dependent, with the highest increase on CVN and the least on PDA. SRRC 2043 and the DeltaodeA mutants were unable to produce sclerotia on CVN. On unsupplemented PDA and CZ, DeltaodeA sclerotial mass was comparable to that of SRRC 2043, but sclerotial number increased significantly to two- to threefold. Supplementing linoleic acid to media, in general, tended to decrease wild type and DeltaodeA sclerotial mass and sclerotial number.

CONCLUSIONS

Linoleic acid stimulates conidial production but has an inhibitory effect on sclerotial development. The relationship between the two processes in A. parasiticus is complex and affected by multiple factors, such as fatty acid composition and nitrogen source.

SIGNIFICANCE AND IMPACT OF STUDY

Conditions that promote sclerotial development differ from those required to promote maximum conidial production. Manipulation of content and availability of linoleic acid at different fungal growth phases might optimize conidial and sclerotial production hence increasing the efficacy of biocompetitive Aspergillus species.

Characterization of the Aspergillus parasiticus Δ12-desaturase gene: a role for lipid metabolism in the Aspergillus–seed interaction

In the mycotoxigenic oilseed pathogens Aspergillus flavus and Aspergillus parasiticus and the model filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives act as important developmental signals that affect asexual conidiospore, sexual ascospore and/or sclerotial development. To dissect the relationship between lipid metabolism and fungal development, an A. parasiticus Δ12-desaturase mutant that was unable to convert oleic acid to linoleic acid and was thus impaired in polyunsaturated fatty acid biosynthesis was generated. The Δ12-desaturase mutant demonstrates delayed spore germination, a twofold reduction in growth, a reduced level of conidiation and complete loss of sclerotial development, compared to the wild-type. Host colonization is impaired, as reflected by a decrease in conidial production on live peanut and corn seed by the mutant compared to the wild-type. Similarly, the previously isolated A. nidulans Δ12-desaturase mutant has reduced colonization capabilities compared to the wild-type. Therefore, desaturation mutants display a key requisite that affords a genetic solution to oilseed crop contamination by mycotoxigenic Aspergillus species: a reduction in the production of conidia, the infectious particle of the pathogenic aspergilli.