Regulation of the expression of the isocitrate lyase gene (acuD) of Aspergillus nidulans

Comparative metabolomics of Phialemonium curvatum as an omnipotent fungus cultivated on crude palm oil versus glucose

BACKGROUND

Sugars and triglycerides are common carbon sources for microorganisms. Nonetheless, a systematic comparative interpretation of metabolic changes upon vegetable oil or glucose as sole carbon source is still lacking. Selected fungi that can grow in acidic mineral salt media (MSM) with vegetable oil had been identified recently. Hence, this study aimed to investigate the overall metabolite changes of an omnipotent fungus and to reveal changes at central carbon metabolism corresponding to both carbon sources.

RESULTS

Targeted and non-targeted metabolomics for both polar and semi-polar metabolites of Phialemonium curvatum AWO2 (DSM 23903) cultivated in MSM with palm oil (MSM-P) or glucose (MSM-G) as carbon sources were obtained. Targeted metabolomics on central carbon metabolism of tricarboxylic acid (TCA) cycle and glyoxylate cycle were analysed using LC-MS/MS-TripleQ and GC-MS, while untargeted metabolite profiling was performed using LC-MS/MS-QTOF followed by multivariate analysis. Targeted metabolomics analysis showed that glyoxylate pathway and TCA cycle were recruited at central carbon metabolism for triglyceride and glucose catabolism, respectively. Significant differences in organic acids concentration of about 4- to 8-fold were observed for citric acid, succinic acid, malic acid, and oxaloacetic acid. Correlation of organic acids concentration and key enzymes involved in the central carbon metabolism was further determined by enzymatic assays. On the other hand, the untargeted profiling revealed seven metabolites undergoing significant changes between MSM-P and MSM-G cultures.

CONCLUSIONS

Overall, this study has provided insights on the understanding on the effect of triglycerides and sugar as carbon source in fungi global metabolic pathway, which might become important for future optimization of carbon flux engineering in fungi to improve organic acids production when vegetable oil is applied as the sole carbon source.

Differential Metabolism of a Two-Carbon Substrate by Members of the Paracoccidioides Genus

The genus Paracoccidioides comprises known fungal pathogens of humans and can be isolated from different infection sites. Metabolic peculiarities in different members of the Paracoccidioides led us to perform proteomic studies in the presence of the two-carbon molecule acetate, which predominates in the nutrient-poor environment of the phagosome. To investigate the expression rates of proteins of different members of Paracoccidioides, including one isolate of P. lutzii (Pb01) and three isolates of P. brasiliensis (Pb03, Pb339, and PbEPM83), using sodium acetate as a carbon source, proteins were quantified using label-free and data-independent liquid chromatography-mass spectrometry. Protein profiles of the isolates were statistically analyzed, revealing proteins that were differentially expressed when the fungus was cultivated in a non-preferential carbon source rather than glucose. A total of 1,160, 1,211, 1,280, and 1,462 proteins were reproducibly identified and relatively quantified in P. lutzii and the P. brasiliensis isolates Pb03, Pb339, and PbEPM83, respectively. Notably, 526, 435, 744, and 747 proteins were differentially expressed among P. lutzii and the P. brasiliensis isolates Pb03, Pb339, and PbEPM83, respectively, with a fold-change equal to or higher than 1.5. This analysis revealed that reorganization of metabolism occurred through the induction of proteins related to gluconeogenesis, glyoxylic/glyoxylate cycle, response to stress, and degradation of amino acids in the four isolates. The following differences were observed among the isolates: higher increases in the expression levels of proteins belonging to the TCA and respiratory chain in PbEPM83 and Pb01; increase in ethanol production in Pb01; utilization of cell wall components for gluconeogenesis in Pb03 and PbEPM83; and increased β-oxidation and methylcitrate cycle proteins in Pb01and PbEPM83. Proteomic profiles indicated that the four isolates reorganized their metabolism in different manners to use acetate as a carbon source.

Phosphorylation is the major mechanism regulating isocitrate lyase activity in Paracoccidioides brasiliensis yeast cells

The glyoxylate cycle plays an essential role for anaplerosis of oxaloacetate during growth of microorganisms on carbon sources such as acetate or fatty acids and has been shown to contribute to virulence of several pathogens. Here, we investigated the transcriptional and post-translational regulation of the glyoxylate cycle key enzyme isocitrate lyase (PbICL) in the human pathogenic fungus Paracoccidioides brasiliensis. Although sequence analyses on fungal isocitrate lyases revealed a high phylogenetic conservation, their regulation seems to differ significantly. Closely related Aspergillus species regulate the glyoxylate cycle at the transcriptional level, whereas Pbicl was constitutively expressed in yeast cells. However, only low PbICL activity was detected when cells were grown in the presence of glucose. Two-dimensional gel analyses with subsequent antibody hybridization revealed constitutive production of PbICL, but low PbICL activity on glucose coincided with extensive protein phosphorylation. Since an in vitro dephosphorylation of PbICL from glucose grown cells strongly increased ICL activity and resembled the phosphorylation pattern of highly active acetate grown cells, post-translational modification seems the main mechanism regulating PbICL activity in yeast cells. In agreement, a transfer of yeast cells from glucose to acetate medium increased PbICL activity without requirement of de novo protein synthesis. Thus, inactivation of PbICL by phosphorylation is reversible, denoting a new strategy for the rapid adaptation to changing environmental conditions.

Differential expression of citA gene encoding the mitochondrial citrate synthase of Aspergillus nidulans in response to developmental status and carbon sources

As an extension of our previous studies on the mitochondrial citrate synthase of Aspergillus nidulans and cloning of its coding gene (citA), we analyzed differential expression of citA in response to the progress of development and change of carbon source. The cDNA consisted of 1,700 nucleotides and was predicted to encode a 474-amino acid protein. By comparing the cDNA sequence with the corresponding genomic sequence, we confirmed that citA gene contains 7 introns and that its transcription starts at position -26 (26-nucleotide upstream from the initiation codon). Four putative CreA binding motifs and three putative stress-response elements (STREs) were found within the 1.45-kb citA promoter region. The mode of citA expression was examined by both Northern blot and confocal microscopy using green fluorescent protein (sGFP) as a vital reporter. During vegetative growth and asexual development, the expression of citA was ubiquitous throughout the whole fungal body including mycelia and conidiophores. During sexual development, the expression of citA was quite strong in cleistothecial shells, but significantly weak in the content of cleistothecia including ascospores. Acetate showed a strong inductive effect on citA expression, which is subjected to carbon catabolite repression (CCR) caused by glucose. The recombinant fusion protein CitA(40)::sGFP (sGFP containing the 40-amino acid N-terminal segment of CitA) was localized into mitochondria, which supports that a mitochondrial targeting signal is included within the 40-amino acid N-terminal segment of CitA.

Fungal metabolism in host niches

Invasive fungal infections of immunocompromised patients cause major problems in modern medicine and only a limited number of effective antifungals are available, making the identification of new drug targets a priority. The inhibition of primary metabolism represents a promising therapeutic strategy, but a better understanding of the metabolic processes during pathogenesis is required. Infection, invasion and maintenance within a host are very dynamic events and fungal metabolism has to adapt to these changes. Glycolysis, gluconeogenesis and starvation all contribute to successful host colonisation, but the temporal and spatial resolution of their specific importance is poorly understood. Knowledge about the metabolic requirements of pathogenic fungi during infection could lead to the identification of new classes of antifungals, which allow the treatment of otherwise life-threatening infections.

Physiological characterisation of acuB deletion in Aspergillus niger

The acuB gene of Aspergillus niger is an ortholog of facB in Aspergillus nidulans. Under carbon-repression conditions, facB is repressed, thereby preventing acetate metabolism when the repressing carbon source is present. Even though facB is reported to be repressed directly by CreA, it is believed that a basal level of FacB activity exists under glucose-repressive conditions. In the present study, the effect of deletion of acuB on the physiology of A. niger was assessed. Differences in organic acid and acetate production, enzyme activities and extracellular amino and non-amino organic acid production were determined under glucose-repressing and -derepressing conditions. Furthermore, consumption of alternative carbon sources (e.g. xylose, citrate, lactate and succinate) was investigated. It was shown that AcuB has pleiotropic effects on the physiology of A. niger. The results indicate that metabolic pathways that are not directly involved in acetate metabolism are influenced by acuB deletion. Clear differences in organic acid consumption and production were detected between the acuB and reference strain. However, the hypothesis that AcuB is responsible for basal AcuA activity necessary for activation of acetate metabolic pathways, even during growth on glucose, could not be confirmed. The experiments demonstrated that also when acuB was deleted, no acetate was formed. Therefore, AcuB cannot be the only activator of AcuA, and another control mechanism has to be available for activating AcuA.

Characterization of an acyl-CoA: carboxylate CoA-transferase from Aspergillus nidulans involved in propionyl-CoA detoxification

Filamentous fungi metabolize toxic propionyl-CoA via the methylcitrate cycle. Disruption of the methylcitrate synthase gene leads to an accumulation of propionyl-CoA and attenuates virulence of Aspergillus fumigatus. However, addition of acetate, but not ethanol, to propionate-containing medium strongly reduces the accumulation of propionyl-CoA and restores growth of the methylcitrate synthase mutant. Therefore, the existence of a CoA-transferase was postulated, which transfers the CoASH moiety from propionyl-CoA to acetate and, thereby, detoxifying the cell. In this study, we purified the responsible protein from Aspergillus nidulans and characterized its biochemical properties. The enzyme used succinyl-, propionyl- and acetyl-CoA as CoASH donors and the corresponding acids as acceptor molecules. Although the protein displayed high sequence similarity to acetyl-CoA hydrolases this activity was hardly detectable. We additionally identified and deleted the coding DNA sequence of the CoA-transferase. The mutant displayed weak phenotypes in the presence of propionate and behaved like the wild type when no propionate was present. However, when a double-deletion mutant defective in both methylcitrate synthase and CoA-transferase was constructed, the resulting strain was unable to grow on media containing acetate and propionate as sole carbon sources, which confirmed the in vivo activity of the CoA-transferase.

The role of the glyoxylate cycle in the symbiotic fungus Tuber borchii: expression analysis and subcellular localization

Expression profiles of isocitrate lyase (TbICL), malate synthase (TbMLS) and fructose-1,6-bisphosphatase (TbFBP) from the mycorrhizal ascomycete Tuber borchii were investigated by real-time RT-PCR in fruiting bodies at different stages of maturation. In addition, a time course experiment was set up to determine how the transcription profile of TbICL, TbMLS and TbFBP in axenic-grown mycelia is affected by different carbon sources. The transcript levels of the three genes in the fruiting bodies were all much higher than those measured in the vegetative stage. The investigation on axenic-grown mycelia revealed that the main positive regulator of TbICL and TbMLS gene expression is the availability of acetate and ethanol, while oleic acid is a too complex substrate for the limited degradative capacities of T. borchii. Immunolabelling on axenic-grown mycelia showed a co-localization of TbICL and the peroxisomal marker protein FOX2. This result demonstrated that in T. borchii ICL is compartmentalized in peroxisomes. The high induction of TbICL, TbMLS and TbFBP transcription and the translocation of lipids in fruiting bodies let us hypothesize that glyoxylate cycle and gluconeogenesis are key metabolic pathways in the recycling of existing cell material and the channelling towards the biosynthesis of new cell components during the maturation of fruiting bodies.

Ability to grow on lipids accounts for the fully virulent phenotype in neutropenic mice of Aspergillus fumigatus null mutants in the key glyoxylate cycle enzymes

Incidence and mortality rates of invasive aspergillosis clearly indicate the need of novel antifungals to treat patients suffering from this disease. Fungal proteins playing a crucial role in pathogenesis and with no orthologue in human cells are considered as primary therapeutic targets for the development of new antifungals with a high therapeutic index, one of the major drawbacks of the standard antifungal therapy, so far. In this work, we have analyzed the role in pathogenesis of the key enzymes of the Aspergillus fumigatus glyxoxylate cycle, isocitrate lyase and malate synthase, two possible candidates to primary therapeutic targets in this fungus. Deletion strains lacking isocitrate lyase (DeltaacuD strains) or malate synthase (DeltaacuE mutants) were constructed in this work. The Neurospora crassa pyr-4 gene was used as the replacing marker in gene deletion experiments. The pathogenicities of DeltaacuD and DeltaacuE mutants were tested in neutropenic mice and compared with those of two reference wild-type isolates A. fumigatus 237 and A. fumigatus 293. Interestingly, virulence and cytological studies clearly indicated the dispensability of the A. fumigatus glyoxylate cycle for pathogenicity. In addition, these results suggested the suitability of the pyr-4 gene as a valuable replacing marker for virulence studies in this fungus, a fact that was further confirmed by gene expression analyses. Finally, growth tests were performed to investigate the germination and growth of the DeltaacuD and DeltaacuE strains in nutrient deprivation environments, resembling the conditions that A. fumigatus conidia face after phagocytosis. Results obtained in this work strongly suggest that the ability to grow on lipids (triglycerides) of A. fumigatus isocitrate lyase and malate synthase deletion strains accounts for their fully virulent phenotype.

Developmental regulation of the glyoxylate cycle in the human pathogen Penicillium marneffei

Penicillium marneffei is a thermally dimorphic opportunistic human pathogen with a saprophytic filamentous hyphal form at 25 degrees C and a pathogenic unicellular yeast form at 37 degrees C. During infection. P. marneffei yeast cells exist intracellularly in macrophages. To cope with nutrient deprivation during the infection process, a number of pathogens employ the glyoxylate cycle to utilize fatty acids as carbon sources. The genes which constitute this pathway have been implicated in pathogenesis. To investigate acetate and fatty acid utilization, the acuD gene encoding a key glyoxylate cycle enzyme (isocitrate lyase) was cloned. The acuD gene is regulated by both carbon source and temperature in P. marneffei, being strongly induced at 37 degrees C even in the presence of a repressing carbon source such as glucose. When introduced into the non-pathogenic monomorphic fungus Aspergillus nidulans, the P. marneffei acuD promoter only responds to carbon source. Similarly, when the A. nidulans acuD promoter is introduced into P. marneffei it only responds to carbon source suggesting that P. marneffei possesses both cis elements and trans-acting factors to control acuD by temperature. The Zn(II)2Cys6 DNA binding motif transcriptional activator FacB was cloned and is responsible for carbon source-, but not temperature-, dependent induction of acuD. The expression of acuD at 37 degrees C is induced by AbaA, a key regulator of morphogenesis in P. marneffei, but deletion of abaA does not completely eliminate temperature-dependent induction, suggesting that acuD and the glyoxylate cycle are regulated by a complex network of factors in P. marneffei which may contribute to its pathogenicity.

Transcription analysis using high-density micro-arrays of Aspergillus nidulans wild-type and creA mutant during growth on glucose or ethanol

Here, we describe how the recently published Aspergillus nidulans genome sequence [Galagan, J.E., Calvo, S.E., Cuomo, C., Li-Jun, M., Wortman, J.R., et al., 2005. Sequencing of Aspergillus nidulans and comparative analysis with A. fumigatus and A. oryzae. Nature 438 (7071), 1105-1115] was used to design a high-density oligo array with probes for 3,278 selected genes using the Febit Geniom One array system. For this purpose, the program OligoWiz II was used to design 24,125 probes to cover the 3,278 selected genes. Subsequently, the Febit system was used to investigate carbon catabolite repression by comparing the gene expression of a creA deleted mutant strain with a reference strain grown either with glucose or ethanol as the sole carbon source. In order to identify co-regulated genes and genes influenced by either the carbon source or CreA, the most significantly regulated genes (p<or=0.01) were grouped in eight clusters based on their expression profile. Analysis of the clusters allowed identification of numerous genes that are presumably not regulated by CreA, or alternatively are either directly or indirectly regulated by CreA. Surprisingly, we found evidence that more than 25% of the genes (54 out of the 200 significantly regulated) that are repressed by glucose are not completely de-repressed during growth on ethanol, as deletion of the creA resulted in increased expression of the genes in question even during growth on ethanol. Thus, the expression profiles obtained in the eight clusters indicate that the carbon catabolite repression is not a simple on/off switch but a more complex system not only dependent on the presence or absence of CreA but also on the carbon source.

Regulatory genes controlling fatty acid catabolism and peroxisomal functions in the filamentous fungus Aspergillus nidulans

The catabolism of fatty acids is important in the lifestyle of many fungi, including plant and animal pathogens. This has been investigated in Aspergillus nidulans, which can grow on acetate and fatty acids as sources of carbon, resulting in the production of acetyl coenzyme A (CoA). Acetyl-CoA is metabolized via the glyoxalate bypass, located in peroxisomes, enabling gluconeogenesis. Acetate induction of enzymes specific for acetate utilization as well as glyoxalate bypass enzymes is via the Zn2-Cys6 binuclear cluster activator FacB. However, enzymes of the glyoxalate bypass as well as fatty acid beta-oxidation and peroxisomal proteins are also inducible by fatty acids. We have isolated mutants that cannot grow on fatty acids. Two of the corresponding genes, farA and farB, encode two highly conserved families of related Zn2-Cys6 binuclear proteins present in filamentous ascomycetes, including plant pathogens. A single ortholog is found in the yeasts Candida albicans, Debaryomyces hansenii, and Yarrowia lipolytica, but not in the Ashbya, Kluyveromyces, Saccharomyces lineage. Northern blot analysis has shown that deletion of the farA gene eliminates induction of a number of genes by both short- and long-chain fatty acids, while deletion of the farB gene eliminates short-chain induction. An identical core 6-bp in vitro binding site for each protein has been identified in genes encoding glyoxalate bypass, beta-oxidation, and peroxisomal functions. This sequence is overrepresented in the 5' region of genes predicted to be fatty acid induced in other filamentous ascomycetes, C. albicans, D. hansenii, and Y. lipolytica, but not in the corresponding genes in Saccharomyces cerevisiae.

Differential expression of thechsEgene encoding a chitin synthase ofAspergillus nidulansin response to developmental status and growth conditions

Expression of chsE encoding one of the five chitin synthases of Aspergillus nidulans was analyzed. Expression of chsE was moderate in conidiophores, but somewhat weaker in vegetative mycelia. During sexual development, chsE was expressed strongly in young cleistothecia and hülle cells, but little in mature sexual structures. Deletion of chsE caused a significant decrease in the chitin content of the cell wall during early sexual development. Expression of chsE was increased by substituting glucose with lactose or by addition of 0.6M KCl or NaCl, but affected little by substituting glucose with sodium acetate. Consequently, chsE was shown to have a mode of expression distinct from those of the other chitin synthase genes, chsA, chsB and chsC.

Differential expression of the chitin synthase genes of Aspergillus nidulans, chsA, chsB, and chsC, in response to developmental status and environmental factors

To understand the role of the chitin synthase genes of Aspergillus nidulans, we analyzed the expression of chsA, chsB, and chsC both by Northern blotting and by a vital reporter system with sgfp encoding a modified version of green fluorescent protein, sGFP. chsA was expressed specifically during asexual differentiation, but not during either vegetative growth or sexual differentiation. The expression of chsB was ubiquitous throughout the fungal body and relatively independent of the change in developmental status of the cells. chsC was expressed moderately during sexual development as well as during the early phase of vegetative growth, but was expressed weakly in old vegetative mycelia and in asexual structures. Furthermore its expression was spatially differentiated, i.e., relatively strong in young cleistothecia and in mature ascospores, but negligible in Hülle cells. Osmostress caused by high concentrations (up to 1.2M) of KCl or NaCl stimulated the expression of chsA and chsC, but not that of chsB. Sodium acetate, especially at high concentration (3%), strongly enhanced the expression of all the three genes. Neither heat shock nor the sugar carbon sources tested (glucose, sucrose, or lactose) affected the expression of any of the three chitin synthase genes.

Isocitrate lyase is essential for pathogenicity of the fungus Leptosphaeria maculans to canola (Brassica napus)

A pathogenicity gene has been identified in Leptosphaeria maculans, the ascomycetous fungus that causes blackleg disease of canola (Brassica napus). This gene encodes isocitrate lyase, a component of the glyoxylate cycle, and is essential for the successful colonization of B. napus. It was identified by a reverse genetics approach whereby a plasmid conferring hygromycin resistance was inserted randomly into the L. maculans genome. Twelve of 516 transformants tested had reduced pathogenicity on cotyledons of B. juncea and B. napus, and 1 of these 12 had a deletion of the isocitrate lyase gene, as well as an insertion of the hygromycin resistance gene. This mutant was unable to grow on fatty acids, including monolaurate, and the isocitrate lyase transcript was not detected. When the wild-type gene was reintroduced into the mutant, growth on monolaurate was restored and pathogenicity was partially restored. L. maculans isocitrate lyase is produced during infection of B. napus cotyledons, while the plant homologue is not. When 2.5% glucose was added to the inoculum of the isocitrate lyase mutant, lesions of sizes similar to those caused by wild-type isolate M1 developed on B. napus cotyledons. These findings suggest that the glyoxylate pathway is essential for disease development by this plant-pathogenic fungus, as has been shown recently for a fungal and bacterial pathogen of animals and a bacterial pathogen of plants. Involvement of the glyoxylate pathway in pathogenesis in animals and plants presents potential drug targets for control of diseases.

Requirement for 3-ketoacyl-CoA thiolase-2 in peroxisome development, fatty acid beta-oxidation and breakdown of triacylglycerol in lipid bodies of Arabidopsis seedlings

3-ketoacyl-CoA thiolase (KAT) (EC: 2.3.1.16) catalyses a key step in fatty acid beta-oxidation. Expression of the Arabidopsis thaliana KAT gene on chromosome 2 (KAT2), which encodes a peroxisomal thiolase, is activated in early seedling growth. We identified a T-DNA insertion in this gene which abolishes its expression and eliminates most of the thiolase activity in seedlings. In the homozygous kat2 mutant, seedling growth is dependent upon exogenous sugar, and storage triacylglycerol (TAG) and lipid bodies persist in green cotyledons. The peroxisomes in cotyledons of kat2 seedlings are very large, the total peroxisomal compartment is dramatically increased, and some peroxisomes contain unusual membrane inclusions. The size and number of plastids and mitochondria are also modified. Long-chain (C16 to C20) fatty acyl-CoAs accumulate in kat2 seedlings, indicating that the mutant lacks long-chain thiolase activity. In addition, extracts from kat2 seedlings have significantly decreased activity with aceto-acetyl CoA, and KAT2 appears to be the only thiolase gene expressed at significant levels during germination and seedling growth, indicating that KAT2 has broad substrate specificity. The kat2 phenotype can be complemented by KAT2 or KAT5 cDNAs driven by the CaMV 35S promoter, showing that these enzymes are functionally equivalent, but that expression of the KAT5 gene in seedlings is too low for effective catabolism of TAG. By comparison with glyoxylate cycle mutants, it is concluded that while gluconeogenesis from fatty acids is not absolutely required to support Arabidopsis seedling growth, peroxisomal beta-oxidation is essential, which is in turn required for breakdown of TAG in lipid bodies.

Exploring infection of wheat and carbohydrate metabolism in Mycosphaerella graminicola transformants with differentially regulated green fluorescent protein expression

A Mycosphaerella graminicola strain transformed with the green fluorescent protein (GFP) downstream of either a carbon source-repressed promoter or a constitutive promoter was used to investigate in situ carbohydrate uptake during penetration of the fungus in wheat leaves. The promoter region of the acu-3 gene from Neurospora crassa encoding isocitrate lyase was used as a carbon source-repressed promoter. The promoter region of the Aspergillus nidulans gpdA gene encoding glyceraldehyde-3-phosphate dehydrogenase was used as a constitutive promoter. Fluorometric measurement of GFP gene expression in liquid cultures of acu-3-regulated transformants indicated that the N. crassa acu-3 promoter functions in M. graminicola as it does in N. crassa, i.e., acetate induced and carbon source repressed. Glucose, fructose, and saccharose triggered the repression, whereas mannitol, xylose, and cell wall polysaccharides did not. Monitoring the GFP level during fungal infection of wheat leaves revealed that acu-3 promoter repression occurred after penetration until sporulation, when newly differentiated pycnidiospores fluoresced. The use of GFP transformants also allowed clear visualization of M. graminicola pathogenesis. No appressoria were formed, but penetration at cell junctions was observed. These results give new insight into the biotrophic status of M. graminicola.

Use of an isocitrate lyase promoter-GFP fusion to monitor carbon metabolism of the plant pathogen Tapesia yallundae during infection of wheat

Abstract Green fluorescent protein (GFP) has been used as a vital marker in a variety of species. Here, we present the use of a GFP-promoter fusion to visualize carbon metabolism in a pathogenic fungus during growth on defined medium and during infection of plants. Isocitrate lyase (ICL), a key enzyme in carbon metabolism, is tightly regulated at the transcriptional level, with high levels of expression during 2-carbon growth and no expression during growth on glucose. A GFP-ICL promoter fusion was used to visualize carbon metabolism in the plant pathogenic fungus Tapesia yallundae during growth in vitro and in the host plant. The ICL promoter from Neurospora crassa retained its native induction and repression characteristics in T. yallundae. Loss of GFP fluorescence from hyphae after repression of the ICL promoter suggested a rapid turnover rate for GFP in T. yallundae. Regulation of this promoter was observed during infection, with expression occurring only on the plant surface, suggesting that 2-carbon metabolism occurs during this phase. These data suggest that GFP is a useful vital marker for the in planta imaging of fungal metabolism.

Isocitrate lyase of Ashbya gossypii--transcriptional regulation and peroxisomal localization

The isocitrate lyase-encoding gene AgICL1 from the filamentous hemiascomycete Ashbya gossypii was isolated by heterologous complementation of a Saccharomyces cerevisiae icl1d mutant. The open reading frame of 1680 bp encoded a protein of 560 amino acids with a calculated molecular weight of 62584. Disruption of the AgICL1 gene led to complete loss of AgIcl1p activity and inability to grow on oleic acid as sole carbon source. Compartmentation of AgIcl1p in peroxisomes was demonstrated both by Percoll density gradient centrifugation and by immunogold labeling of ultrathin sections using specific antibodies. This fitted with the peroxisomal targeting signal AKL predicted from the C-terminal DNA sequence. Northern blot analysis with mycelium grown on different carbon sources as well as AgICL1 promoter replacement with the constitutive AgTEF promoter revealed a regulation at the transcriptional level. AgICL1 was subject to glucose repression, derepressed by glycerol, partially induced by the C2 compounds ethanol and acetate, and fully induced by soybean oil.

Promoter analysis of the acetate-inducible isocitrate lyase gene (acu-3) from Neurospora crassa

Analysis of the promoter region of the acetate-induced isocitrate lyase gene (acu-3) of Neurospora crassa was undertaken. A series of deletions in the 5' non-transcribed region were constructed and the effects of these mutations on the enzyme levels following growth on sucrose and transfer to acetate were measured. Sequences within the region -603 to -271 relative to the transcription start site appear essential for transcription. The region -950 to -1278 is required for sucrose repression, which is consistent with previous protein/DNA gel retardation results of protein extracts from N. crassa cultured on sucrose. Protein extracts from acetate-induced mycelia identify alternative promoter regions apparently involved in acetate-induced gene transcription.

FacB, the Aspergillus nidulans activator of acetate utilization genes, binds dissimilar DNA sequences

The facB gene is required for acetate induction of acetamidase (amdS) and the acetate utilization enzymes acetyl-CoA synthase (facA), isocitrate lyase (acuD) and malate synthase (acuE) in Aspergillus nidulans. The facB gene encodes a transcriptional activator with a GAL4-type Zn(II)2Cys6 zinc binuclear cluster DNA-binding domain which is shown to be required for DNA binding. In vitro DNA-binding sites for FacB in the 5' regions of the amdS, facA, acuD and acuE genes have been identified. Mutations in amdS FacB DNA-binding sites affected expression of an amdS-lacZ reporter in vivo and altered the affinity of in vitro DNA binding. This study shows that the FacB Zn(II)2Cys6 cluster binds to dissimilar sites which show similarity in form but not sequence with DNA-binding sites of other Zn(II)2Cys6 proteins. Sequences with homology to FacB sites are found in the 5' regions of genes regulated by the closely related yeast Zn(II)2Cys6 protein CAT8.

Molecular characterization of mutants of the acetate regulatory gene facB of Aspergillus nidulans

The facB gene of Aspergillus nidulans encodes a DNA binding transcriptional activator required for growth on acetate as a sole carbon source. FacB contains N-terminal GAL4-like Zn(II)2Cys6 (or C6 zinc) binuclear cluster DNA binding and leucine zipper-like heptad repeat motifs and central and C-terminal acidic alpha-helical regions. facB recessive loss of function mutants are deficient in acetate induction of acetyl-CoA synthase, isocitrate lyase, malate synthase, acetamidase, and NADP-isocitrate dehydrogenase. Characterization of lesions in facB mutant alleles has localized important functional regions of the FacB protein. Two extreme mutants are shown to lack the C-terminal region of the protein. Two temperature sensitive mutants contain amino acid substitutions in the DNA binding domain and are shown to affect acetate induction of amdS-lacZ expression and confer temperature sensitive in vitro DNA binding. Two temperature sensitive facB mutations result in thermolability of acetyl-CoA synthase, isocitrate lyase, and malate synthase but not acetamidase or NADP-isocitrate dehydrogenase in crude extracts. This suggests that FacB may have a structural role in acetate metabolism in addition to its regulatory function.

Carbon repression in Aspergilli

Many microorganisms prefer easily metabolizable carbon sources over alternative, less readily metabolized carbon sources. One of the mechanisms to achieve this is repression of the synthesis of enzymes related to catabolism of the alternative carbon sources, i.e. carbon repression. It is now clear that in Aspergillus nidulans and Aspergillus niger the repressor protein CREA plays a major role in carbon repression. CREA inhibits transcription of many target genes by binding to specific sequences in the promoter of these genes. Unfortunately there is little information on other components of the signalling pathway that triggers repression by CREA. In this review we summarize the current understanding of carbon repression in Aspergilli.

Correlation of isocitrate lyase activity and riboflavin formation in the riboflavin overproducer Ashbya gossypii

Isocitrate lyase (ICL) was assayed during batch cultivations of Ashbya gossypii on soybean oil or glucose as carbon source. On soybean oil, a correlation between enzyme activity and riboflavin synthesis was observed. On glucose as carbon source, riboflavin overproduction started in the late growth phase when glucose was exhausted. ICL activity appeared in parallel and reached a maximum of 0.41 U (mg protein)_-1. This suggested synthesis of vitamin B₂ from the intracellular reserve fat. ICL specific activity correlated with the enzyme concentration detected by specific antibodies. Itaconate, an efficient inhibitor of ICL, was used as an antimetabolite to screen mutants with enhanced ICL activity. Cultivations of an itaconate-resistant mutant on soybean oil revealed a 15% increase in enzyme specific activity and a 25-fold increase in riboflavin yield compared to the wild-type. On the other hand, growth experiments on glucose resulted in an eightfold increase in riboflavin yield but showed a 33% reduction in ICL specific activity compared to the wild-type grown on the same medium. These results support the idea of an ICL bottleneck in the riboflavin overproducer A. gossypii when plant oil is used as the substrate.

Inhibition of purified isocitrate lyase identified itaconate and oxalate as potential antimetabolites for the riboflavin overproducer Ashbya gossypii

A specific isocitrate lyase (ICL) activity of 0.17 U (mg protein)^-1 was detected in cultures of the riboflavin-producing fungus Ashbya gossypii during growth on soybean oil. Enzyme activity was not detectable during growth on glucose [<0.005 U (mg protein)^-1], indicating a regulation. The enzyme was purified 108-fold by means of ammonium sulphate fractionation, gel filtration and cation-exchange chromatography. SDS-PAGE of the purified protein showed a homogeneous band with an M _r of 66000. The M _r of 254000 determined by gel-filtration chromatography indicated a tetrameric structure of the native protein. The enzyme was found to have a pH optimum for the isocitrate cleavage of 7.0, and the K _m for threo-DL-isocitrate was determined as 550 μ. Enzyme activity was Mg^2+- dependent. In regulation studies ICL was weakly inhibited by central metabolites. A concentration of 10 mM phosphoenolpyruvate or 6-phosphogluconate revealed a residual activity of more than 40%. On the other hand, oxalate (K _i: 4 μM) and itaconate (K _i: 170 μM) showed a strong inhibition and may therefore be interesting as antimetabolites.

Glucose-induced inactivation of isocitrate lyase in Aspergillus nidulans

The existence of a second mechanism of catabolite control of isocitrate lyase of Aspergillus nidulans, in addition to the carbon catabolite repression phenomenon recently reported was analysed. Isocitrate lyase was rapidly and specifically inactivated by glucose. The inactivation was irreversible at all stages in the presence of cycloheximide, showing that reactivation depends on de novo protein synthesis. In addition, analysis of glucose-induced inactivation of isocitrate lyase in a creAd-30 strain showed that the creA gene is not involved in this process.

Analysis of the regulation of the Aspergillus nidulans acuD gene, encoding isocitrate lyase, by construction of a hybrid promoter

In order to confirm functionally that a 208 bp fragment of the 5'-flanking sequence of the acuD gene of Aspergillus nidulans is the region responsible for acetate inducibility and catabolite repression, a hybrid promoter was constructed by insertion of this fragment into the promoter of the (highly expressed) oliC gene of A. nidulans. Analysis of expression of the lacZ reporter gene fused to the oliC/acuD promoter showed induction by acetate at much higher levels than wild-type acuD expression. Acetate inducibility of the hybrid promoter was dependent on the facB gene, demonstrating that a facB-dependent upstream activating sequence (UAS) for acetate must be located in the 208 bp acuD fragment. In parallel, partial relief of the transcriptional repression of acetate inducibility by sucrose and glucose was observed in a creA- background, showing that the 208 bp acuD fragment also responds to the creA gene. In addition, the results show that combination of a regulatory element from a low-expression promoter (acuD) with a high-expression constitutive promoter (oliC) leads to amplification of the level of regulated expression.