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

Bioproduction of riboflavin: a bright yellow history

Riboflavin (vitamin B₂) is an essential nutrient for humans and animals that must be obtained from the diet. To ensure an optimal supply, riboflavin is used on a large scale as additive in the food and feed industries. Here, we describe a historical overview of the industrial process of riboflavin production starting from its discovery and the need to produce the vitamin in bulk at prices that would allow for their use in human and animal nutrition. Riboflavin was produced industrially by chemical synthesis for many decades. At present, the development of economical and eco-efficient fermentation processes, which are mainly based on Bacillus subtilis and Ashbya gossypii strains, has replaced the synthetic process at industrial scale. A detailed account is given of the development of the riboflavin overproducer strains as well as future prospects for its improvement.

The Ubiquitin Ligase SCF(Ucc1) Acts as a Metabolic Switch for the Glyoxylate Cycle

Despite the crucial role played by the glyoxylate cycle in the virulence of pathogens, seed germination in plants, and sexual development in fungi, we still have much to learn about its regulation. Here, we show that a previously uncharacterized SCF(Ucc1) ubiquitin ligase mediates proteasomal degradation of citrate synthase in the glyoxylate cycle to maintain metabolic homeostasis in glucose-grown cells. Conversely, transcription of the F box subunit Ucc1 is downregulated in C2-compound-grown cells, which require increased metabolic flux for gluconeogenesis. Moreover, in vitro analysis demonstrates that oxaloacetate regenerated through the glyoxylate cycle induces a conformational change in citrate synthase and inhibits its recognition and ubiquitination by SCF(Ucc1), suggesting the existence of an oxaloacetate-dependent positive feedback loop that stabilizes citrate synthase. We propose that SCF(Ucc1)-mediated regulation of citrate synthase acts as a metabolic switch for the glyoxylate cycle in response to changes in carbon source, thereby ensuring metabolic versatility and flexibility.

Organization of organelles within hyphae of Ashbya gossypii revealed by electron tomography

Ashbya gossypii grows as multinucleated and constantly elongating hyphae. Nuclei are in continuous forward and backward motion, also move during mitosis, and frequently bypass each other. Whereas these nuclear movements are well documented, comparatively little is known about the density and morphology of organelles which very likely influence these movements. To understand the three-dimensional subcellular organization of hyphae at high resolution, we performed large-scale electron tomography of the tip regions in A. gossypii. Here, we present a comprehensive space-filling model in which most membrane-limited organelles including nuclei, mitochondria, endosomes, multivesicular bodies, vacuoles, autophagosomes, peroxisomes, and vesicles are modeled. Nuclei revealed different morphologies and protrusions filled by the nucleolus. Mitochondria are very abundant and form a tubular network with a polarized spherical fraction. The organelles of the degradative pathways show a clustered organization. By analyzing vesicle-like bodies, we identified three size classes of electron-dense vesicles (∼200, ∼150, and ∼100 nm) homogeneously distributed in the cytoplasm which most likely represent peroxisomes. Finally, coated and uncoated vesicles with approximately 40-nm diameters show a polarized distribution toward the hyphal tip with the coated vesicles preferentially localizing at the hyphal periphery.

Contributions of carnitine acetyltransferases to intracellular acetyl unit transport in Candida albicans

Transport of acetyl-CoA between intracellular compartments is mediated by carnitine acetyltransferases (Cats) that reversibly link acetyl units to the carrier molecule carnitine. The genome of the opportunistic pathogenic yeast Candida albicans encodes several (putative) Cats: the peroxisomal and mitochondrial Cat2 isoenzymes encoded by a single gene and the carnitine acetyltransferase homologs Yat1 and Yat2. To determine the contributions of the individual Cats, various carnitine acetyltransferase mutant strains were constructed and subjected to phenotypic and biochemical analyses on different carbon sources. We show that mitochondrial Cat2 is required for the intramitochondrial conversion of acetylcarnitine to acetyl-CoA, which is essential for a functional tricarboxylic acid cycle during growth on oleate, acetate, ethanol, and citrate. Yat1 is cytosolic and contributes to acetyl-CoA transport from the cytosol during growth on ethanol or acetate, but its activity is not required for growth on oleate. Yat2 is also cytosolic, but we were unable to attribute any function to this enzyme. Surprisingly, peroxisomal Cat2 is essential neither for export of acetyl units during growth on oleate nor for the import of acetyl units during growth on acetate or ethanol. Oxidation of fatty acids still takes place in the absence of peroxisomal Cat2, but biomass formation is absent, and the strain displays a growth delay on acetate and ethanol that can be partially rescued by the addition of carnitine. Based on our results, we present a model for the intracellular flow of acetyl units under various growth conditions and the roles of each of the Cats in this process.

Importance of malate synthase in the glyoxylate cycle of Ashbya gossypii for the efficient production of riboflavin

The glyoxylate cycle is an anabolic pathway that is necessary for growth on nonfermentable carbon sources such as vegetable oils and is important for riboflavin production by the filamentous fungus Ashbya gossypii. The aim of this study was to identify malate synthase in the glyoxylate cycle of A. gossypii and to investigate its importance in riboflavin production from rapeseed oil. The ACR268C gene was identified as the malate synthase gene that encoded functional malate synthase in the glyoxylate cycle. The ACR268C gene knockout mutant lost malate synthase activity, and its riboflavin production and oil consumption were 10- and 2-fold lower, respectively, than the values of the wild-type strain. In contrast, the ACR268C gene-overexpressing strain showed a 1.6-fold increase in the malate synthase activity and 1.7-fold higher riboflavin production than the control strain. These results demonstrate that the malate synthase in the glyoxylate cycle has an important role not only in riboflavin production but also in oil consumption.

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.

Dynamin-like protein-dependent formation of Woronin bodies in Saccharomyces cerevisiae upon heterologous expression of a single protein

Filamentous ascomycetes harbor Woronin bodies and glyoxysomes, two types of microbodies, within one cell at the same time. The dominant protein of the Neurospora crassa Woronin body, HEX1, forms a hexagonal core crystal via oligomerization and evidence has accumulated that Woronin bodies bud off from glyoxysomes. We analyzed whether HEX1 is sufficient to induce Woronin body formation upon heterologous expression in Saccharomyces cerevisiae, an organism devoid of this specialized organelle. In wild-type strain BY4742, initial import of HEX1 into existing peroxisomes enabled the formation of organelles with a hexagonal crystal. The observed structures mimicked the shape of genuine Woronin bodies, but exhibited a lower density and were significantly larger. Double-immunofluorescence analysis revealed that hexagonal HEX1 structures only occasionally co-localized with peroxisomal marker proteins, indicating that the Woronin-body-like structures are well separated from peroxisomes. In cells lacking Vps1p and Dnm1p, dynamin-like proteins required for the division of peroxisomes, the Woronin-body-like organelles remained attached to peroxisomes. The data indicate that Woronin bodies emerge after the formation of a HEX1 core crystal within peroxisomes followed by Vps1p- and Dnm1p-mediated fission.

Isolation of peroxisomes from tissues and cells by differential and density gradient centrifugation

Peroxisome purification depends on a two-step procedure: differential centrifugation to prepare a light mitochondrial fraction and fractionation on a density-gradient medium preferably iodixanol or Nycodenz, to isolate the peroxisome enriched fraction. The iodixanol gradient may be a preformed continuous gradient or a self-generating gradient. Alternatively a continuous Nycodenz gradient or a simple Nycodenz barrier may be used for the second step. The unit contains protocols for peroxisome isolation from rat liver, tissue culture cells (HepG2 cells), and yeast spheroplasts. The extent of endoplasmic reticulum contamination of the prep can be assessed using an assay for the marker enzyme NADPH-cytochrome creductase.

The peroxin PEX14 of Neurospora crassa is essential for the biogenesis of both glyoxysomes and Woronin bodies

In the filamentous fungus Neurospora crassa, glyoxysomes and Woronin bodies coexist in the same cell. Because several glyoxysomal matrix proteins and also HEX1, the dominant protein of Woronin bodies, possess typical peroxisomal targeting signals, the question arises as to how protein targeting to these distinct yet related types of microbodies is achieved. Here we analyzed the function of the Neurospora ortholog of PEX14, an essential component of the peroxisomal import machinery. PEX14 interacted with both targeting signal receptors and was localized to glyoxysomes but was virtually absent from Woronin bodies. Nonetheless, a pex14Delta mutant not only failed to grow on fatty acids because of a defect in glyoxysomal beta-oxidation but also suffered from cytoplasmic bleeding, indicative of a defect in Woronin body-dependent septal pore plugging. Inspection of pex14Delta mutant hyphae by fluorescence and electron microscopy indeed revealed the absence of Woronin bodies. When these cells were subjected to subcellular fractionation, HEX1 was completely mislocalized to the cytosol. Expression of GFP-HEX1 in wild-type mycelia caused the staining of Woronin bodies and also of glyoxysomes in a targeting signal-dependent manner. Our data support the view that Woronin bodies emerge from glyoxysomes through import of HEX1 and subsequent fission.

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.

A central role for the peroxisomal membrane in glyoxylate cycle function

The glyoxylate cycle provides the means to convert C2-units to C4-precursors for biosynthesis, allowing growth on fatty acids and C2-compounds. The conventional view that the glyoxylate cycle is contained within peroxisomes in fungi and plants is no longer valid. Glyoxylate cycle enzymes are located both inside and outside the peroxisome. Thus, the operation of the glyoxylate cycle requires transport of several intermediates across the peroxisomal membrane. Glyoxylate cycle progression is also dependent upon mitochondrial metabolism. An understanding of the operation and regulation of the glyoxylate cycle, and its integration with cellular metabolism, will require further investigation of the participating metabolite transporters in the peroxisomal membrane.

Peroxisome function regulates growth on glucose in the basidiomycete fungus Cryptococcus neoformans

The function of the peroxisomes was examined in the pathogenic basidiomycete Cryptococcus neoformans. Recent studies reveal the glyoxylate pathway is required for virulence of diverse microbial pathogens of plants and animals. One exception is C. neoformans, in which isocitrate lyase (encoded by ICL1) was previously shown not to be required for virulence, and here this was extended to exclude also a role for malate synthase (encoded by MLS1). The role of peroxisomes, in which the glyoxylate pathway enzymes are localized in many organisms, was examined by mutation of two genes (PEX1 and PEX6) encoding AAA (ATPases associated with various cellular activities)-type proteins required for peroxisome formation. The pex1 and pex6 deletion mutants were unable to localize the fluorescent DsRED-SKL protein to peroxisomal punctate structures, in contrast to wild-type cells. pex1 and pex6 single mutants and a pex1 pex6 double mutant exhibit identical phenotypes, including abolished growth on fatty acids but no growth difference on acetate. Because both icl1 and mls1 mutants are unable to grow on acetate as the sole carbon source, these findings demonstrate that the glyoxylate pathway can function efficiently outside the peroxisome in C. neoformans. The pex1 mutant exhibits wild-type virulence in a murine inhalation model and in an insect host, demonstrating that peroxisomes are not required for virulence under these conditions. An unusual phenotype of the pex1 and pex6 mutants was that they grew poorly with glucose as the carbon source, but nearly wild type with galactose, which suggested impaired hexokinase function and that C. neoformans peroxisomes might function analogously to the glycosomes of the trypanosomid parasites. Deletion of the hexokinase HXK2 gene reduced growth in the presence of glucose and suppressed the growth defect of the pex1 mutant on glucose. The hexokinase 2 protein of C. neoformans contains a predicted peroxisome targeting signal (type 2) motif; however, Hxk2 fused to fluorescent proteins was not localized to peroxisomes. Thus, we hypothesize that glucose or glycolytic metabolites are utilized in the peroxisome by an as yet unidentified enzyme or regulate a pathway required by the fungus in the absence of peroxisomes.

Peroxisomal fatty acid beta-oxidation is not essential for virulence of Candida albicans

Phagocytic cells form the first line of defense against infections by the human fungal pathogen Candida albicans. Recent in vitro gene expression data suggest that upon phagocytosis by macrophages, C. albicans reprograms its metabolism to convert fatty acids into glucose by inducing the enzymes of the glyoxylate cycle and fatty acid beta-oxidation pathway. Here, we asked whether fatty acid beta-oxidation, a metabolic pathway localized to peroxisomes, is essential for fungal virulence by constructing two C. albicans double deletion strains: a pex5Delta/pex5Delta mutant, which is disturbed in the import of most peroxisomal enzymes, and a fox2Delta/fox2Delta mutant, which lacks the second enzyme of the beta-oxidation pathway. Both mutant strains had strongly reduced beta-oxidation activity and, accordingly, were unable to grow on media with fatty acids as a sole carbon source. Surprisingly, only the fox2Delta/fox2Delta mutant, and not the pex5Delta/pex5Delta mutant, displayed strong growth defects on nonfermentable carbon sources other than fatty acids (e.g., acetate, ethanol, or lactate) and showed attenuated virulence in a mouse model for systemic candidiasis. The degree of virulence attenuation of the fox2Delta/fox2Delta mutant was comparable to that of the icl1Delta/icl1Delta mutant, which lacks a functional glyoxylate cycle and also fails to grow on nonfermentable carbon sources. Together, our data suggest that peroxisomal fatty acid beta-oxidation is not essential for virulence of C. albicans, implying that the attenuated virulence of the fox2Delta/fox2Delta mutant is largely due to a dysfunctional glyoxylate cycle.

Subcellular localization of glyoxylate cycle key enzymes involved in oxalate biosynthesis of wood-destroying basidiomycete Fomitopsis palustris grown on glucose

This study investigated the subcellular localization of key enzymes of the glyoxylate cycle, i.e. isocitrate lyase (ICL; EC 4.1.3.1) and malate synthase (EC 2.3.3.9), that function constitutively in coordination with oxalate biosynthesis of glucose-grownFomitopsis palustris. The ICL purified previously fromF. palustrisis termed FPICL1. Subcellular fractionation analysis of the cell homogenate by the sucrose density-gradient method showed that both key enzymes were present in peroxisomes, whereas acetyl-CoA synthase (EC 6.2.1.1) and oxalate-producing oxaloacetate acetylhydrolase (EC 3.7.1.1) were cytosolic. The peroxisomal localization of FPICL1 was further confirmed by electron microscopic and immunocytochemical analysis with anti-FPICL1 antibody. In addition, the peroxisomal target signal, composed of SKL at the C terminus of the cDNA encoding FPICL1, was found, which also suggests that FPICL1 is peroxisomal. Accordingly, it is postulated that transportation of succinate from peroxisomes to mitochondria, and vice versa, for the transportation of isocitrate or citrate, occurs in glucose-grownF. palustrisfor the constitutive metabolic coordination of the TCA and glyoxylate cycles with oxalate biosynthesis.

Yarrowia lipolytica mutants devoid of pyruvate carboxylase activity show an unusual growth phenotype

We have cloned and characterized the gene PYC1, encoding the unique pyruvate carboxylase in the dimorphic yeast Yarrowia lipolytica. The protein putatively encoded by the cDNA has a length of 1,192 amino acids and shows around 70% identity with pyruvate carboxylases from other organisms. The corresponding genomic DNA possesses an intron of 269 bp located 133 bp downstream of the starting ATG. In the branch motif of the intron, the sequence CCCTAAC, not previously found at this place in spliceosomal introns of Y. lipolytica, was uncovered. Disruption of the PYC1 gene from Y. lipolytica did not abolish growth in glucose-ammonium medium, as is the case in other eukaryotic microorganisms. This unusual growth phenotype was due to an incomplete glucose repression of the function of the glyoxylate cycle, as shown by the lack of growth in that medium of double pyc1 icl1 mutants lacking both pyruvate carboxylase and isocitrate lyase activity. These mutants grew when glutamate, aspartate, or Casamino Acids were added to the glucose-ammonium medium. The cDNA from the Y. lipolytica PYC1 gene complemented the growth defect of a Saccharomyces cerevisiae pyc1 pyc2 mutant, but introduction of either the S. cerevisiae PYC1 or PYC2 gene into Y. lipolytica did not result in detectable pyruvate carboxylase activity or in growth on glucose-ammonium of a Y. lipolytica pyc1 icl1 double mutant.

Overexpression of isocitrate lyase is an important strategy in the survival of Pseudomonas fluorescens exposed to aluminum

Isocitrate lyase, ICL (EC 4.1.3.1), an enzyme that cleaves isocitrate into succinate, and glyoxylate appears to play a pivotal role in the detoxification of aluminum (Al) in Pseudomonas fluorescens. Here, we present evidence that the 4-fold increase in ICL activity observed in Al-stressed cells is due to the overexpression of this enzyme. Blue-Native-PAGE, Western blotting, and spectrophotometric experiments revealed that ICL is optimally expressed at 35 h of growth in Al-stressed cells. However, following the immobilization of Al, at 60 h of growth, the level of the enzyme decreases markedly. This enzyme that exists as a homotetramer with a molecular mass of approximately 133 kDa appears to be transcriptionally regulated. The overexpression of ICL may be a specific response to Al-stress as P. fluorescens grown in the presence of such metals as Ga3+, Pb2+, and Ca2+ does not undergo any significant increase in ICL activity. Thus, these findings support the notion that the overexpression of ICL plays a pivotal role in the survival and in the increased oxalogenesis observed in Al-stressed P. fluorescens.

TheICL1 gene ofPichia pastoris, transcriptional regulation and use of its promoter

We cloned and characterized a gene encoding isocitrate lyase from the methylotrophic yeast Pichia pastoris. This gene was isolated from a P. pastoris genomic library using a homologous PCR hybridization probe, amplified with two sets of degenerate primers designed from conserved regions in yeast isocitrate lyases. The cloned gene was sequenced and consists of an open reading frame of 1563 bp encoding a protein of 551 amino acids. The molecular mass of the protein is calculated to be 60.6 kDa with high sequence similarity to isocitrate lyase from other organisms. There is a 64% identity between amino acid sequences of P. pastoris Icl and Saccharomyces cerevisiae Icl. Northern blot analyses showed that, as in S. cerevisiae, the steady-state ICL1 mRNA levels depend on the carbon source used for cell growth. Expression in P. pastoris of the dextranase gene (dexA) from Penicillium minioluteum under control of the ICL1 promoter proved that P(ICL1) is a good alternative for the expression of heterologous proteins in this methylotrophic yeast. The sequence presented here has been deposited in the EMBL data library under Accession No. AJ272040.

An IQGAP-related protein, encoded by AgCYK1, is required for septation in the filamentous fungus Ashbya gossypii

In filamentous ascomycetes hyphae are compartmentalized by septation in which the cytoplasm of the compartments are interconnected via septal pores. Thus, septation in filamentous fungi is different from cytokinesis in yeast like fungi. We have identified an Ashbya gossypii orthologue of the Saccharomyces cerevisiae CYK1 gene which belongs to the IQGAP-protein family. In contrast to S. cerevisiae disruption of AgCYK1 yields viable mutant strains that exhibit wildtype-like polarized hyphal growth rates. In the Agcyk1 mutant cortical actin patches localize to growing hyphal tips like wildtype, however, mutant hyphae are totally devoid of actin rings at presumptive septal sites. Septation in wildtype results in the formation of chitin rings. Agcyk1 mutant hyphae are aseptate and do not accumulate chitin in their cell walls. Agcyk1 mutant strains are completely asporogenous indicating that septation is essential for the formation of sporangia in A. gossypii. AgCyk1p-GFP localizes to sites of future septation as a ring prior to chitin depositioning. Furthermore, decrease in Cyk1p-ring diameter was found to be a prerequisite for the accumulation of chitin and septum formation.

Transcriptional regulation of 3,4-dihydroxy-2-butanone 4-phosphate synthase

The filamentous hemiascomycete Ashbya gossypii is a strong riboflavin overproducer. A striking but as yet uninvestigated phenomenon is the fact that the overproduction of this vitamin starts when growth rate declines, which means that most of the riboflavin is produced in the stationary phase, the so-called production phase. The specific activity of 3,4-dihydroxy-2-butanone 4-phosphate (DHBP) synthase, the first enzyme in the biosynthetic pathway for riboflavin, was determined during cultivation and an increase during the production phase was found. Furthermore, an increase of RIB3 mRNA, encoding DHBP synthase, was observed by competitive RT-PCR in the production phase. The mRNAs of two housekeeping genes, ACT1 (encoding actin) and TEF (encoding translation elongation factor-1 alpha), served as standards in the RT-PCR. Reporter studies with a RIB3 promoter-lacZ fusion showed an increase of beta-galactosidase specific activity in the production phase. This investigation verified that the increase of RIB3 mRNA in the production phase is caused by an induction of promoter activity. These data suggest that the time course of riboflavin overproduction of A. gossypii is correlated with a transcriptional regulation of the DHBP synthase.

The glyoxylate cycle in an arbuscular mycorrhizal fungus. Carbon flux and gene expression

The arbuscular mycorrhizal (AM) symbiosis is responsible for huge fluxes of photosynthetically fixed carbon from plants to the soil. Lipid, which is the dominant form of stored carbon in the fungal partner and which fuels spore germination, is made by the fungus within the root and is exported to the extraradical mycelium. We tested the hypothesis that the glyoxylate cycle is central to the flow of carbon in the AM symbiosis. The results of (13)C labeling of germinating spores and extraradical mycelium with (13)C(2)-acetate and (13)C(2)-glycerol and analysis by nuclear magnetic resonance spectroscopy indicate that there are very substantial fluxes through the glyoxylate cycle in the fungal partner. Full-length sequences obtained by polymerase chain reaction from a cDNA library from germinating spores of the AM fungus Glomus intraradices showed strong homology to gene sequences for isocitrate lyase and malate synthase from plants and other fungal species. Quantitative real-time polymerase chain reaction measurements show that these genes are expressed at significant levels during the symbiosis. Glyoxysome-like bodies were observed by electron microscopy in fungal structures where the glyoxylate cycle is expected to be active, which is consistent with the presence in both enzyme sequences of motifs associated with glyoxysomal targeting. We also identified among several hundred expressed sequence tags several enzymes of primary metabolism whose expression during spore germination is consistent with previous labeling studies and with fluxes into and out of the glyoxylate cycle.

Riboflavin, overproduced during sporulation of Ashbya gossypii, protects its hyaline spores against ultraviolet light

Riboflavin (vitamin B2), essential in tiny amounts as a precursor for oxidoreductase coenzymes, is a yellow pigment. Although it causes cytotoxicity via photoinduced damage of macromolecules, several microorganisms are striking overproducers. A question, unanswered for decades, is whether riboflavin overproducers can benefit from this property. Here, we report an ultraviolet (UV) protective effect of riboflavin. The spores of Ashbya gossypii, a riboflavin-overproducing fungus, are more sensitive to UV than those of Aspergillus nidulans. The addition of riboflavin to suspensions improves the UV resistance of both spore types. Interestingly, we show that regulation of sporulation and riboflavin overproduction in A. gossypii are linked. In batch culture, both were elevated when growth ceased. At constant growth rates, obtained in a chemostat culture, neither was elevated. Supplementation of cultures by cAMP, a known stress signal, negatively affected sporulation as well as riboflavin overproduction, establishing a second, independent argument for the linkage.

Cytoplasmic dynein is required to oppose the force that moves nuclei towards the hyphal tip in the filamentous ascomycete Ashbya gossypii

We have followed the migration of GFP-labelled nuclei in multinucleate hyphae of Ashbya gossypii. For the first time we could demonstrate that the mode of long range nuclear migration consists of oscillatory movements of nuclei with, on average, higher amplitudes in the direction of the growing tip. We could also show that mitotic division proceeds at a constant rate of 0. 64 microm/minute which differs from the biphasic kinetics described for the yeast Saccharomyces cerevisiae. Furthermore we were able to identify the microtubule-based motor dynein as a key element in the control of long range nuclear migration. For other filamentous fungi it had already been demonstrated that inactivating mutations in dynein led to severe problems in nuclear migration, i.e. generation of long nuclei-free hyphal tips and clusters of nuclei throughout the hyphae. This phenotype supported the view that dynein is important for the movement of nuclei towards the tip. In A. gossypii the opposite seems to be the case. A complete deletion of the dynein heavy chain gene leads to nuclear clusters exclusively at the hyphal tips and to an essentially nucleus-free network of hyphal tubes and branches. Anucleate hyphae and branches in the vicinity of nuclear clusters show actin cables and polarized actin patches, as well as microtubules. The slow growth of this dynein null mutant could be completely reverted to wild-type-like growth in the presence of benomyl, which can be explained by the observed redistribution of nuclei in the hyphal network.

Cell polarity and hyphal morphogenesis are controlled by multiple rho-protein modules in the filamentous ascomycete Ashbya gossypii

Polarized cell growth requires a polarized organization of the actin cytoskeleton. Small GTP-binding proteins of the Rho-family have been shown to be involved in the regulation of actin polarization as well as other processes. Hyphal growth in filamentous fungi represents an ideal model to investigate mechanisms involved in generating cell polarity and establishing polarized cell growth. Since a potential role of Rho-proteins has not been studied so far in filamentous fungi we isolated and characterized the Ashbya gossypii homologs of the Saccharomyces cerevisiae CDC42, CDC24, RHO1, and RHO3 genes. The AgCDC42 and AgCDC24 genes can both complement conditional mutations in the S. cerevisiae CDC42 and CDC24 genes and both proteins are required for the establishment of actin polarization in A. gossypii germ cells. Agrho1 mutants show a cell lysis phenotype. Null mutant strains of Agrho3 show periodic swelling of hyphal tips that is overcome by repolarization and polar hyphal growth in a manner resembling the germination pattern of spores. Thus different Rho-protein modules are required for distinct steps during polarized hyphal growth of A. gossypii.

A PAK-like protein kinase is required for maturation of young hyphae and septation in the filamentous ascomycete Ashbya gossypii

Filamentous fungi grow by hyphal extension, which is an extreme example of polarized growth. In contrast to yeast species, where polarized growth of the tip of an emerging bud is temporally limited, filamentous fungi exhibit constitutive polarized growth of the hyphal tip. In many fungi, including Ashbya gossypii, polarized growth is reinforced by a process called hyphal maturation. Hyphal maturation refers to the developmental switch from slow-growing hyphae of young mycelium to fast-growing hyphae of mature mycelium. This process is essential for efficient expansion of mycelium. We report for the first time on the identification and characterization of a fungal gene important for hyphal maturation. This novel A. gossypii gene encodes a presumptive PAK (p21-activated kinase)-like kinase. Its closest homolog is the S. cerevisiae Cla4 protein kinase; the A. gossypii protein is therefore called AgCla4p. Agcla4 deletion strains are no longer able to perform the developmental switch from young to mature hyphae, and GFP (green fluorescent protein)-tagged AgCla4p localizes with much higher frequency in mature hyphal tips than in young hyphal tips. Both results support the importance of AgCla4p in hyphal maturation. AgCla4p is also required for septation, indicated by the inability of Agcla4 deletion strains to properly form actin rings and chitin rings. Despite the requirement of AgCla4p for the development of fast-growing hyphae, AgCla4p is not necessary for actin polarization per se, because tips enriched in cortical patches and hyphae with a fully developed network of actin cables can be seen in Agcla4 deletion strains. The possibility that AgCla4p may be involved in regulatory mechanisms that control the dynamics of the actin patches and/or actin cables is discussed.

Determination of cell polarity in germinated spores and hyphal tips of the filamentous ascomycete Ashbya gossypii requires a rhoGAP homolog

In the filamentous ascomycete Ashbya gossypii, like in other filamentous fungi onset of growth in dormant spores occurs as an isotropic growth phase generating spherical germ cells. Thereafter, a switch to polarized growth results in the formation of the first hyphal tip. The initial steps of hyphal tip formation in filamentous fungi, therefore, resemble processes taking place prior to and during bud emergence of unicellular yeast-like fungi. We investigated whether phenotypic similarities between these distinct events extended to the molecular level. To this end we isolated and characterized the A. gossypii homolog of the Saccharomyces cerevisiae BEM2 gene which is part of a network of rho-GTPases and their regulators required for bud emergence and bud growth in yeast. Here we show that the AgBem2 protein contains a GAP- (GTPase activating protein) domain for rho-like GTPases at its carboxy terminus, and that this part of AgBem2p is required for complementation of an Agbem2 null strain. Germination of spores resulted in enlarged Agbem2 germ cells that were unable to generate the bipolar branching pattern found in wild-type germ cells. In addition, mutant hyphae were swollen due to defects in polarized cell growth indicated by the delocalized distribution of chitin and cortical actin patches. Surprisingly, the complete loss of cell polarity which lead to spherical hyphal tips was overcome by the establishment of new cell polarities and the formation of multiple new hyphal tips. In conclusion these results and other findings demonstrate that establishment of cell polarity, maintenance of cell polarity, and polarized hyphal growth in filamentous fungi require members of Ρ-GTPase modules.

PCR-based gene targeting in the filamentous fungus Ashbya gossypii

We have investigated a PCR-based approach for one-step gene targeting in the filamentous fungus Ashbya gossypii. Short guide sequences with 40-46 bp of homology to two sequences of a targeted gene, provided by PCR, were sufficient to mediate homologous recombination. The PCR products used for transformation were generated from the newly constructed chimeric selection marker GEN3. This consists of the open reading frame of the Escherichia coli kanR gene under the control of promoter and terminator sequences of the Saccharomyces cerevisiae TEF2 gene and allows selection of G418/geneticin-resistant transformants. Verification of gene targeting was performed either by PCR or by DNA hybridization analyses, and in all 18 cases tested, correct targeting was confirmed. This approach was used for the complete deletion of the open reading frame of the A. gossypii RHO4 gene for which a double-strand sequence was available as information source for the design of PCR primers. We also demonstrated successful partial deletion of four other ORFs using single-read sequences (SRS) as sole information for the design of targeting primers. A gossypii is the first filamentous fungus in which a PCR-based gene disruption technique has been established. Since short target guide sequences are sufficient to direct homologous integration into the A. gossypii genome it is not necessary to obtain and sequence large DNA fragments from a target locus to provide the long flanking homology regions usually required for efficient targeting of cloned disruption cassettes in filamentous fungi. Thus functional analysis of A. gossypii genes is already possible, based on single-pass sequence information.