Physiological consequence of disruption of the VMA1 gene in the riboflavin overproducer Ashbya gossypii

Effect of Potato Glycoside Alkaloids on Energy Metabolism of Fusarium solani

Fusarium solani is one of the primary pathogens causing root rot of wolfberry. The aims of this study were to investigate the inhibitory effect of potato glycoside alkaloids (PGA) on F. solani and its energy metabolism. In this study, the effects of PGA treatment on the growth and development of F. solani were investigated and the changes in the glycolytic pathway (EMP), ATPase activity, mitochondrial complex activity, mitochondrial structure, and energy charge level were analyzed to elucidate the possible antifungal mechanism of PGA on F. solani. The results showed that PGA treatment inhibited the colony growth, biomass, and spore germination of F. solani. PGA treatment reduced the glucose content and Hexokinase (HK) activity of F. solani, but increased the activity of Fructose-6-Phosphate Kinase (PFK) and Pyruvate Kinase (PK) and promoted the accumulation of pyruvic acid. In addition, PGA treatment inhibited the activities of H⁺-ATPase, Ca²⁺-ATPase, and mitochondrial complex IV, increased the mitochondrial inner membrane Ca²⁺ content and mitochondrial membrane permeability transition pore, and decreased the contents of ATP, ADP, and AMP as well as the energy charge. These results indicate that PGA treatment inhibits the growth and development of F. solani, activates the glycolysis pathway, inhibits ATPase activity and mitochondrial complex activity, and destroys the structure and function of mitochondrial membrane, resulting in a lower energy charge level.

Characterization of a Riboflavin-Producing Mutant of Bacillus subtilis Isolated by Droplet-Based Microfluidics Screening

Bacillus subtilis is one of the commonly used industrial strains for riboflavin production. High-throughput screening is useful in biotechnology, but there are still an insufficient number of articles focusing on improving the riboflavin production of B. subtilis by this powerful tool. With droplet-based microfluidics technology, single cells can be encapsulated in droplets. The screening can be carried out by detecting the fluorescence intensity of secreted riboflavin. Thus, an efficient and high-throughput screening method suitable for riboflavin production strain improvement could be established. In this study, droplet-based microfluidics screening was applied, and a more competitive riboflavin producer U3 was selected from the random mutation library of strain S1. The riboflavin production and biomass of U3 were higher than that of S1 in flask fermentation. In addition, the results of fed-batch fermentation showed that the riboflavin production of U3 was 24.3 g/L, an 18% increase compared with the parent strain S1 (20.6 g/L), and the yield (g riboflavin/100 g glucose) increased by 19%, from 7.3 (S1) to 8.7 (U3). Two mutations of U3 (sinR^G89R and icd^D28E) were identified through whole genome sequencing and comparison. Then they were introduced into BS168DR (parent of S1) for further analysis, which also caused riboflavin production to increase. This paper provides protocols for screening riboflavin-producing B. subtilis with droplet-based microfluidics technology and reveals mutations in riboflavin overproduction strains.

Metabolic engineering of non-conventional yeasts for construction of the advanced producers of biofuels and high-value chemicals

Non-conventional yeasts, i.e. yeasts different from Saccharomyces cerevisiae, represent heterogenous group of unicellular fungi consisting of near 1500 species. Some of these species have interesting and sometimes unique properties like ability to grow on methanol, n-alkanes, ferment pentose sugars xylose and l-arabinose, grow at high temperatures (50°С and more), overproduce riboflavin (vitamin B2) and others. These unique properties are important for development of basic science; moreover, some of them possess also significant applied interest for elaboration of new biotechnologies. Current paper represents review of the recent own results and of those of other authors in the field of non-conventional yeast study for construction of the advanced producers of biofuels (ethanol, isobutanol) from lignocellulosic sugars glucose and xylose or crude glycerol (Ogataea polymorpha, Magnusiomyces magnusii) and vitamin B2 (riboflavin) from glucose and cheese whey (Candida famata).

A Straightforward Approach to Synthesize 7-Aminocephalosporanic Acid In Vivo in the Cephalosporin C Producer Acremonium chrysogenum

The pharmaceutical industry has developed various highly effective semi-synthetic cephalosporins, which are generated by modifying the side chains of the core molecule 7-aminocephalosporanic acid (7-ACA). In industrial productions, the 7-ACA nucleus is obtained in vitro from cephalosporin C (CPC) by chemical or enzymatic processes, which are waste intensive and associated with high production costs. Here, we used a transgenic in vivo approach to express bacterial genes for cephalosporin C acylase (CCA) in the CPC producer Acremonium chrysogenum. Western blot and mass spectrometry analyses verified that the heterologous enzymes are processed into α- and β-subunits in the fungal cell. Extensive HPLC analysis detected substrates and products of CCAs in both fungal mycelia and culture supernatants, with the highest amount of 7-ACA found in the latter. Using different incubation times, temperatures, and pH values, we explored the optimal conditions for the active bacterial acylase to convert CPC into 7-ACA in the culture supernatant. We calculated that the best transgenic fungal strains exhibit a one-step conversion rate of the bacterial acylase of 30%. Our findings can be considered a remarkable contribution to supporting future pharmaceutical manufacturing processes with reduced production costs.

Strategies to Increase the Production of Biosynthetic Riboflavin

Riboflavin is widely regarded as an essential nutrient that is involved in biological oxidation in vivo. In addition to preventing and treating acyl-CoA dehydrogenase deficiency in patients with keratitis, stomatitis, and glossitis, riboflavin is also closely related to the treatment of radiation mucositis and cardiovascular disease. Chemical synthesis has been the dominant method for producing riboflavin for approximately 50 years. Nevertheless, due to the intricate synthesis process, relatively high cost, and high risk of pollution, alternative methods of chemical syntheses, such as the fermentation method, began to develop and eventually became the main methods for producing riboflavin. At present, there are three types of strains used in industrial riboflavin production: Ashbya gossypii, Candida famata, and Bacillus subtilis. Additionally, many recent studies have been conducted on Escherichia coli and Lactobacillus. Fermentation increases the yield of riboflavin using genetic engineering technology to modify and induce riboflavin production in the strain, as well as to regulate the metabolic flux of the purine pathway and pentose phosphate pathway (PP pathway), thereby optimizing the culture process. This article briefly introduces recent progress in the fermentation of riboflavin.

Recent Advances in Construction of the Efficient Producers of Riboflavin and Flavin Nucleotides (FMN, FAD) in the Yeast Candida famata

The approaches used by the authors to design the Candida famata strains capable to overproduce riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) are described. The metabolic engineering approaches include overexpression of SEF1 gene encoding positive regulator of riboflavin biosynthesis, IMH3 (coding for IMP dehydrogenase) orthologs from another species of flavinogenic yeast Debaryomyces hansenii, and the homologous genes RIB1 and RIB7 encoding GTP cyclohydrolase II and riboflavin synthase, the first and the last enzymes of riboflavin biosynthesis pathway, respectively. Overexpression of the above mentioned genes in the genetically stable riboflavin overproducer AF-4 obtained by classical selection resulted in fourfold increase of riboflavin production in shake flask experiments.Overexpression of engineered enzymes phosphoribosyl pyrophosphate synthetase and phosphoribosyl pyrophosphate amidotransferase catalyzing the initial steps of purine nucleotide biosynthesis enhances riboflavin synthesis in the flavinogenic yeast C. famata even more.Recombinant strains of C. famata containing FMN1 gene from D. hansenii encoding riboflavin kinase under control of the strong constitutive TEF1 promoter were constructed. Overexpression of the FMN1 gene in the riboflavin-producing mutant led to the 30-fold increase of the riboflavin kinase activity and 400-fold increase of FMN production in the resulting recombinant strains which reached maximally 318.2 mg/L.FAD overproducing strains of C. famata were also constructed. This was achieved by overexpression of FAD1 gene from D. hansenii in C. famata FMN overproducing strain. The 7- to 15-fold increase in FAD synthetase activity as compared to the wild-type strain and FAD accumulation into cultural medium were observed. The maximal FAD titer 451.5 mg/L was achieved.

Unlocking Nature's Biosynthetic Power-Metabolic Engineering for the Fermentative Production of Chemicals

Fermentation as a production method for chemicals is especially attractive, as it is based on cheap renewable raw materials and often exhibits advantages in terms of costs and sustainability. The tremendous development of technology in bioscience has resulted in an exponentially increasing knowledge about biological systems and has become the main driver for innovations in the field of metabolic engineering. Progress in recombinant DNA technology, genomics, and computational methods open new, cheaper, and faster ways to metabolically engineer microorganisms. Existing biosynthetic pathways for natural products, such as vitamins, organic acids, amino acids, or secondary metabolites, can be discovered and optimized efficiently, thereby enabling competitive commercial production processes. Novel biosynthetic routes can now be designed by the rearrangement of nature's unlimited number of enzymes and metabolic pathways in microbial strains. This expands the range of chemicals accessible by biotechnology and has yielded the first commercial products, while new fermentation technologies targeting novel active ingredients, commodity chemicals, and CO₂ -fixation methods are on the horizon.

Role of the regulatory genes SEF1, VMA1 and SFU1 in riboflavin synthesis in the flavinogenic yeast Candida famata (Candida flareri)

Riboflavin or vitamin B₂ is an essential dietary component for humans and animals that is the precursor of flavin coenzymes flavin mononucleotide and flavin adenine dinucleotide involved in numerous enzymatic reactions. The flavinogenic yeast Candida famata overproduces riboflavin under iron starvation; however, regulation of this process is poorly understood. Regulatory gene SEF1 encoding transcription activator has been identified. Its deletion blocks yeast ability to overproduce riboflavin under iron starvation. It was shown here that the SEF1 promoters from other flavinogenic (Candida albicans) and non-flavinogenic (Candida tropicalis) yeasts fused with the open reading frame (ORF) of SEF1 gene from C. famata are able to restore riboflavin oversynthesis in sef1Δ mutants. It is known that in the pathogenic flavinogenic yeast C. albicans, Sfu1 (GATA-type transcription factor) represses SEF1. Here, we found that deletion of SFU1 gene in wild-type C. famata leads to riboflavin oversynthesis. Moreover, it was shown that disruption of VMA1 gene (coding for vacuolar ATPase subunit A) also results in riboflavin oversynthesis in C. famata.

Cpvma1, a Vacuolar H+-ATPase Catalytic Subunit of Cryphonectria parasitica, is Essential for Virulence and Hypovirus RNA Accumulation

The vacuolar H⁺-ATPases (V-ATPases) are conserved ATP-dependent proton pumps that acidify intracellular compartments in eukaryotic cells. The role of Cpvma1, a V-ATPase catalytic subunit A of Cryphonectria parasitica, was investigated by generating cpvma1-overexpressing and cpvma1-silenced strains. The mutant strains were evaluated for phenotypic characteristics, V-ATPase activity, response to elevated pH and Ca²⁺ in the medium, virulence on chestnut, and accumulation of hypovirus RNA in the cells. Compared with the wild-type strain, cpvma1-overexpressing strains showed no significant difference in phenotype; however, cpvma1-silenced strains exhibited a phenotype of reduced growth rate, lower level of sporulation, and a marked decrease in V-ATPase activity and virulence. In addition, silencing of cpvma1 increased sensitivity to elevated pH and Ca²⁺, implicating an important role for Cpvma1 in pH adaptation and Ca²⁺ homeostasis. Furthermore, silencing of cpvma1 resulted in significantly decreased accumulation of hypoviral RNA. Taken together, our results indicate that Cpvma1 plays an important role in the regulation of phenotypic traits and virulence and the accumulation of hypovirus RNA in C. parasitica.

Engineering Ashbya gossypii strains for de novo lipid production using industrial by-products

Ashbya gossypii is a filamentous fungus that naturally overproduces riboflavin, and it is currently exploited for the industrial production of this vitamin. The utilization of A. gossypii for biotechnological applications presents important advantages such as the utilization of low-cost culture media, inexpensive downstream processing and a wide range of molecular tools for genetic manipulation, thus making A. gossypii a valuable biotechnological chassis for metabolic engineering. A. gossypii has been shown to accumulate high levels of lipids in oil-based culture media; however, the lipid biosynthesis capacity is rather limited when grown in sugar-based culture media. In this study, by altering the fatty acyl-CoA pool and manipulating the regulation of the main ∆9 desaturase gene, we have obtained A. gossypii strains with significantly increased (up to fourfold) de novo lipid biosynthesis using glucose as the only carbon source in the fermentation broth. Moreover, these strains were efficient biocatalysts for the conversion of carbohydrates from sugarcane molasses to biolipids, able to accumulate lipids up to 25% of its cell dry weight. Our results represent a proof of principle showing the promising potential of A. gossypii as a competitive microorganism for industrial biolipid production using cost-effective feed stocks.

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 filamentous fungus Ashbya gossypii as a competitive industrial inosine producer

Inosine is a nucleoside with growing biotechnological interest due to its recently attributed beneficial health effects and as a convenient precursor of the umami flavor. At present, most of the industrial inosine production relies on bacterial fermentations. In this work, we have metabolically engineered the filamentous fungus Ashbya gossypii to obtain strains able to excrete high amounts of inosine to the culture medium. We report that the disruption of only two key genes of the purine biosynthetic pathway efficiently redirect the metabolic flux, increasing 200-fold the excretion of inosine with respect to the wild type, up to 2.2 g/L. These results allow us to propose A. gossypii as a convenient candidate for large-scale nucleoside production, especially in view of the several advantages that Ashbya has with respect to the bacterial systems used at present for the industrial production of this food additive. Biotechnol. Bioeng. 2016;113: 2060-2063. © 2016 Wiley Periodicals, Inc.

Guanine nucleotide binding to the Bateman domain mediates the allosteric inhibition of eukaryotic IMP dehydrogenases

Inosine-5′-monophosphate dehydrogenase (IMPDH) plays key roles in purine nucleotide metabolism and cell proliferation. Although IMPDH is a widely studied therapeutic target, there is limited information about its physiological regulation. Using Ashbya gossypii as a model, we describe the molecular mechanism and the structural basis for the allosteric regulation of IMPDH by guanine nucleotides. We report that GTP and GDP bind to the regulatory Bateman domain, inducing octamers with compromised catalytic activity. Our data suggest that eukaryotic and prokaryotic IMPDHs might have developed different regulatory mechanisms, with GTP/GDP inhibiting only eukaryotic IMPDHs. Interestingly, mutations associated with human retinopathies map into the guanine nucleotide-binding sites including a previously undescribed non-canonical site and disrupt allosteric inhibition. Together, our results shed light on the mechanisms of the allosteric regulation of enzymes mediated by Bateman domains and provide a molecular basis for certain retinopathies, opening the door to new therapeutic approaches.

IMP dehydrogenase (IMPDH) plays essential roles in purine metabolism and cell proliferation. Here Buey et al. describe a guanine nucleotides regulated molecular mechanism for allosteric communication between the regulatory and catalytic domains of IMPDH.

Increased riboflavin production by manipulation of inosine 5'-monophosphate dehydrogenase in Ashbya gossypii

Guanine nucleotides are the precursors of essential biomolecules including nucleic acids and vitamins such as riboflavin. The enzyme inosine-5'-monophosphate dehydrogenase (IMPDH) catalyzes the ratelimiting step in the guanine nucleotide de novo biosynthetic pathway and plays a key role in controlling the cellular nucleotide pools. Thus, IMPDH is an important metabolic bottleneck in the guanine nucleotide synthesis, susceptible of manipulation by means of metabolic engineering approaches. Herein, we report the functional and structural characterization of the IMPDH enzyme from the industrial fungus Ashbya gossypii. Our data show that the overexpression of the IMPDH gene increases the metabolic flux through the guanine pathway and ultimately enhances 40 % riboflavin production with respect to the wild type. Also, IMPDH disruption results in a 100-fold increase of inosine excretion to the culture media. Our results contribute to the developing metabolic engineering toolbox aiming at improving the production of metabolites with biotechnological interest in A. gossypii.

Increased production of inosine and guanosine by means of metabolic engineering of the purine pathway in Ashbya gossypii

Background

Inosine and guanosine monophosphate nucleotides are convenient sources of the umami flavor, with attributed beneficial health effects that have renewed commercial interest in nucleotide fermentations. Accordingly, several bacterial strains that excrete high levels of inosine and guanosine nucleosides are currently used in the food industry for this purpose.

Results

In the present study, we show that the filamentous fungus Ashbya gossypii, a natural riboflavin overproducer, excretes high amounts of inosine and guanosine nucleosides to the culture medium. Following a rational metabolic engineering approach of the de novo purine nucleotide biosynthetic pathway, we increased the excreted levels of inosine up to 27-fold.

Conclusions

We generated Ashbya gossypii strains with improved production titers of inosine and guanosine. Our results point to Ashbya gossypii as the first eukaryotic microorganism representing a promising candidate, susceptible to further manipulation, for industrial nucleoside fermentation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0234-4) contains supplementary material, which is available to authorized users.

Construction and fed-batch cultivation of Candida famata with enhanced riboflavin production

Riboflavin (vitamin B2) is an essential nutrition component serving as a precursor of coenzymes FMN and FAD that are involved mostly in reactions of oxidative metabolism. Riboflavin is produced in commercial scale and is used in feed and food industries, and in medicine. The yeast Candida famata (Candida flareri) belongs to the group of so called "flavinogenic yeasts" which overproduce riboflavin under iron limitation. Three genes SEF1, RIB1 and RIB7 coding for a putative transcription factor, GTP cyclohydrolase II and riboflavin synthase, respectively were simultaneously overexpressed in the background of a non-reverting riboflavin producing mutant AF-4, obtained earlier in our laboratory using methods of classical selection (Dmytruk et al. (2011), Metabolic Engineering 13, 82-88). Cultivation conditions of the constructed strain were optimized for shake-flasks and bioreactor cultivations. The constructed strain accumulated up to 16.4g/L of riboflavin in optimized medium in a 7L laboratory bioreactor during fed-batch fermentation.

Analysis of the cell wall integrity pathway of Ashbya gossypii

Fungal cells are exposed to rapidly changing environmental conditions, in particular with regard to the osmotic potential. This requires constant remodeling of the cell wall and, therefore, the cell wall integrity (CWI) MAP-kinase pathway plays a major role in shaping the fungal cell wall to protect from adverse external stresses. To provide a comprehensive functional analysis of the Ashbya gossypii CWI pathway we generated a set of ten deletion mutants in conserved components including the cell surface sensors AgWSC1 and AgMID2, a putative Rho1-guanine nucleotide exchange factor, AgTUS1, the protein kinase C, AgPKC1, the MAP-kinases AgBCK1, AgMKK1 and AgMPK1, and transcription factors known to be involved in CWI signaling AgRLM1, AgSWI4 and AgSWI6. Deletion of AgPKC1 shows a severe growth defect with frequent tip cell lysis. Deletion of components of the MAP-kinase module generates a pronounced colony lysis phenotype in older regions of the mycelium. Cytoplasmic leakage was assayed using alkaline phosphatase and β-galactosidase release assays. This indicated that the lysis phenotypes of CWI pathway mutants may be useful to facilitate the isolation of riboflavin from A. gossypii. Remarkably, the Agwsc1 mutant showed a strong (up to 8-fold) increase of riboflavin in the growth medium compared to the parental strain.

Involvement of MoVMA11, a Putative Vacuolar ATPase c' Subunit, in Vacuolar Acidification and Infection-Related Morphogenesis of Magnaporthe oryzae

Many functions of vacuole depend on the activity of vacuolar ATPase which is essential to maintain an acidic lumen and create the driving forces for massive fluxes of ions and metabolites through vacuolar membrane. In filamentous fungus Magnaportheoryzae, subcellular colocalization and quinacrine staining suggested that the V1V0 domains of V-ATPase were fully assembled and the vacuoles were kept acidic during infection-related developments. Targeted gene disruption of MoVMA11 gene, encoding the putative c' subunit of V-ATPase, impaired vacuolar acidification and mimicked the phenotypes of yeast V-ATPase mutants in the poor colony morphology, abolished asexual and sexual reproductions, selective carbon source utilization, and increased calcium and heavy metals sensitivities, however, not in the typical pH conditional lethality. Strikingly, aerial hyphae of the MoVMA11 null mutant intertwined with each other to form extremely thick filamentous structures. The results also implicated that MoVMA11 was involved in cell wall integrity and appressorium formation. Abundant non-melanized swollen structures and rare, small appressoria without penetration ability were produced at the hyphal tips of the ΔMovma11 mutant on onion epidermal cells. Finally, the MoVMA11 null mutant lost pathogenicity on both intact and wounded host leaves. Overall, our data indicated that MoVMA11, like other fungal VMA genes, is associated with numerous cellular functions and highlighted that V-ATPase is essential for infection-related morphogenesis and pathogenesis in M. oryzae.

Yap1-dependent oxidative stress response provides a link to riboflavin production in Ashbya gossypii

Ashbya gossypii is a natural overproducer of riboflavin. Overproduction of riboflavin can be induced by environmental stress, e.g. nutritional or oxidative stress. The Yap-protein family has a well-documented role in stress response. Particularly, Yap1 has a major role in directing the oxidative stress responses. The A. gossypii YAP-family consists of only three genes in contrast to its closest relative Eremothecium cymbalariae, which has four YAP-homologs. Gene order at Eremothecium YAP-loci is conserved with the reconstructed yeast ancestor. AgYap1p is unique amongst Yap-homologs as it lacks the cysteine-rich domains (CRDs). AgYAP1 expression is inducible and GFP-AgYap1 localizes to the nucleus. Agyap1 mutants displayed higher sensitivity against oxidative stress - H(2)O(2) and menadione - and are strongly reduced in riboflavin production. High levels of cAMP, which also reduce riboflavin production, show a synergistic effect on this sensitivity. AgYAP1 and a chimera of AgYAP1 (with the DNA-binding domain) and ScYAP1 (with the CRDs) can both complement the Scyap1 oxidative stress sensitivity. This suggests that the DNA-binding sites of ScYap1 are conserved in A. gossypii. Expression of AgRIB4, which contains three putative Yap1-binding sites, assayed via a lacZ-reporter gene was strongly reduced in an Agyap1 mutant suggesting a direct involvement of AgYap1 in riboflavin production. Furthermore, our data show that application of H(2)O(2) stress leads to an increase in riboflavin production in a Yap1-dependent manner.

Genome evolution in the eremothecium clade of the Saccharomyces complex revealed by comparative genomics

We used comparative genomics to elucidate the genome evolution within the pre-whole-genome duplication genus Eremothecium. To this end, we sequenced and assembled the complete genome of Eremothecium cymbalariae, a filamentous ascomycete representing the Eremothecium type strain. Genome annotation indicated 4712 gene models and 143 tRNAs. We compared the E. cymbalariae genome with that of its relative, the riboflavin overproducer Ashbya (Eremothecium) gossypii, and the reconstructed yeast ancestor. Decisive changes in the Eremothecium lineage leading to the evolution of the A. gossypii genome include the reduction from eight to seven chromosomes, the downsizing of the genome by removal of 10% or 900 kb of DNA, mostly in intergenic regions, the loss of a TY3-Gypsy-type transposable element, the re-arrangement of mating-type loci, and a massive increase of its GC content. Key species-specific events are the loss of MNN1-family of mannosyltransferases required to add the terminal fourth and fifth α-1,3-linked mannose residue to O-linked glycans and genes of the Ehrlich pathway in E. cymbalariae and the loss of ZMM-family of meiosis-specific proteins and acquisition of riboflavin overproduction in A. gossypii. This reveals that within the Saccharomyces complex genome, evolution is not only based on genome duplication with subsequent gene deletions and chromosomal rearrangements but also on fungi associated with specific environments (e.g. involving fungal-insect interactions as in Eremothecium), which have encountered challenges that may be reflected both in genome streamlining and their biosynthetic potential.

Genetic control of biosynthesis and transport of riboflavin and flavin nucleotides and construction of robust biotechnological producers

Riboflavin [7,8-dimethyl-10-(1'-d-ribityl)isoalloxazine, vitamin B₂] is an obligatory component of human and animal diets, as it serves as the precursor of flavin coenzymes, flavin mononucleotide, and flavin adenine dinucleotide, which are involved in oxidative metabolism and other processes. Commercially produced riboflavin is used in agriculture, medicine, and the food industry. Riboflavin synthesis starts from GTP and ribulose-5-phosphate and proceeds through pyrimidine and pteridine intermediates. Flavin nucleotides are synthesized in two consecutive reactions from riboflavin. Some microorganisms and all animal cells are capable of riboflavin uptake, whereas many microorganisms have distinct systems for riboflavin excretion to the medium. Regulation of riboflavin synthesis in bacteria occurs by repression at the transcriptional level by flavin mononucleotide, which binds to nascent noncoding mRNA and blocks further transcription (named the riboswitch). In flavinogenic molds, riboflavin overproduction starts at the stationary phase and is accompanied by derepression of enzymes involved in riboflavin synthesis, sporulation, and mycelial lysis. In flavinogenic yeasts, transcriptional repression of riboflavin synthesis is exerted by iron ions and not by flavins. The putative transcription factor encoded by SEF1 is somehow involved in this regulation. Most commercial riboflavin is currently produced or was produced earlier by microbial synthesis using special selected strains of Bacillus subtilis, Ashbya gossypii, and Candida famata. Whereas earlier RF overproducers were isolated by classical selection, current producers of riboflavin and flavin nucleotides have been developed using modern approaches of metabolic engineering that involve overexpression of structural and regulatory genes of the RF biosynthetic pathway as well as genes involved in the overproduction of the purine precursor of riboflavin, GTP.

Identification of the genes affecting the regulation of riboflavin synthesis in the flavinogenic yeast Pichia guilliermondii using insertion mutagenesis

Pichia guilliermondii is a representative of a group of so-called flavinogenic yeast species that overproduce riboflavin (vitamin B(2)) in response to iron limitation. Using insertion mutagenesis, we isolated P. guilliermondii mutants overproducing riboflavin. Analysis of nucleotide sequence of recombination sites revealed that insertion cassettes integrated into the genome disrupting P. guilliermondii genes similar to the VMA1 gene of Ashbya gossypii and Saccharomyces cerevisiae and FES1 and FRA1 genes of S. cerevisiae. The constructed P. guilliermondiiΔvma1-17 mutant possessed five- to sevenfold elevated riboflavin production and twofold decreased iron cell content as compared with the parental strain. Pichia guilliermondiiΔfra1-45 mutant accumulated 1.8-2.2-fold more iron in the cells and produced five- to sevenfold more riboflavin as compared with the parental strain. Both Δvma1-17 and Δfes1-77 knockout strains could not grow at 37 °C in contrast to the wild-type strain and the Δfra1-45 mutant. Increased riboflavin production by the wild-type strain was observed at 37 °C. Although the Δfes1-77 mutant did not overproduce riboflavin, it showed partial complementation when crossed with previously isolated P. guilliermondii riboflavin-overproducing mutant rib80-22. Complementation analysis revealed that Δvma1-17 and Δfra1-45 mutants are distinct from previously reported riboflavin-producing mutants hit1-1, rib80-22 and rib81-31 of this yeast.

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.

Phosphoribosyl pyrophosphate synthetase activity affects growth and riboflavin production in Ashbya gossypii

Background

Phosphoribosyl pyrophosphate (PRPP) is a central compound for cellular metabolism and may be considered as a link between carbon and nitrogen metabolism. PRPP is directly involved in the de novo and salvage biosynthesis of GTP, which is the immediate precursor of riboflavin. The industrial production of this vitamin using the fungus Ashbya gossypii is an important biotechnological process that is strongly influenced by substrate availability.

Results

Here we describe the characterization and manipulation of two genes of A. gossypii encoding PRPP synthetase (AGR371C and AGL080C). We show that the AGR371C and AGL080C gene products participate in PRPP synthesis and exhibit inhibition by ADP. We also observed a major contribution of AGL080C to total PRPP synthetase activity, which was confirmed by an evident growth defect of the Δagl080c strain. Moreover, we report the overexpression of wild-type and mutant deregulated isoforms of Agr371cp and Agl080cp that significantly enhanced the production of riboflavin in the engineered A. gossypii strains.

Conclusion

It is shown that alterations in PRPP synthetase activity have pleiotropic effects on the fungal growth pattern and that an increase in PRPP synthetase enzymatic activity can be used to enhance riboflavin production in A. gossypii.

The Histoplasma capsulatum vacuolar ATPase is required for iron homeostasis, intracellular replication in macrophages and virulence in a murine model of histoplasmosis

Histoplasma capsulatum is a dimorphic fungal pathogen that survives and replicates within macrophages (Mphi). To identify specific genes required for intracellular survival, we utilized Agrobacterium tumefaciens-mediated mutagenesis, and screened for H. capsulatum insertional mutants that were unable to survive in human Mphi. One colony was identified that had an insertion within VMA1, the catalytic subunit A of the vacuolar ATPase (V-ATPase). The vma1 mutant (vma1::HPH) grew normally on iron-replete medium, but not on iron-deficient media. On iron-deficient medium, the growth of the vma1 mutant was restored in the presence of wild-type (WT) H. capsulatum yeasts, or the hydroxamate siderophore, rhodotorulic acid. However, the inability to replicate within Mphi was only partially restored by the addition of exogenous iron. The vma1::HPH mutant also did not grow as a mold at 28 degrees C. Complementation of the mutant (vma/VMA1) restored its ability to replicate in Mphi, grow on iron-poor medium and grow as a mold at 28 degrees C. The vma1::HPH mutant was avirulent in a mouse model of histoplasmosis, whereas the vma1/VMA1 strain was as pathogenic as WT yeasts. These studies demonstrate the importance of V-ATPase function in the pathogenicity of H. capsulatum, in iron homeostasis and in fungal dimorphism.

Overexpression of the riboflavin biosynthetic pathway in Pichia pastoris

BACKGROUND

High cell density cultures of Pichia pastoris grown on methanol tend to develop yellow colored supernatants, attributed to the release of free flavins. The potential of P. pastoris for flavin overproduction is therefore given, but not pronounced when the yeast is grown on glucose. The aim of this study is to characterize the relative regulatory impact of each riboflavin synthesis gene. Deeper insight into pathway control and the potential of deregulation is established by overexpression of the single genes as well as a combined deregulation of up to all six riboflavin synthesis genes.

RESULTS

Overexpression of the first gene of the riboflavin biosynthetic pathway (RIB1) is already sufficient to obtain yellow colonies and the accumulation of riboflavin in the supernatant of shake flask cultures growing on glucose. Sequential deregulation of all the genes, by exchange of their native promoter with the strong and constitutive glyceraldehyde-3-phosphate dehydrogenase promoter (PGAP) increases the riboflavin accumulation significantly.

CONCLUSION

The regulation of the pathway is distributed over more than one gene. High cell density cultivations of a P. pastoris strain overexpressing all six RIB genes allow the accumulation of 175 mg/L riboflavin in the supernatant. The basis for rational engineering of riboflavin production in P. pastoris has thus been established.

Growth stress triggers riboflavin overproduction in Ashbya gossypii

The filamentous fungus Ashbya gossypii is used for riboflavin biosynthesis on an industrial scale, but even the wild type displays overproduction. Because riboflavin overproduction was known to start at the transition between growth and stationary phase, it was suspected that overproduction was induced at low growth rates. However, chemostatic cultivations performed at different growth rates did not result in any detectable riboflavin formation. In this study, we report that it was not the final growth rate that triggered riboflavin overproduction but a decline in growth rate. Therefore, continuous fermenter cultivations with dilution rate shifts were performed. Peaks of riboflavin overproduction were observed in the wild type and in a RIB3placZ reporter strain after downshifts in dilution rate. Accumulation of riboflavin correlated with an increased expression of lacZ reporter activity. The step size of the downshifts corresponded to the peak size of riboflavin formation and reporter activity. Expression of further RIB genes encoding riboflavin biosynthetic enzymes was analyzed by RT-PCR. RIB mRNA levels of the ribulose-5-phosphate branch of the divided riboflavin biosynthesis pathway (RIB3, RIB4, and RIB5) were found to increase in the riboflavin production phase, whereas the RIB2 and RIB7 mRNA levels belonging to the GTP branch remained constant. We propose that a decline in growth rate triggers the increased expression of RIB3, RIB4, and RIB5 resulting in riboflavin overproduction. Because although a reduction in oxygen supply, temperature increase or decrease, or salt stress did affect growth, but neither did lead to riboflavin overproduction nor did induce RIB3 reporter expression, we conclude that declining nutrition must be the stress stimulus. Because about half of the cells in the hyphae of Ashbya gossypii did not accumulate riboflavin, the regulatory response on the cellular level can be estimated to be at least twice as great in comparison to what we detected as overall signals.

Purine biosynthesis, riboflavin production, and trophic-phase span are controlled by a Myb-related transcription factor in the fungus Ashbya gossypii

Ashbya gossypii is a natural riboflavin overproducer used in the industrial production of the vitamin. We have isolated an insertional mutant exhibiting higher levels of riboflavin production than the wild type. DNA analysis of the targeted locus in the mutant strain revealed that a syntenic homolog of the Saccharomyces cerevisiae BAS1 gene, a member of the Myb family of transcription factors, was inactivated. Directed gene disruption of AgBAS1 confirmed the phenotype observed for the insertional mutant, and the Deltabas1 mutant also showed auxotrophy for adenine and several growth defects, such as a delay in the germination of the spores and an abnormally prolonged trophic phase. Additionally, we demonstrate that the DNA-binding domain of AgBas1p is able to bind to the Bas1-binding motifs in the AgADE4 promoter; we also show a clear nuclear localization of a green fluorescent protein-Bas1 fusion protein. Real-time quantitative PCR analyses comparing the wild type and the Deltabas1 mutant revealed that AgBAS1 was responsible for the adenine-mediated regulation of the purine and glycine pathways, since the transcription of the ADE4 and SHM2 genes was virtually abolished in the Deltabas1 mutant. Furthermore, the transcription of ADE4 and SHM2 in the Deltabas1 mutant did not diminish during the transition from the trophic to the productive phase did not diminish, in contrast to what occurred in the wild-type strain. A C-terminal deletion in the AgBAS1 gene, comprising a hypothetical regulatory domain, caused constitutive activation of the purine and glycine pathways, enhanced riboflavin overproduction, and prolonged the trophic phase. Taking these results together, we propose that in A. gossypii, AgBAS1 is an important transcription factor that is involved in the regulation of different physiological processes, such as purine and glycine biosynthesis, riboflavin overproduction, and growth.

The where, when, and how of organelle acidification by the yeast vacuolar H+-ATPase

All eukaryotic cells contain multiple acidic organelles, and V-ATPases are central players in organelle acidification. Not only is the structure of V-ATPases highly conserved among eukaryotes, but there are also many regulatory mechanisms that are similar between fungi and higher eukaryotes. These mechanisms allow cells both to regulate the pHs of different compartments and to respond to changing extracellular conditions. The Saccharomyces cerevisiae V-ATPase has emerged as an important model for V-ATPase structure and function in all eukaryotic cells. This review discusses current knowledge of the structure, function, and regulation of the V-ATPase in S. cerevisiae and also examines the relationship between biosynthesis and transport of V-ATPase and compartment-specific regulation of acidification.

A proteome analysis of the yeast response to the herbicide 2,4-dichlorophenoxyacetic acid

The intensive use of herbicides may give rise to a number of toxicological problems in non-target organisms and has led to the emergence of resistant weeds. To gain insights into the mechanisms of adaptation to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), we have identified variations in protein expression level in the eukaryotic experimental model Saccharomyces cerevisiae exposed to herbicide aggression, based on two-dimensional gel electrophoresis. We show results suggesting that during the adaptation period preceding the resumption of inhibited exponential growth under herbicide stress, the antioxidant enzyme Ahp1p and the heat shock proteins Hsp12p and Ssb2p (or Ssb1p) are present in higher amounts. The increased level of other enzymes involved in protein (Cdc48p) and mRNA (Dcp1p) degradation, in carbohydrate metabolism (Eno1p, Eno2p and Glk1p) and in vacuolar H(+)-ATPase (V-ATPase) function (Vma1p and Vma2p, two subunits of the peripheral catalytic sector) was also registered. V-ATPase is involved in the homeostasis of intracellular pH and in the compartmentalization of amino acids and other metabolites in the vacuole. The increased expression of amino acid biosynthetic enzymes (Arg1p, Aro3p, Aro8p, Gdh1p, His4p, Ilv3p and Met6p), also suggested by comparative analysis of the proteome, was correlated with the reduction of amino acid concentration registered in both the vacuole and the cytosol of 2,4-D-stressed cells, possibly due to the disturbance of vacuolar and plasma membrane functions by the lipophilic acid herbicide.

Metabolic engineering of the purine pathway for riboflavin production in Ashbya gossypii

Purine nucleotides are essential precursors for living organisms because they are involved in many important processes, such as nucleic acid synthesis, energy supply, and the biosynthesis of several amino acids and vitamins such as riboflavin. GTP is the immediate precursor for riboflavin biosynthesis, and its formation through the purine pathway is subject to several regulatory mechanisms in different steps. Extracellular purines repress the transcription of most genes required for de novo ATP and GTP synthesis. Additionally, three enzymes of the pathway, phosphoribosyl pyrophosphate (PRPP) amidotransferase, adenylosuccinate synthetase, and IMP dehydrogenase, are subject to feedback inhibition by their end products. Here we report the characterization and manipulation of the committed step in the purine pathway of the riboflavin overproducer Ashbya gossypii. We report that phosphoribosylamine biosynthesis in A. gossypii is negatively regulated at the transcriptional level by extracellular adenine. Furthermore, we show that ATP and GTP exert a strong inhibitory effect on the PRPP amidotransferase from A. gossypii. We constitutively overexpressed the AgADE4 gene encoding PRPP amidotransferase in A. gossypii, thereby abolishing the adenine-mediated transcriptional repression. In addition, we replaced the corresponding residues (aspartic acid310, lysine333, and alanine417) that have been described to be important for PRPP amidotransferase feedback inhibition in other organisms by site-directed mutagenesis. With these manipulations, we managed to enhance metabolic flow through the purine pathway and to increase the production of riboflavin in the triple mutant strain 10-fold (228 mg/liter).

Expression of alanine:glyoxylate aminotransferase gene from Saccharomyces cerevisiae in Ashbya gossypii

Two plasmids containing an autonomously replicating sequence from Saccharomyces cerevisiae were constructed. Using these vectors, the AGX1 gene encoding alanine:glyoxylate aminotransferase (AGT) from S. cerevisiae, which converts glyoxylate into glycine but is not present in Ashbya gossypii, was expressed in A. gossypii. Geneticin-resistant transformants with the plasmid having the kanamycin resistance gene under the control of the translation elongation factor 1 alpha (TEF) promoter and terminator from A. gossypii were obtained with a transformation efficiency of approximately 10-20 transformants per microgram of plasmid DNA. The specific AGT activities of A. gossypii pYPKTPAT carrying the AGX1 gene in glucose- and rapeseed-oil-containing media were 40 and 160 mU mg-1 of wet mycelial weight, respectively. The riboflavin concentrations of A. gossypii pYPKTPAT carrying AGX1 gene in glucose- and rapeseed-oil-containing media were 20 and 150 mg l-1, respectively. In the presence of 50 mM glyoxylate, the riboflavin concentration and the specific riboflavin concentration of A. gossypii pYPKTPAT were 2- and 1.3-fold those of A. gossypii pYPKT without the AGX1 gene.

Characterization of Schizosaccharomyces pombe mutants defective in vacuolar acidification and protein sorting

The vacuolar H+-ATPases (V-ATPases) are ATP-dependent proton pumps responsible for acidification of intracellular compartments in eukaryotic cells. To investigate the functional roles of the V-ATPase in Schizosaccharomyces pombe, the gene vma1 encoding subunit A or vma3 encoding subunit c was disrupted. Both deletion mutants lost the capacity for vacuolar acidification in vivo, and showed sensitivity to neutral pH or high concentrations of divalent cations including Ca2+. The delivery of FM4-64 to the vacuolar membrane and accumulation of Lucifer Yellow CH were strongly inhibited in the vma1 and vma3 mutants. Moreover, deletion of the S. pombe vma1+ or vma3+ gene resulted in pleiotropic phenotypes consistent with lack of vacuolar acidification, including the missorting of vacuolar carboxypeptidase Y, abnormal vacuole morphology, and mating defects. These findings suggest that V-ATPase is essential for endocytosis, ion and pH homeostasis, and for intracellular targeting of vacuolar proteins and vacuolar biogenesis in S. pombe.

Spectrofluorimetric method for the estimation of total lipids in Eremothecium ashbyii fungal filaments using Nile blue and avoiding interference of autofluorescent riboflavin

A rapid, simple and sensitive spectrofluorimetric technique was developed for monitoring total lipids in hyphae of the riboflavin-overproducing fungus Eremothecium ashbyii using the fluorescent probe Nile blue in an aqueous system, avoiding the interference due to autofluorescent riboflavin. The existing methodologies for lipid estimation are tedious, requiring large biomass, solvent extraction and gravimetry. E. ashbyii is a hemiascomycete fungus which accumulates lipids in its mycelia prior to flavinogenesis. This study defines the conditions (wavelength selection and sensitivity) for the spectrofluorimetric quantification of lipids in situ in the macerated mycelia of this fungus in the presence of intracellular autofluorescent riboflavin without the need to extract the lipids from the mycelia. The fluorescent intensity was linear with the lipid concentration of the mycelia (by gravimetry) under three different growth conditions using glucose, olive oil and sunflower oil as carbon sources. This spectrofluorimetic method of lipid estimation can be applied to other fungi and microorganisms.

Degeneration of a homing endonuclease and its target sequence in a wild yeast strain

Mobile introns and inteins self-propagate by 'homing', a gene conversion process initiated by site-specific homing endonucleases. The VMA intein, which encodes the PI-SceI endonuclease in Saccharomyces cerevisiae, is present in several different yeast strains. Surprisingly, a wild wine yeast (DH1-1A) contains not only the intein(+) allele, but also an inteinless allele that has not undergone gene conversion. To elucidate how these two alleles co-exist, we characterized the endonuclease encoded by the DH1-1A intein(+) allele and the target site in the intein(-) allele. Sequence analysis reveals seven mutations in the 31 bp recognition sequence, none of which occurs at positions that are individually critical for activity. However, binding and cleavage of the sequence by PI-SceI is reduced 10-fold compared to the S.cerevisiae target. The PI-SceI analog encoded by the DH1-1A intein(+) allele contains 11 mutations at residues in the endonuclease and protein splicing domains. None affects protein splicing, but one, a R417Q substitution, accounts for most of the decrease in DNA cleavage and DNA binding activity of the DH1-1A protein. Loss of activity in the DH1-1A endonuclease and target site provides one explanation for co-existence of the intein(+) and intein(-) alleles.

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.

Cytosolic Ca2+ homeostasis is a constitutive function of the V-ATPase in Saccharomyces cerevisiae

The vacuole is the major site of intracellular Ca(2+) storage in yeast and functions to maintain cytosolic Ca(2+) levels within a narrow physiological range via a Ca(2+) pump (Pmc1p) and a H(+)/Ca(2+) antiporter (Vcx1p) driven by the vacuolar H(+)-ATPase (V-ATPase). We examined the function of the V-ATPase in cytosolic Ca(2+) homeostasis by comparing responses to a brief Ca(2+) challenge of a V-ATPase mutant (vma2Delta) and wild-type cells treated with the V-ATPase inhibitor concanamycin A. The kinetics of the Ca(2+) response were determined using transgenic aequorin as an in vivo cytosolic Ca(2+) reporter system. In wild-type cells, the V-ATPase-driven Vcx1p was chiefly responsible for restoring cytosolic Ca(2+) concentrations after a brief pulse. In cells lacking V-ATPase activity, brief exposure to elevated Ca(2+) compromised viability, even when there was little change in the final cytosolic Ca(2+) concentration. vma2Delta cells were more efficient at restoring cytosolic [Ca(2+)] after a pulse than concanamycin-treated wild-type cells, suggesting long term loss of V-ATPase triggers compensatory mechanisms. This compensation was dependent on calcineurin, and was mediated primarily by Pmc1p.

Molecular characterization of FMN1, the structural gene for the monofunctional flavokinase of Saccharomyces cerevisiae

Flavokinase catalyzes the transfer of the gamma-phosphoryl group of ATP to riboflavin to form the flavocoenzyme FMN. Consistent patterns of sequence similarities have identified the open reading frame of unknown function YDR236c as a candidate to encode flavokinase in Saccharomyces cerevisiae. In order to determine whether the product of this gene corresponds to yeast flavokinase, its coding region was amplified from S. cerevisiae genomic DNA by polymerase chain reaction and expressed in Escherichia coli. The purified form of the expressed recombinant protein efficiently catalyzed the formation of FMN from riboflavin and ATP. In contrast to bifunctional prokaryotic flavokinase/FAD synthetase enzymes, the yeast enzyme did not show accompanying FAD synthetase activity. Deletion of YDR236c produced yeast mutants unable to grow on rich medium; however, the growth of the ydr236cDelta mutants could be rescued by the addition of FMN to the medium. Overexpression of YDR236c caused a 50-fold increase in flavokinase specific activity in yeast cells. These findings demonstrate that YDR236c corresponds to the gene encoding a monofunctional flavokinase in yeast, which we propose to be designated as FMN1. The FMN1 gene codes for a 25-kDa protein with characteristics of signals for import into mitochondria. By immunoblotting analysis of Saccharomyces subcellular fractions, we provide evidence that the Fmn1 protein is localized in microsomes and in mitochondria. Analysis of submitochondrial fractions revealed that the mitochondrial form of Fmn1p is an integral protein of the inner membrane exposing its COOH-terminal domain to the matrix space. A similarity search in the data base banks revealed the presence of sequences homologous to yeast flavokinase in the genome of several eukaryotic organisms such as Schizosaccharomyces pombe, Arabidopsis thaliana, Drosophila melanogaster, Caenorhabditis elegans, and humans.

Disruption of vma-1, the gene encoding the catalytic subunit of the vacuolar H(+)-ATPase, causes severe morphological changes in Neurospora crassa

By using the process of Repeat-induced Point mutation (Selker, E. U., and Garrett, P. W. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 6870-6874), we inactivated vma-1, the gene encoding subunit A of the V-ATPase of Neurospora crassa. Two vma-1 mutant strains were characterized. One was mutated at multiple sites, did not make a protein product, and produced spores that only rarely germinated. The other had four point mutations, made a protein product, and produced viable spores. Neither strain had detectable V-ATPase activity. The vma-1 mutant strains did not grow in medium buffered to pH 7.0 or above or in medium supplemented with the cation Zn(2+). They were completely resistant to inhibition by concanamycin C, supporting our hypothesis that the V-ATPase is the in vivo target of this antibiotic. Inactivation of the vma-1 gene had a pronounced effect on morphology and development of the organism. In the mutants tip growth was inhibited, and multiple branching was induced. The vma-1 mutant strains could not differentiate conidia or perithecia. They could grow slowly as mycelia and could donate nuclei in a sexual cross. A mutation in the plasma membrane ATPase, which suppressed the sensitivity of wild type N. crassa to concanamycin, also proved effective in suppressing the sensitivity of a vma-1 null mutant to basic pH but did not correct the morphological defects.

Alternative mechanisms of vacuolar acidification in H(+)-ATPase-deficient yeast

Acidification of the endosomal/lysosomal pathway by the vacuolar-type proton translocating ATPase (V-ATPase) is necessary for a variety of essential eukaryotic cellular functions. Nevertheless, yeasts lacking V-ATPase activity (Deltavma) are viable when grown at low pH, suggesting alternative methods of organellar acidification. This was confirmed by directly measuring the vacuolar pH by ratio fluorescence imaging. When Deltavma yeasts were cultured and tested in the acidic conditions required for growth of V-ATPase-deficient mutants, the vacuolar pH was 5.9. Fluid-phase pinocytosis of acidic extracellular medium cannot account for these observations, because the V-ATPase-independent vacuolar acidification was unaffected in mutants deficient in endocytosis. Similarly, internalization of the plasmalemmal H(+)-ATPase (Pma1p) was ruled out, because overexpression of Pma1p failed to complement the Deltavma phenotype and did not potentiate the vacuolar acidification. To test whether weak electrolytes present in the culture medium could ferry acid equivalents to the vacuole, wild-type and the Deltavma yeasts were subjected to sudden changes in extracellular pH. In both cell types, the vacuoles rapidly alkalinized when external pH was raised from 5.5 (the approximate pH of the culture medium) to 7.5 and re-acidified when the yeasts were returned to a medium of pH 5.5. Importantly, these rapid pH changes were only observed when NH(4)(+), routinely added as a nitrogen source, was present. The NH(4)(+)-dependent acidification was not due to efflux of NH(3) from the vacuole, as cells equilibrated to pH 7.5 in the absence of weak electrolytes rapidly acidified when challenged with an acidic medium containing NH(4)(+). These findings suggest that although NH(3) can act as a cell-permeant proton scavenger, NH(4)(+) may function as a protonophore, facilitating equilibration of the pH across the plasma and vacuolar membranes of yeast. The high concentration of NH(4)(+) frequently added as a nitrogen source to yeast culture media together with effective NH(4)(+) transporters thereby facilitate vacuolar acidification when cells are suspended in acidic solutions.