Dimorphism in Yarrowia lipolytica: filament formation is suppressed by nitrogen starvation and inhibition of respiration

Genome-wide CRISPR-Cas9 screen reveals a persistent null-hyphal phenotype that maintains high carotenoid production in Yarrowia lipolytica

Yarrowia lipolytica is a metabolic engineering host of growing industrial interest due to its ability to metabolize hydrocarbons, fatty acids, glycerol, and other renewable carbon sources. This dimorphic yeast undergoes a stress-induced transition to a multicellular hyphal state, which can negatively impact biosynthetic activity, reduce oxygen and nutrient mass transfer in cell cultures, and increase culture viscosity. Identifying mutations that prevent the formation of hyphae would help alleviate the bioprocess challenges that they create. To this end, we conducted a genome-wide CRISPR screen to identify genetic knockouts that prevent the transition to hyphal morphology. The screen identified five mutants with a null-hyphal phenotype-ΔRAS2, ΔRHO5, ΔSFL1, ΔSNF2, and ΔPAXIP1. Of these hits, only ΔRAS2 suppressed hyphal formation in an engineered lycopene production strain over a multiday culture. The RAS2 knockout was also the only genetic disruption characterized that did not affect lycopene production, producing more than 5 mg L^-1 OD^-1 from a heterologous pathway with enhanced carbon flux through the mevalonate pathway. These data suggest that a ΔRAS2 mutant of Y. lipolytica could prove useful in engineering a metabolic engineering host of the production of carotenoids and other biochemicals.

Molecular and comparative genomic analyses reveal evolutionarily conserved and unique features of the Schizosaccharomyces japonicus mycelial growth and the underlying genomic changes

Fungal pathogens, from phytopathogenic fungus to human pathogens, are able to alternate between the yeast-like form and filamentous forms. This morphological transition (dimorphism) is in close connection with their pathogenic lifestyles and with their responses to changing environmental conditions. The mechanisms governing these morphogenetic conversions are still not fully understood. Therefore, we studied the filamentous growth of the less-known, non-pathogenic dimorphic fission yeast, S. japonicus, which belongs to an ancient and early evolved branch of the Ascomycota. Its RNA sequencing revealed that several hundred genes were up- or down-regulated in the hyphae compared to the yeast-phase cells. These genes belonged to different GO categories, confirming that mycelial growth is a rather complex process. The genes of transport- and metabolic processes appeared especially in high numbers among them. High expression of genes involved in glycolysis and ethanol production was found in the hyphae, while other results pointed to the regulatory role of the protein kinase A (PKA) pathway. The homologues of 49 S. japonicus filament-associated genes were found by sequence alignments also in seven distantly related dimorphic and filamentous species. The comparative genomic analyses between S. japonicus and the closely related but non-dimorphic S. pombe shed some light on the differences in their genomes. All these data can contribute to a better understanding of hyphal growth and those genomic rearrangements that underlie it.

Sec14 family of lipid transfer proteins in yeasts

The hydrophobicity of lipids prevents their free movement across the cytoplasm. To achieve highly heterogeneous and precisely regulated lipid distribution in different cellular membranes, lipids are transported by lipid transfer proteins (LTPs) in addition to their transport by vesicles. Sec14 family is one of the most extensively studied groups of LTPs. Here we provide an overview of Sec14 family of LTPs in the most studied yeast Saccharomyces cerevisiae as well as in other selected non-Saccharomyces yeasts-Schizosaccharomyces pombe, Kluyveromyces lactis, Candida albicans, Candida glabrata, Cryptococcus neoformans, and Yarrowia lipolytica. Discussed are specificities of Sec14-domain LTPs in various yeasts, their mode of action, subcellular localization, and physiological function. In addition, quite few Sec14 family LTPs are target of antifungal drugs, serve as modifiers of drug resistance or influence virulence of pathologic yeasts. Thus, they represent an important object of study from the perspective of human health.

Investigating the Smuts: Common Cues, Signaling Pathways, and the Role of MAT in Dimorphic Switching and Pathogenesis

The corn smut fungus Ustilago maydis serves as a model species for studying fungal dimorphism and its role in phytopathogenic development. The pathogen has two growth phases: a saprobic yeast phase and a pathogenic filamentous phase. Dimorphic transition of U. maydis involves complex processes of signal perception, mating, and cellular reprogramming. Recent advances in improvement of reference genomes, high-throughput sequencing and molecular genetics studies have been expanding research in this field. However, the biology of other non-model species is frequently overlooked. This leads to uncertainty regarding how much of what is known in U. maydis is applicable to other dimorphic fungi. In this review, we will discuss dimorphic fungi in the aspects of physiology, reproductive biology, genomics, and molecular genetics. We also perform comparative analyses between U. maydis and other fungi in Ustilaginomycotina, the subphylum to which U. maydis belongs. We find that lipid/hydrophobicity is a potential common cue for dimorphic transition in plant-associated dimorphic fungi. However, genomic profiles alone are not adequate to explain dimorphism across different fungi.

Genetic and bioprocess engineering to improve squalene production in Yarrowia lipolytica

Squalene is the precursor for triterpene-based natural products and steroids-based drugs. It has been widely used as pharmaceutical intermediates and personal care products. The aim of this work is to test the feasibility of engineering Yarrowia lipolytica as a potential host for squalene production. The bottleneck of the pathway was removed by overexpressing native HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase. With the recycling of NADPH from the mannitol cycle, the engineered strain produced about 180.3 mg/L and 188.2 mg/L squalene from glucose or acetate minimal media. By optimizing the C/N ratio, controlling the media pH and mitigating acetyl-CoA flux competition from lipogenesis, the engineered strain produced 502.7 mg/L squalene, a 28-fold increase over the parental strain (17.2 mg/L). This work may serve as a baseline to harness Y. lipolytica as an oleaginous cell factory for sustainable production of squalene or terpenoids-based chemicals and natural products.

Multicellular growth of the Basidiomycota phytopathogen fungus Sporisorium reilianum induced by acid conditions

Fungi are considered model organisms for the analysis of important phenomena of eukaryotes. For example, some of them have been described as models to understand the phenomenon of multicellularity acquisition by different unicellular organisms phylogenetically distant. Interestingly, in this work, we describe the multicellular development in the model fungus S. reilianum. We observed that Sporisorium reilianum, a Basidiomycota cereal pathogen that at neutral pH grows with a yeast-like morphology during its saprophytic haploid stage, when incubated at acid pH grew in the form of multicellular clusters. The multicellularity observed in S. reilianum was of clonal type, where buds of "stem" cells growing as yeasts remain joined by their cell wall septa, after cytokinesis. The elaboration and analysis of a regulatory network of S. reilianum showed that the putative zinc finger transcription factor CBQ73544.1 regulates a number of genes involved in cell cycle, cellular division, signal transduction pathways, and biogenesis of cell wall. Interestingly, homologous of these genes have been found to be regulated during Saccharomyces cerevisiae multicellular growth. In adddition, some of these genes were found to be negatively regulated during multicellularity of S. reilianum. With these data, we suggest that S. reilianum is an interesting model for the study of multicellular development.

Green and sustainable succinic acid production from crude glycerol by engineered Yarrowia lipolytica via agricultural residue based in situ fibrous bed bioreactor

In situ fibrous bed bioreactor (isFBB) for efficient succinic acid (SA) production by Yarrowia lipolytica was firstly developed in our former study. In this study, agricultural residues including wheat straw, corn stalk and sugarcane bagasse were investigated for the improvement of isFBB, and sugarcane bagasse was demonstrated to be the best immobilization material. With crude glycerol as the sole carbon source, optimization for isFBB batch fermentation was carried out. Under the optimal conditions of 20g sugarcane bagasse as immobilization material, 120gL^-1 crude glycerol as carbon source and 4Lmin^-1 of aeration rate, the resultant SA concentration was 53.6gL^-1 with an average productivity of 1.45gL^-1h^-1 and a SA yield of 0.45gg^-1. By feeding crude glycerol, SA titer up to 209.7gL^-1 was obtained from fed batch fermentation, which was the highest value that ever reported.

Online flow cytometry, an interesting investigation process for monitoring lipid accumulation, dimorphism, and cells' growth in the oleaginous yeast Yarrowia lipolytica JMY 775

This study aims to understand and better control the main biological mechanisms and parameters modulating the various phenomena affecting Yarrowia lipolytica JMY775 and its lipids accumulation. The results obtained in this study stress forward that the use of an original tool, consisting of coupling bioreactors to online flow cytometry, is highly efficient. Throughout 48 h of culturing, this emerging process allowed an online continuous observation of the effects of pH and/or aeration on the cell growth and dimorphism and lipid accumulation by Y. lipolytica. This present study showed clearly that online flow cytometry is an advantageous tool for the real-time monitoring of microbial culture at a single-cell level. Indeed, the present investigation showed for the first time that profiling of the various phenomena and their monitoring upon culture time is now possible by coupling online cytometry with culture bioreactors.

Regulation of amino-acid metabolism controls flux to lipid accumulation in Yarrowia lipolytica

Yarrowia lipolytica is a promising microbial cell factory for the production of lipids to be used as fuels and chemicals, but there are few studies on regulation of its metabolism. Here we performed the first integrated data analysis of Y. lipolytica grown in carbon and nitrogen limited chemostat cultures. We first reconstructed a genome-scale metabolic model and used this for integrative analysis of multilevel omics data. Metabolite profiling and lipidomics was used to quantify the cellular physiology, while regulatory changes were measured using RNAseq. Analysis of the data showed that lipid accumulation in Y. lipolytica does not involve transcriptional regulation of lipid metabolism but is associated with regulation of amino-acid biosynthesis, resulting in redirection of carbon flux during nitrogen limitation from amino acids to lipids. Lipid accumulation in Y. lipolytica at nitrogen limitation is similar to the overflow metabolism observed in many other microorganisms, e.g. ethanol production by Sacchromyces cerevisiae at nitrogen limitation.

L-Phenylalanine catabolism and 2-phenylethanol synthesis in Yarrowia lipolytica--mapping molecular identities through whole-proteome quantitative mass spectrometry analysis

A world-wide effort is now being pursued towards the development of flavors and fragrances (F&F) production independently from traditional sources, as well as autonomously from depleting fossil fuel supplies. Biotechnological production of F&F by microbes has emerged as a vivid solution to the current market limitations. Amongst a wide variety of fragrant chemicals, 2-PE is of significant interest to both scientific and industrial community. Although the general overview of the 2-PE synthesis pathway is commonly known, involvement of particular molecular identities in this pathway has not been elucidated in Yarrowia lipolytica to date. The aim of this study was mapping molecular identities involved in 2-PE synthesis in Y. lipolytica. To acquire a comprehensive landscape of the proteins that are directly and indirectly involved in L-Phe degradation and 2-PE synthesis, we took advantage of comprehensibility and sensitivity of high-throughput LC-MS/MS-quantitative analysis. Amongst a number of proteins involved in amino acid turnover and the central carbon metabolism, enzymes involved in L-Phe conversion to 2-PE have been identified. Results on yeast-to-hyphae transition in relation to the character of the provided nitrogen source have been presented.

A role for the rap GTPase YlRsr1 in cellular morphogenesis and the involvement of YlRsr1 and the ras GTPase YlRas2 in bud site selection in the dimorphic yeast Yarrowia lipolytica

Yarrowia lipolytica is a dimorphic yeast species that can grow in the ovoid yeast form or in the elongated pseudohyphal or hyphal form depending on the growth conditions. Here, we show that the Rap GTPase Rsr1 of Y. lipolytica (YlRsr1) plays an important role in cellular morphogenesis in this microorganism. Cells deleted for YlRSR1 exhibited impaired polarized growth during yeast-form growth. Pseudohyphal and hyphal development were also abnormal. YlRsr1 is also important for cell growth, since the deletion of YlRSR1 in cells lacking the Ras GTPase YlRas2 caused lethality. Y. lipolytica cells bud in a bipolar pattern in which the cells produce the new buds at the two poles. YlRsr1 plays a prominent role in this bud site selection process. YlRsr1's function in bud site selection absolutely requires the cycling of YlRsr1 between the GTP- and GDP-bound states but its function in cellular morphogenesis does not, suggesting that the two processes are differentially regulated. Interestingly, the Ras GTPase YlRas2 is also involved in the control of bud site selection, as Ylras2Δ cells were severely impaired in bipolar bud site selection. The GTP/GDP cycling and the plasma membrane localization of YlRas2 are important for YlRas2's function in bud site selection. However, they are not essential for this process, suggesting that the mechanism by which YlRas2 acts is different from that of YlRsr1. Our results suggest that YlRsr1 is regulated by the GTPase-activating protein (GAP) YlBud2 and partially by YlCdc25, the potential guanine nucleotide exchange factor (GEF) for YlRas2.

Transcriptomic analysis of the dimorphic transition of Ustilago maydis induced in vitro by a change in pH

Dimorphism is the property of fungi to grow as budding yeasts or mycelium, depending on the environmental conditions. This phenomenon is important as a model of differentiation in eukaryotic organisms, and since a large number of fungal diseases are caused by dimorphic fungi, its study is important for practical reasons. In this work, we examined the transcriptome during the dimorphic transition of the basidiomycota phytopathogenic fungus Ustilago maydis using microarrays, utilizing yeast and mycelium monomorphic mutants as controls. This way, we thereby identified 154 genes of the fungus that are specifically involved in the dimorphic transition induced by a pH change. Of these, 82 genes were up-regulated, and 72 were down-regulated. Differential categorization of these genes revealed that they mostly belonged to the classes of metabolism, cell cycle and DNA processing, transcription and protein fate, transport and cellular communication, stress, cell differentiation and biogenesis of cellular components, while a significant number of them corresponded to unclassified proteins. The data reported in this work are important for our understanding of the molecular bases of dimorphism in U. maydis, and possibly of other fungi.

The RAD52 ortholog of Yarrowia lipolytica is essential for nuclear integrity and DNA repair

Yarrowia lipolytica (Yl) is a dimorphic fungus that has become a well-established model for a number of biological processes, including secretion of heterologous and chimerical proteins. However, little is known on the recombination machinery responsible for the integration in the genome of the exogenous DNA encoding for those proteins. We have carried out a phenotypic analysis of rad52 deletants of Y. lipolytica. YlRad52 exhibited 20-30% identity with Rad52 homologues of other eukaryotes, including Saccharomyces cerevisiae and Candida albicans. Ylrad52-Δ strains formed colonies on YPD-agar plates which were spinier and smaller than those from wild type, whereas in YPD liquid cultures they exhibited a decreased grow rate and contained cells with aberrant morphology and fragmented chromatin, supporting a role for homologous recombination (HR) in genome stability under nondamaging conditions. In addition, Ylrad52 mutants showed moderate to high sensitivity to UV light, oxidizing agents and compounds that cause single- (SSB) and double-strand breaks (DSB), indicating an important role for Rad52 in DNA repair. These findings extend to Yl previous observations indicating that RAD52 is a crucial gene for DNA repair in other fungi, including S. cerevisiae, C. albicans and Schizosaccharomyces pombe.

The TEA/ATTS transcription factor YlTec1p represses the yeast-to-hypha transition in the dimorphic yeast Yarrowia lipolytica

Tec1p in the budding yeast Saccharomyces cerevisiae is important for dimorphic transition. In this study, we identified a homologue of Tec1p, YlTec1p, in the distantly related dimorphic yeast Yarrowia lipolytica. YlTec1p contains an evolutionarily conserved TEA/ATTS DNA-binding domain. Expression of YlTEC1 in S. cerevisiae tec1Δ cells rescued the invasive growth defect and activated a FLO11-lacZ reporter, indicating that YlTec1p is functionally related to Tec1p. However, YlTEC1 expression failed to activate an FRE-lacZ reporter, suggesting that these two transcription factors are different. YlTEC1 plays a negative role in the yeast-to-hypha transition in Y. lipolytica based on gene deletion and overexpression studies. We show that YlTec1p activates rather than represses gene expression in Y. lipolytica by yeast one-hybrid assay, and YlTec1p is critical for the activation of FLO11-lacZ in Y. lipolytica. In addition, YlTec1p localized to the nucleus and its nuclear localization decreased during hyphal growth. We speculate that YlTec1p may normally regulate the expression of a set of target genes that may prevent rather than promote hyphal development in Y. lipolytica. Our study also suggests that YlTEC1 may not be largely regulated by the cAMP-protein kinase A pathway.

Design of an efficient medium for heterologous protein production in Yarrowia lipolytica: case of human interferon alpha 2b

Background

The non conventional yeast Yarrowia lipolytica has aroused a strong industrial interest for heterologous protein production. However most of the studies describing recombinant protein production by this yeast rely on the use of complex media, such media are not convenient for large scale production particularly for products intended for pharmaceutical applications. In addition medium composition can also affect the production yield. Hence it is necessary to design an efficient medium for therapeutic protein expression by this host.

Results

Five different media, including four minimal media and a complex medium, were assessed in shake flasks for the production of human interferon alpha 2b (hIFN α2b) by Y. lipolytica under the control of POX2 promoter inducible with oleic acid. The chemically defined medium SM4 formulated by Invitrogen for Pichia pastoris growth was the most suitable. Using statistical experimental design this medium was further optimized. The selected minimal medium consisting in SM4 supplemented with 10 mg/l FeCl₃, 1 g/l glutamate, 5 ml/l PTM1 (Pichia Trace Metals) solution and a vitamin solution composed of myo-inositol, thiamin and biotin was called GNY medium. Compared to shake flask, bioreactor culture in GNY medium resulted in 416-fold increase of hIFN α2b production and 2-fold increase of the biological activity.

Furthermore, SM4 enrichment with 5 ml/l PTM1 solution contributed to protect hIFN α2b against the degradation by the 28 kDa protease identified by zymography gel in culture supernatant. The screening of the inhibitory effect of the trace elements present in PTM1 solution on the activity of this protease was achieved using a Box-Behnken design. Statistical data analysis showed that FeCl₃ and MnSO₄ had the most inhibitory effect.

Conclusion

We have designed an efficient medium for large scale production of heterologous proteins by Y. lipolytica. The optimized medium GNY is suitable for the production of hIFN α2b with the advantage that no complex nitrogen sources with non-defined composition were required.

Tay1 protein, a novel telomere binding factor from Yarrowia lipolytica

Inspection of the complete genome of the yeast Yarrowia lipolytica for the presence of genes encoding homologues of known telomere-binding proteins surprisingly revealed no counterparts of typical yeast Myb domain-containing telomeric factors including Rap1 or Taz1. Instead, we identified a gene, YALIOD10923g, encoding a protein containing two Myb domains, exhibiting a high degree of similarity to the Myb domain of human telomeric proteins TRF1 and TRF2 and homologous to an essential fission yeast protein Mug152 whose expression is elevated during meiosis. The protein, which we named Tay1p (telomere-associated in Yarrowia lipolytica 1), was purified for biochemical studies. Using a model Y. lipolytica telomere, we demonstrate that the protein preferentially binds to Y. lipolytica telomeric tracts. Tay1p binds along the telomeric tract as dimers and larger oligomers, and it is able to remodel the telomeric DNA into both looped structures and synaptic complexes of two model telomere DNAs. The ability of Tay1p to induce dimerization of telomeres in vitro goes in line with its oligomeric nature, where each oligomer can employ several Myb domains to form intermolecular telomere clusters. We also provide experimental evidence that Tay1p may be associated with Y. lipolytica telomeres in vivo. Together with its homologues from Schizosaccharomyces pombe and several basidiomycetous fungi (Sánchez-Alonso, P., and Guzman, P. (2008) Fungal Genet. Biol. 45, S54-S62), Tay1p constitutes a novel family of putative telomeric factors whose analysis may be instrumental in understanding the function and evolution of double-stranded DNA telomeric proteins.

Lack of the catalytic subunit of telomerase leads to growth defects accompanied by structural changes at the chromosomal ends in Yarrowia lipolytica

Comparative analysis of the telomeres of distantly related species has proven to be helpful for identifying novel components involved in telomere maintenance. We therefore initiated such a study in the nonconventional yeast Yarrowia lipolytica. Its genome encodes only a small fraction of the proteins that are typically associated with telomeres in other yeast models, indicating that its telomeres may employ noncanonical means for their stabilization and maintenance. In this report, we have measured the size of the telomeric fragments in wild-type strains, and characterized the catalytic subunit of telomerase (YlEst2p). In silico analysis of the YlEst2 amino acid sequence revealed the presence of domains typical for telomerase reverse transcriptases. Disruption of YlEST2 is not lethal, but results in retarded growth accompanied by a rapid loss of the telomeric sequences. This phenotype is associated with structural changes at the chromosomal ends in the ΔYlest2 mutants, likely the circularization of all six chromosomes. An apparent absence of several typical telomere-associated factors, as well as the presence of an efficient means of telomerase-independent telomere maintenance, qualify Y. lipolytica as an attractive model for the study of telomere maintenance mechanisms and a promising source of novel players in telomere dynamics.

Triplicate genes for mitochondrial ADP/ATP carriers in the aerobic yeast Yarrowia lipolytica are regulated differentially in the absence of oxygen

Yarrowia lipolytica is a strictly aerobic fungus, which differs from the extensively studied model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe with respect to its physiology, genetics and dimorphic growth habit. We isolated and sequenced cDNA and genomic clones (YlAAC1) from Y. lipolytica that encode a mitochondrial ADP/ATP carrier. The YlAAC1 gene can complement the S. cerevisiae Deltaaac2 deletion mutant. Southern hybridization, analysis of Yarrowia clones obtained in the course of the Genolevures project, and further sequencing revealed the existence of two paralogs of the YlAAC1 gene, which were named YlAAC2 and YlAAC3, respectively. Phylogenetic analysis showed that YlAAC1 and YlAAC2 were more closely related to each other than to YlAAC3, and are likely to represent the products of a recent gene duplication. All three Y. lipolytica YlAAC genes group together on the phylogenetic tree, suggesting that YlAAC3 is derived from a more ancient duplication within the Y. lipolytica lineage. A similar branching pattern for the three ScAAC paralogs in the facultative anaerobe S. cerevisiae demonstrates that two rounds of duplication of AAC genes occurred independently at least twice in the evolution of hemiascomycetous yeasts. Surprisingly, in both the aerobic Y. lipolytica and the facultative anaerobe S. cerevisiae, the three paralogs are differentially regulated in the absence of oxygen. Apparently, Y. lipolytica can sense hypoxia and down-regulate target genes in response.

Expression of vitreoscilla hemoglobin improves growth and levels of extracellular enzyme in Yarrowia lipolytica

Enhancement in oxygen uptake by high-cell-density cultivations has been achieved previously by expression of the bacterial hemoglobin gene from Vitreoscilla. The Vitreoscilla hemoglobin (VHb) gene was expressed in the yeast Yarrowia lipolytica to study the effect of expression in this commercially important yeast. The expression of VHb in this yeast was found to enhance growth, contrary to reported observations in wild-type Saccharomyces cerevisiae in which there was no significant growth enhancement. VHb-expressing Y. lipolytica exhibited higher specific growth rate, enhanced oxygen uptake rate, and higher respiratory activity. We report the beneficial effects of VHb expression on growth under microaerobic as well as under nonlimiting dissolved oxygen conditions. Earlier studies in Y. lipolytica have demonstrated inhibition of mycelia formation by respiratory inhibitors and poor nitrogen source, conditions poor for growth. VHb(+) Y. lipolytica cells were more efficient at forming mycelia, indicating better utilization of available oxygen as compared with the VHb(-) cells. Expression of VHb was also found to increase the levels of enzyme ribonuclease secreted into the medium, a property that may be beneficial for producing heterologous proteins in Y. lipolytica.

Isolation and regeneration of protoplasts from the yeast and mycelial form of the dimorphic zygomycete Benjaminiella poitrasii: role of chitin metabolism for morphogenesis during regeneration

Experimental parameters for isolation and regeneration of protoplasts from the mycelial and yeast form cells of the dimorphic zygomycete Benjamininiella poitrasii are reported. Using a chitosanase containing preparation from Streptomyces sp. MCl we obtained protoplasts after 5 h incubation with a yield of 2+/-0.3 x 10(6) ml(-1) and 3+/-0.4 x 10(7) ml(-1) for the mycelial and yeast form, respectively. During regeneration under conditions triggering dimorphism the two morphological forms were observed after 36 h. Initially, for 10-12 h only an irregular mass was formed as a result of deregulated cell wall synthesis. Among the tested inhibitors influencing cell wall metabolism, chitin metabolism inhibitors showed distinctive effects on the regeneration of protoplasts suggesting that the respective enzymes significantly contribute to determining the morphogenesis of the dimorphic fungus B. poitrasii.

Presence of organic sources of nitrogen is critical for filament formation and pH-dependent morphogenesis in Yarrowia lipolytica

Yeast dimorphism is an attractive model for the study of cell morphogenesis and differentiation. The non-conventional yeast Yarrowia lipolytica was chosen to characterise the regulation of dimorphic transition by extracellular pH and by the presence of organic sources of nitrogen. Organic nitrogen sources appear to be required for the morphogenic effect of pH. Two sets of mutants defective in either pH-dependent or nitrogen source-dependent signalling pathway were analysed. The results suggest that the latter but not the former is required for both normal filament formation on solid medium and pH-dependent dimorphic behaviour of Y. lipolytica in liquid medium. We propose that in this organism pH affects the formation of hyphae indirectly by modulation of availability and/or utilisation of transportable sources of nitrogen.

Control of pseudohyphae formation in Saccharomyces cerevisiae

Pseudohyphal growth in both haploid and diploid strains of Saccharomyces cerevisiae reflects concerted changes in different cellular processes: budding pattern, cell elongation and cell adhesion. These changes are triggered by environmental signals and are controlled by several pathways which act in parallel. Nitrogen deprivation, and possibly other stresses, activate a MAP kinase cascade which has the transcription factor Ste12 as its final target. A cAMP-dependent pathway, in which the protein kinase Tpk2 plays a specific role, is also required for the morphogenetic switch. Both pathways contribute to modulate the expression of the MUC1/FLO11 gene which encodes a cell-surface flocculin required for pseudohyphal and invasive growth. The MAP kinase cascade could also control the activity of the cyclin/Cdc28 complexes which affect both the budding pattern of yeast and cell elongation. A further protein which stimulates filamentous growth in S. cerevisiae is Phd1; although its mode of action is unknown, it may be regulated by a cAMP-dependent protein kinase, as occurs with the homologous protein Efg1 from Candida albicans, which is required for the formation of true hyphae. Morphogenesis in different yeast genera share common elements, but there are also important differences. Although a complete picture cannot yet be drawn, partial models may be proposed for the interaction of the regulatory pathways, both in the case of S. cerevisiae and in that of C. albicans.

The respiratory complex I in yeast: isolation of a gene NUO51 coding for the nucleotide-binding subunit of NADH:ubiquinone oxidoreductase from the obligately aerobic yeast Yarrowia lipolytica

We have isolated a gene NUO51 coding for a homologue of the nucleotide-binding subunit of mitochondrial respiratory chain linked NADH:ubiquinone oxidoreductase from the obligately aerobic yeast Yarrowia lipolytica. DNA sequencing revealed a 1464 bp open reading frame encoding a protein with predicted molar mass of about 53.7 kDa. The sequence is highly conserved with its counterparts from filamentous fungi and represents the first yeast homologue of the NADH-binding subunit (51 kDa) of the respiratory complex 1. In addition, PFGE and Southern hybridization analysis indicate that NUO51 is a single copy gene in the genome of Y. lipolytica. The expression of NUO51 by Northern blot analysis was also examined.