Molecular cloning and analysis of the yeast flocculation gene FLO1

The influence of flocculation upon global gene transcription in a yeast CYC8 mutant

The transcriptome from a Saccharomyces cerevisiae tup1 deletion mutant was one of the first comprehensive yeast transcriptomes published. Subsequent transcriptomes from tup1 and cyc8 mutants firmly established the Tup1-Cyc8 complex as predominantly acting as a repressor of gene transcription. However, transcriptomes from tup1/cyc8 gene deletion or conditional mutants would all have been influenced by the striking flocculation phenotypes that these mutants display. In this study, we have separated the impact of flocculation from the transcriptome in a cyc8 conditional mutant to reveal those genes (i) subject solely to Cyc8p-dependent regulation, (ii) regulated by flocculation only and (iii) regulated by Cyc8p and further influenced by flocculation. We reveal a more accurate list of Cyc8p-regulated genes that includes newly identified Cyc8p-regulated genes that were masked by the flocculation phenotype and excludes genes which were indirectly influenced by flocculation and not regulated by Cyc8p. Furthermore, we show evidence that flocculation exerts a complex and potentially dynamic influence upon global gene transcription. These data should be of interest to future studies into the mechanism of action of the Tup1-Cyc8 complex and to studies involved in understanding the development of flocculation and its impact upon cell function.

Gene mapping methodology powered by induced genome rearrangements

Phenotypic variation occurs through genome rearrangements and mutations in certain responsible genes; however, systematic gene identification methodologies based on genome rearrangements have not been fully established. Here, we explored the loci responsible for the given phenotype using the TAQing system and compared it with a conventional mutagenesis-based method. Two yeast strains with different genetic backgrounds and flocculation phenotypes were fused and genomic rearrangements were induced by transient DNA breaks. Then, selection pressure was applied and multiple mutants were generated, showing different flocculation abilities. We also raised mutants with altered cohesiveness due to spontaneous mutations during long-term recursive passages of haploid strains without TAQing treatment. Comparative genomic analysis of the TAQed mutants revealed three chromosomal regions harboring pivotal flocculation genes, whereas conventional mutagenesis generated a more diverse list of candidate loci after prolonged selection. The combined use of these approaches will accelerate the identification of genes involved in complex phenotypes.

Yeast Flocculin: Methods for Quantitative Analysis of Flocculation in Yeast Cells

Flocculation, the clump forming property of yeast, has long been appreciated in breweries and utilized as an off-cost method to enable the reuse of yeast cells. Members of the flocculin protein family were identified as the adherent proteins on the cell surface responsible for flocculation, and their properties have been investigated. Crystal structures of the adhesion domain of flocculins revealed their unique mode of ligand binding where a calcium ion is located in the middle of the interface between flocculin and the interacting sugar. Here we describe the most commonly used flocculation assay. The method is simple and easy, yet it is the most direct and reliable assay to evaluate the flocculation cellular phenotype.

Surface Display—An Alternative to Classic Enzyme Immobilization

Enzyme immobilization to solid matrices often presents a challenge due to protein conformation sensitivity, desired enzyme purity, and requirements for the particular carrier properties and immobilization technique. Surface display of enzymes at the cell walls of microorganisms presents an alternative that has been the focus of many research groups worldwide in different fields, such as biotechnology, energetics, pharmacology, medicine, and food technology. The range of systems by which a heterologous protein can be displayed at the cell surface allows the appropriate one to be found for almost every case. However, the efficiency of display systems is still quite low. The most frequently used yeast for the surface display of proteins is Saccharomyces cerevisiae. However, apart from its many advantages, Saccharomyces cerevisiae has some disadvantages, such as low robustness in industrial applications, hyperglycosylation of some heterologous proteins, and relatively low efficiency of surface display. Thus, in the recent years the display systems for alternative yeast hosts with better performances including Pichia pastoris, Hansenula polymorpha, Blastobotrys adeninivorans, Yarrowia lipolytica, Kluyveromyces marxianus, and others have been developed. Different strategies of surface display aimed to increase the amount of displayed protein, including new anchoring systems and new yeast hosts are reviewed in this paper.

Cell Wall and Whole Cell Proteomes Define Flocculation and Fermentation Behavior of Yeast

Flocculation is one of the most important characteristics of brewing yeast as it allows for the easy and cheap removal of cells after fermentation. The genes responsible for both the Flo1 and NewFlo flocculation phenotypes are well characterized. However, the relationship between Flo protein abundance and flocculation efficiency is poorly understood. In this present study, we used mass spectrometry proteomics to compare the cell wall and whole cell proteomes of commercial yeast strains with diverse flocculation behaviors. We found that the relative abundance of Flo1/5 or Flo10 in the cell wall was correlated with the ability of these yeast strains to flocculate. Analysis of whole cell proteomes identified differences in the proteomes of yeast strains and identified the potential for high metabolic diversity. Characterization of the cell wall and whole cell proteomes during fermentation showed high levels of Flo10 in cells that settled early during fermentation. Our data reveal the diversity of the cell wall and global proteomes of brewing yeast, highlighting the potential biochemical diversity present in yeast that can be utilized in the production of fermented beverages.

A novel chimaeric flocculation protein enhances flocculation in Saccharomyces cerevisiae

Yeast flocculation is the reversible formation of multicellular complexes mediated by lectin-like cell wall proteins binding to neighbouring cells. Strong flocculation can improve the inhibitor tolerance and fermentation performance of yeast cells in second generation bioethanol production. The strength of flocculation increases with the size of the flocculation protein and is strain dependent. However, the large number of internal repeats in the sequence of FLO1 from Saccharomyces cerevisiae S288c makes it difficult to recombinantly express the gene to its full length. In the search for novel flocculation genes resulting in strong flocculation, we discovered a DNA sequence, FLONF, that gives NewFlo phenotype flocculation in S. cerevisiae CEN.PK 113-7D. The nucleotide sequence of the internal repeats of FLONF differed from those of FLO1. We hypothesized that a chimaeric flocculation gene made up of a FLO1 variant derived from S. cerevisiae S288c and additional repeats from FLONF from S. cerevisiae CCUG 53310 would be more stable and easier to amplify by PCR. The constructed gene, FLOw, had 22 internal repeats compared to 18 in FLO1. Expression of FLOw in otherwise non-flocculating strains led to strong flocculation. Despite the length of the gene, the cassette containing FLOw could be easily amplified and transformed into yeast strains of different genetic background, leading to strong flocculation in all cases tested. The developed gene can be used as a self-immobilization technique or to obtain rapidly sedimenting cells for application in e.g. sequential batches without need for centrifugation.

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A novel gene sequence conferring NewFlo phenotype flocculation was isolated.

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The chimaeric gene FLOw was constructed from parts of FLO1 and FLONF.

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Overexpression of FLOw gave strong flocculation in several S. cerevisiae strains.

Enhancing the performance of brewing yeasts

Beer production is one of the oldest known traditional biotechnological processes, but is nowadays facing increasing demands not only for enhanced product quality, but also for improved production economics. Targeted genetic modification of a yeast strain is one way to increase beer quality and to improve the economics of beer production. In this review we will present current knowledge on traditional approaches for improving brewing strains and for rational metabolic engineering. These research efforts will, in the near future, lead to the development of a wider range of industrial strains that should increase the diversity of commercial beers.

Sas3 and Ada2(Gcn5)-dependent histone H3 acetylation is required for transcription elongation at the de-repressed FLO1 gene

The Saccharomyces cerevisiae FLO1 gene encodes a cell wall protein that imparts cell-cell adhesion. FLO1 transcription is regulated via the antagonistic activities of the Tup1-Cyc8 co-repressor and Swi-Snf co-activator complexes. Tup1-Cyc8 represses transcription through the organization of strongly positioned, hypoacetylated nucleosomes across gene promoters. Swi-Snf catalyzes remodeling of these nucleosomes in a mechanism involving histone acetylation that is poorly understood. Here, we show that FLO1 de-repression is accompanied by Swi-Snf recruitment, promoter histone eviction and Sas3 and Ada2(Gcn5)-dependent histone H3K14 acetylation. In the absence of H3K14 acetylation, Swi-Snf recruitment and histone eviction proceed, but transcription is reduced, suggesting these processes, while essential, are not sufficient for de-repression. Further analysis in the absence of H3K14 acetylation reveals RNAP II recruitment at the FLO1 promoter still occurs, but RNAP II is absent from the gene-coding region, demonstrating Sas3 and Ada2-dependent histone H3 acetylation is required for transcription elongation. Analysis of the transcription kinetics at other genes reveals shared mechanisms coupled to a distinct role for histone H3 acetylation, essential at FLO1, downstream of initiation. We propose histone H3 acetylation in the coding region provides rate-limiting control during the transition from initiation to elongation which dictates whether the gene is permissive for transcription.

Display of heterologous proteins on the Saccharomyces cerevisiae surface display system using a single constitutive expression vector

In this study, we constructed a novel and simple yeast surface display system with a single expression vector. The newly established system uses a bidirectional expression vector carrying the AGA1 gene driven by the PGK1 promoter in one direction and the AGA2-expression cassette driven by the TEF1 promoter in the reverse direction, and uses the geneticin, a G418-resistant gene, as the selection marker for transformants. Because all the display elements are put into one expression vector, the new system is much simpler to use, and there is no need for any genetic modification of the host strains; therefore, the new system can be used in wild type as well as laboratory strains of Saccharomyces cerevisiae. The display efficiency of heterologous proteins using the new system has been confirmed by displaying enhanced green fluorescent protein and Eimeria tenella (a chicken protozoan parasite) microneme protein2 (EtMic2) on several S. cerevisiae strains. We also tested the new system with an aga2 mutant strain of S. cerevisiae. The results indicate that the native expressed Aga2 protein has no effect on the display efficiency of heterologous proteins.

Development of new tolerant strains to hydrophilic and hydrophobic organic solvents by the yeast surface display methodology

Yeast surface display is a research methodology based on anchoring functional proteins and peptides onto the surface of the cells of this eukaryotic organism. Its development has resulted in the construction of a good number of new whole-cell biocatalysts with diverse applications in biotechnology, pharmacy, and medicine. In this work, we describe the design of new yeast strains in which several proteins and peptides have been introduced at the N-terminal position of protein agglutinin Aga2p. In all cases, proteins were correctly expressed and displayed on the cell surface according to the western blot, fluorescence microscopy, and fluorescence-activated cell sorting (FACS) analyses. The introduction of a glycosylable, Ser/Thr-rich protein (S1) resulted in improved resistance to ethanol, nonane, and dimethyl sulfoxide (DMSO) stress. The protein with a very high hydrophobic content (S2d) proved to confer tolerance to acetonitrile, ethanol, nonane, salt, and sodium dodecyl sulfate (SDS). The introduction of five leucine residues at the N-terminal position of S1 and S2 resulted in similar or increased resistance to the above-mentioned stress conditions. The adverse effects described in a previous work, when these residues were introduced into the N-terminus of Aga2p, with no other protein acting as a spacer, were not observed. Indeed, these strains grew better in the presence of hydrophilic solvents such as acetonitrile and ethanol. The new strains reported in this work have biotechnological potentiality given their behavior under adverse conditions of interest for biocatalytic and industrial processes.

The contribution of Pir protein family to yeast cell surface display

Proteins with internal repeats (Pir) in the Baker's yeast are located on the cell wall and include four highly homologous members. Recently, Pir proteins have become increasingly used as anchor proteins in yeast cell surface display systems. These display systems are classified into three types: N-terminal fusion, C-terminal fusion, and inserted fusion. In addition to the GPI (glycosylphosphatidyl inositol) and the FL/FS anchor proteins, these three Pir-based systems significantly increase the choices for target proteins to be displayed. Furthermore, Pir proteins can also be used as a fusion partner for target proteins to be effectively secreted into culture medium. Here, we summarize the development and application of Pir proteins as anchor proteins.

Yeast arming by the Aga2p system: effect of growth conditions in galactose on the efficiency of the display and influence of expressing leucine-containing peptides

The amino or carboxy-terminal regions of certain cell wall proteins are capable of anchoring foreign proteins or peptides on the cell wall of the yeast Saccharomyces cerevisiae. This possibility has resulted in the development of a methodology known as yeast display which has powerful applications in biotechnology, pharmacy, and medicine. This work describes the results of experiments in which the agglutinin Aga2p protein is used as an anchor and several leucine-based peptides have been introduced into its N-terminal or C-terminal position. We found that the sequence of these peptides can affect plasmid stability, growth kinetics, and levels of the fusion protein displayed, and we analyzed how the incubation conditions influence these parameters. Besides, we show that the introduction of these small peptides can modify the properties of cell cover; in particular, fusing five or ten leucine residues to the Aga2p protein results in greater hydrophobicity of the cell wall and also in increased resistance to the presence of the organic solvents acetonitrile and ethanol and to high salt concentrations. The introduction of the RLRLL sequence also results in higher resistance to the exposure of yeast cells to NaCl stress.

Deletion of intragenic tandem repeats in unit C of FLO1 of Saccharomyces cerevisiae increases the conformational stability of flocculin under acidic and alkaline conditions

Flocculation is an attractive property for Saccaromyces cerevisiae, which plays important roles in fermentation industry and environmental remediation. The process of flocculation is mediated by a family of cell surface flocculins. As one member of flocculins, Flo1 is characterized by four families of repeats (designated as repeat units A, B, C and D) in the central domain. It is generally accepted that variation of repeat unit A in length in Flo1 influences the degree of flocculation or specificity for sugar recognization. However, no reports were observed for other repeat units. Here, we compared the flocculation ability and its sensitivity to environmental factors between yeast strain YSF1 carrying the intact FLO1 gene and yeast strains carrying the derived forms of FLO1 with partial or complete deletion of repeats in unit C. No obvious differences in flocculation ability and specificity of carbohydrate recognition were observed among these yeast strains, which indicates the truncated flocculins can stride across the cell wall and cluster the N-terminal domain on the surface of yeast cells as the intact Flo1 thereby improving intercellular binding. However, yeast strains with the truncated flocculins required more mannose to inhibit completely the flocculation, displayed broad tolerance of flocculation to pH fluctuation, and the fewer the repeats in unit C, the stronger adaptability of flocculation to pH change, which was not relevant to the position of deletion. This suggests that more stable active conformation is obtained for flocculin by deletion the repeat unit C in the central domain of Flo1, which was validated further by the higher hydrophobicity on the surface of cells of YSF1c with complete deletion of unit C under neutral and alkaline conditions and the stabilization of GFP conformation by fusion with flocculin with complete deletion of unit C in the central domain.

Cloning, expression, and purification of the N-terminal domain of the Flo1 flocculation protein from Saccharomyces cerevisiae in Pichia pastoris

Saccharomyces cerevisiae flocculation is governed by FLO genes, encoding Flo proteins (flocculins). Flo proteins are cell wall proteins consisting of three domains, sticking out of the cell wall and interacting with other yeast cells using their N-terminal mannose-binding domain. Until recently, flocculation research was focused on the genetic and cellular level. To extend the knowledge about flocculation to the protein level, we isolated the N-terminal domain of the Flo1p (N-Flo1p) that contains the mannose-binding domain, which is responsible for the strong interaction (flocculation) of S. cerevisiae cells. To obtain a high production yield and a more uniform and lower glycosylation of N-Flo1p, it was cloned in Pichia pastoris. The expression and the purification of N-Flo1p were optimised towards a one-step purification protocol. The activity of the protein, i.e. the binding of the purified protein to mannose using fluorescence spectroscopy, was demonstrated.

Plasmid-mediate transfer of FLO1 into industrial Saccharomyces cerevisiae PE-2 strain creates a strain useful for repeat-batch fermentations involving flocculation-sedimentation

The flocculation gene FLO1 was transferred into the robust industrial strain Saccharomyces cerevisiae PE-2 by the lithium acetate method. The recombinant strain showed a fermentation performance similar to that of the parental strain. In 10 repeat-batch cultivations in VHG medium with 345 g glucose/L and cell recycling by flocculation-sedimentation, an average final ethanol concentration of 142 g/L and an ethanol productivity of 2.86 g/L/h were achieved. Due to the flocculent nature of the recombinant strain it is possible to reduce the ethanol production cost because of lower centrifugation and distillation costs.

Flocculation in ale brewing strains of Saccharomyces cerevisiae: re-evaluation of the role of cell surface charge and hydrophobicity

Flocculation is an eco-friendly process of cell separation, which has been traditionally exploited by the brewing industry. Cell surface charge (CSC), cell surface hydrophobicity (CSH) and the presence of active flocculins, during the growth of two (NCYC 1195 and NCYC 1214) ale brewing flocculent strains, belonging to the NewFlo phenotype, were examined. Ale strains, in exponential phase of growth, were not flocculent and did not present active flocculent lectins on the cell surface; in contrast, the same strains, in stationary phase of growth, were highly flocculent (>98%) and presented a hydrophobicity of approximately three to seven times higher than in exponential phase. No relationship between growth phase, flocculation and CSC was observed. For comparative purposes, a constitutively flocculent strain (S646-1B) and its isogenic non-flocculent strain (S646-8D) were also used. The treatment of ale brewing and S646-1B strains with pronase E originated a loss of flocculation and a strong reduction of CSH; S646-1B pronase E-treated cells displayed a similar CSH as the non-treated S646-8D cells. The treatment of the S646-8D strain with protease did not reduce CSH. In conclusion, the increase of CSH observed at the onset of flocculation of ale strains is a consequence of the presence of flocculins on the yeast cell surface and not the cause of yeast flocculation. CSH and CSC play a minor role in the auto-aggregation of the ale strains since the degree of flocculation is defined, primarily, by the presence of active flocculins on the yeast cell wall.

Choosing the right lifestyle: adhesion and development in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae is a eukaryotic microorganism that is able to choose between different unicellular and multicellular lifestyles. The potential of individual yeast cells to switch between different growth modes is advantageous for optimal dissemination, protection and substrate colonization at the population level. A crucial step in lifestyle adaptation is the control of self- and foreign adhesion. For this purpose, S. cerevisiae contains a set of cell wall-associated proteins, which confer adhesion to diverse biotic and abiotic surfaces. Here, we provide an overview of different aspects of S. cerevisiae adhesion, including a detailed description of known lifestyles, recent insights into adhesin structure and function and an outline of the complex regulatory network for adhesin gene regulation. Our review shows that S. cerevisiae is a model system suitable for studying not only the mechanisms and regulation of cell adhesion, but also the role of this process in microbial development, ecology and evolution.

Display of adenoregulin with a novel Pichia pastoris cell surface display system

Two Pichia pastoris cell surface display vectors were constructed. The vectors consisted of the flocculation functional domain of Flo1p with its own secretion signal sequence or the alpha-factor secretion signal sequence, a polyhistidine (6xHis) tag for detection, an enterokinase recognition site, and the insertion sites for target proteins. Adenoregulin (ADR) is a 33-amino-acid antimicrobial peptide isolated from Phyllomedusa bicolor skin. The ADR was expressed and displayed on the Pichia pastoris KM71 cell surface with the system reported. The displayed recombinant ADR fusion protein was detected by fluorescence microscopy and confocal laser scanning microscopy (CLSM). The antimicrobial activity of the recombinant adenoregulin was detected after proteolytic cleavage of the fusion protein on cell surface. The validity of the Pichia pastoris cell surface display vectors was proved by the displayed ADR.

The N-terminal domain of the Flo1 flocculation protein from Saccharomyces cerevisiae binds specifically to mannose carbohydrates

Saccharomyces cerevisiae cells possess a remarkable capacity to adhere to other yeast cells, which is called flocculation. Flocculation is defined as the phenomenon wherein yeast cells adhere in clumps and sediment rapidly from the medium in which they are suspended. These cell-cell interactions are mediated by a class of specific cell wall proteins, called flocculins, that stick out of the cell walls of flocculent cells. The N-terminal part of the three-domain protein is responsible for carbohydrate binding. We studied the N-terminal domain of the Flo1 protein (N-Flo1p), which is the most important flocculin responsible for flocculation of yeast cells. It was shown that this domain is both O and N glycosylated and is structurally composed mainly of β-sheets. The binding of N-Flo1p to D-mannose, α-methyl-D-mannoside, various dimannoses, and mannan confirmed that the N-terminal domain of Flo1p is indeed responsible for the sugar-binding activity of the protein. Moreover, fluorescence spectroscopy data suggest that N-Flo1p contains two mannose carbohydrate binding sites with different affinities. The carbohydrate dissociation constants show that the affinity of N-Flo1p for mono- and dimannoses is in the millimolar range for the binding site with low affinity and in the micromolar range for the binding site with high affinity. The high-affinity binding site has a higher affinity for low-molecular-weight (low-MW) mannose carbohydrates and no affinity for mannan. However, mannan as well as low-MW mannose carbohydrates can bind to the low-affinity binding site. These results extend the cellular flocculation model on the molecular level.

Regulation of the display ratio of enzymes on the Saccharomyces cerevisiae cell surface by the immunoglobulin G and cellulosomal enzyme binding domains

We constructed a novel cell surface display system to control the ratio of target proteins on the Saccharomyces cerevisiae cell surface, using two pairs of protein-protein interactions. One protein pair is the Z domain of protein A derived from Staphylococcus aureus and the Fc domain of human immunoglobulin G. The other is the cohesin (Coh) and dockerin (Dock) from the cellulosome of Clostridium cellulovorans. In this proposed displaying system, the scaffolding proteins (fusion proteins of Z and Coh) were displayed on the cell surface by fusing with the 3' half of alpha-agglutinin, and the target proteins fused with Fc or Dock were secreted. As a target protein, a recombinant Trichoderma reesei endoglucanase II (EGII) was secreted into the medium and immediately displayed on the yeast cell surface via the Z and Fc domains. Display of EGII on the cell surface was confirmed by hydrolysis of beta-glucan as a substrate, and EGII activity was detected in the cell pellet fraction. Finally, two enzymes, EGII and Aspergillus aculeatus beta-glucosidase 1, were codisplayed on the cell surface via Z-Fc and Dock-Coh interactions, respectively. As a result, the yeast displaying two enzymes hydrolyzed beta-glucan to glucose very well. These results strongly indicated that the proposed strategy, the simultaneous display of two enzymes on the yeast cell surface, was accomplished by quantitatively controlling the display system using affinity binding.

Construction of flocculent Kluyveromyces marxianus strains suitable for high-temperature ethanol fermentation

Flocculating yeasts are highly useful in fermentation processes because these cells can be separated easily from the fermentation mash. However, native yeasts are usually non-flocculating, including Kluyveromyces marxianus, which exhibits a potent high-temperature ethanol fermentation ability. We describe here the construction of flocculent K. marxianus strains via the introduction of the FLO1, FLO5, FLO9, and FLO10 genes from Saccharomyces cerevisiae. The S. cerevisiae FLO genes were overexpressed by upstream insertion of the constitutive TDH3 promoter, resulting in flocculent S. cerevisiae strains. These TDH3p-FLO sequences were then amplified by PCR and introduced directly into a K. marxianus strain. These K. marxianus strains showed a flocculation phenotype, indicating that the introduced S. cerevisiae TDH3 promoter and all FLO genes were functional in this strain. Moreover, a flocculating K. marxianus strain showed the same ethanol production profile as that of its wild-type parent. The K. marxianus flocculating strains we generated should be useful in the future development of cost-effective fed-batch and continuous fermentation systems at high temperatures.

Chromosomal location of Lg-FLO1 in bottom-fermenting yeast and the FLO5 locus of industrial yeast

AIMS

To determine the chromosomal location and entire sequence of Lg-FLO1, the expression of which causes the flocculation of bottom-fermenting yeast.

METHODS AND RESULTS

Two cosmid clones carrying DNA from a bottom-fermenting yeast chromosome VIII right-arm end were selected by colony hybridization. Sequencing revealed that the clones contained DNA derived from a Saccharomyces cerevisiae type chromosome VIII and a Saccharomyces bayanus type chromosome VIII, both from bottom-fermenting yeast.

CONCLUSIONS

Lg-FLO1 is located on the S. cerevisiae type chromosome VIII at the same position as the FLO5 gene of the laboratory yeast S. cerevisiae S288c. The unique chromosome VIII structure of bottom-fermenting yeast is conserved among other related strains. FLO5 and Lg-FLO1 promoter sequences are identical except for the presence of three 42 bp repeats in the latter, which are associated with gene activity. Flocculin genes might have been generated by chromosomal recombination at these repeats.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of the exact chromosomal location and entire sequence of Lg-FLO1. This information will be useful in the brewing industry for the identification of normal bottom-fermenting yeast. Moreover, variations in the FLO5 locus among strains are thought to reflect yeast evolution.

Improvement of Saccharomyces yeast strains used in brewing, wine making and baking

Yeast was the first microorganism domesticated by mankind. Indeed, the production of bread and alcoholic beverages such as beer and wine dates from antiquity, even though the fact that the origin of alcoholic fermentation is a microorganism was not known until the nineteenth century. The use of starter cultures in yeast industries became a common practice after methods for the isolation of pure yeast strains were developed. Moreover, effort has been undertaken to improve these strains, first by classical genetic methods and later by genetic engineering. In general, yeast strain development has aimed at improving the velocity and efficiency of the respective production process and the quality of the final products. This review highlights the achievements in genetic engineering of Saccharomyces yeast strains applied in food and beverage industry.

Efficient display of active lipase LipB52 with a Pichia pastoris cell surface display system and comparison with the LipB52 displayed on Saccharomyces cerevisiae cell surface

Background

For industrial bioconversion processes, the utilization of surface-displayed lipase in the form of whole-cell biocatalysts is more advantageous, because the enzymes are displayed on the cell surface spontaneously, regarded as immobilized enzymes.

Results

Two Pichia pastoris cell surface display vectors based on the flocculation functional domain of FLO with its own secretion signal sequence or the α-factor secretion signal sequence were constructed respectively. The lipase gene lipB52 fused with the FLO gene was successfully transformed into Pichia pastoris KM71. The lipase LipB52 was expressed under the control of the AOX1 promoter and displayed on Pichia pastoris KM71 cell surface with the two Pichia pastoris cell surface display vectors. Localization of the displayed LipB52 on the cell surface was confirmed by the confocal laser scanning microscopy (CLSM). The LipB52 displayed on the Pichia pastoris cell surface exhibited activity toward p-nitrophenol ester with carbon chain length ranging from C₁₀ to C₁₈, and the optimum substrate was p-nitrophenol-caprate (C₁₀), which was consistent with it displayed on the Saccharomyces cerevisiae EBY100 cell surface. The hydrolysis activity of lipase LipB52 displayed on Pichia pastoris KM71-pLHJ047 and KM71-pLHJ048 cell surface reached 94 and 91 U/g dry cell, respectively. The optimum temperature of the displayed lipases was 40°C at pH8.0, they retained over 90% activity after incubation at 60°C for 2 hours at pH 7.0, and still retained 85% activity after incubation for 3 hours.

Conclusion

The LipB52 displayed on the Pichia pastoris cell surface exhibited better stability than the lipase LipB52 displayed on Saccharomyces cerevisiae cell surface. The displayed lipases exhibited similar transesterification activity. But the Pichia pastoris dry cell weight per liter (DCW/L) ferment culture was about 5 times than Saccharomyces cerevisiae, the lipase displayed on Pichia pastoris are more suitable for whole-cell biocatalysts than that displayed on Saccharomyces cerevisiae cell surface.

Cell surface display of functionally active lipases from Yarrowia lipolytica in Pichia pastoris

The lipase genes of Yarrowia lipolytica, LIPY7 and LIPY8, fused with FLO-flocculation domain sequence from Saccharomyces cerevisiae at their N-termini, were expressed in Pichia pastoris KM71. Following the induction with methanol, the recombinant proteins were displayed on the cell surface of P. pastoris, as confirmed by the confocal laser scanning microscopy. The LipY7p and LipY8p were anchored on P. pastoris via the flocculation functional domain of Flo1p. The surface-displayed lipases were characterized for their application as the whole-cell biocatalyst. These lipases can also be cleaved off from their anchor by enterokinase treatment to yield functionally active proteins in the supernatant offering an alternative purification method for LipY7p and LipY8p.

Construction of a flocculating yeast for fuel ethanol production

The expression vector pYX212 harboring FLO1 gene and kanMX gene was transformed into Saccharomyces. cerevisiae ZWA46. The transformant, ZWA46-F2, showed strong and stable flocculation ability during 20 serial batch cultivations. The flocculation onset of the strain is in the early stationary growth phase, not coincident with the glucose depletion in the culture medium. The flocculation ability of the transformant showed no difference with the initial pH ranging from 3.5 to 6.0. Furthermore, the ethanol concentration and other properties of the transformant strain ZWA46-F2 were similar to those of the wild-type strain ZWA46.

A biochemical guide to yeast adhesins: glycoproteins for social and antisocial occasions

Fungi are nonmotile eukaryotes that rely on their adhesins for selective interaction with the environment and with other fungal cells. Glycosylphosphatidylinositol (GPI)-cross-linked adhesins have essential roles in mating, colony morphology, host-pathogen interactions, and biofilm formation. We review the structure and binding properties of cell wall-bound adhesins of ascomycetous yeasts and relate them to their effects on cellular interactions, with particular emphasis on the agglutinins and flocculins of Saccharomyces and the Als proteins of Candida. These glycoproteins share common structural motifs tailored to surface activity and biological function. After being secreted to the outer face of the plasma membrane, they are covalently anchored in the wall through modified GPI anchors, with their binding domains elevated beyond the wall surface on highly glycosylated extended stalks. N-terminal globular domains bind peptide or sugar ligands, with between millimolar and nanomolar affinities. These affinities and the high density of adhesins and ligands at the cell surface determine microscopic and macroscopic characteristics of cell-cell associations. Central domains often include Thr-rich tandemly repeated sequences that are highly glycosylated. These domains potentiate cell-to-cell binding, but the molecular mechanism of such an association is not yet clear. These repeats also mediate recombination between repeats and between genes. The high levels of recombination and epigenetic regulation are sources of variation which enable the population to continually exploit new niches and resources.

Brewing yeast genomes and genome-wide expression and proteome profiling during fermentation

The genome structure, ancestry and instability of the brewing yeast strains have received considerable attention. The hybrid nature of brewing lager yeast strains provides adaptive potential but yields genome instability which can adversely affect fermentation performance. The requirement to differentiate between production strains and assess master cultures for genomic instability has led to significant adoption of specialized molecular tool kits by the industry. Furthermore, the development of genome-wide transcriptional and protein expression technologies has generated significant interest from brewers. The opportunity presented to explore, and the concurrent requirement to understand both, the constraints and potential of their strains to generate existing and new products during fermentation is discussed.

Intrinsic kinetics of continuous growth and ethanol production of a flocculating fusant yeast strain SPSC01

The intrinsic kinetics of continuous yeast cell growth and ethanol production for a self-flocculating fusant yeast strain SPSC01 was investigated by means of mechanically dispersing the flocs and correspondingly established floc size distribution on-line monitoring technique using the focused beam reflectance measurement system, through which the floc intra-particle mass transfer limitation was effectively eliminated, but its ethanol formation metabolism was not affected. Modified kinetic models were developed, which can be used to predict the continuous kinetic behaviors of SPSC01, especially when low dilution rates are applied and limiting substrate concentrations are undetectable and almost all kinetic models developed previously are failed in predicting corresponding kinetic behaviors. Both substrate and product inhibitions reported for freely suspended yeast cell ethanol production were also observed for SPSC01 when high gravity media were fed and relatively high levels of residual sugar and ethanol presented. Model parameters were evaluated through numerical calculation method and validated by experimental data mu = 0.584C(s)/0.155 + C(s) + C(2)(s)/160.7(1 -P/125)(3.68) + 0.004 for growth, nu = 1.998C(s)/0.427 + C(s) + C(2)(s)/366.7(1- P/125)(1.72) + 0.060 for ethanol production These intrinsic kinetic models can be further used to develop the observed kinetic models that quantitatively correlate the impact of the self-flocculating yeast cell size distributions on their apparent rates for yeast cell growth, substrate uptake and ethanol production and optimize the ethanol production process.

Amplified fragment length polymorphism of the AWA1 gene of sake yeasts for identification of sake yeast strains

Sake yeasts are used for sake brewing and have a crucial role in the quality of sake, since they produce not only ethanol but also various compounds that provide sake flavors. Therefore, the appropriate selection and monitoring of a strain used in sake mash is important. However, the identification of specific sake yeast strains has been difficult, because sake yeasts have similar characteristics in taxonomic and physiological analyses. We found amplified fragment length polymorphisms (AFLPs) in the PCR products of the AWA1 gene of sake yeast strains. The AWA1 gene encodes a cell wall protein that is responsible for foam formation in sake mash. This polymorphism of the AWA1 gene can be used for the identification of sake yeast strains.

Identification and characterization of an SKN7 homologue in Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated fungal pathogen that primarily infects the central nervous system of immunocompromised individuals, causing life-threatening meningoencephalitis. The capacity of C. neoformans to subvert host defenses and disseminate by intracellular parasitism of alveolar macrophages in the immune-compromised host has led to studies to evaluate genes associated with C. neoformans resistance to oxidative stress. In the present study, we identify and characterize a C. neoformans homologue to SKN7, a transcription factor in Saccharomyces cerevisiae that regulates the oxidative stress response, cell cycle, and cell wall biosynthesis. To examine the contribution of SKN7 in the pathogenesis of fungal infections, we created skn7 mutants via targeted disruption. The skn7 mutants were observed to be more susceptible to reactive oxygen species in vitro and were significantly less virulent than the wild-type strain and a reconstituted strain as measured by cumulative survival in the mouse inhalational model. The Skn7 protein was observed to be important for expression of thioredoxin reductase in response to oxidative challenge. Interestingly, skn7 mutants were also observed to flocculate following in vitro culture, a novel phenotype not observed in skn7 mutants derived from other fungi. These findings demonstrate that SKN7 contributes to the virulence composite but is not required for pathogenicity in C. neoformans. In addition, flocculation of C. neoformans skn7 mutants suggests a potentially unique function of SKN7 not previously observed in other cryptococcal strains or skn7 mutants.

Direct production of ethanol from raw corn starch via fermentation by use of a novel surface-engineered yeast strain codisplaying glucoamylase and alpha-amylase

Direct and efficient production of ethanol by fermentation from raw corn starch was achieved by using the yeast Saccharomyces cerevisiae codisplaying Rhizopus oryzae glucoamylase and Streptococcus bovis alpha-amylase by using the C-terminal-half region of alpha-agglutinin and the flocculation functional domain of Flo1p as the respective anchor proteins. In 72-h fermentation, this strain produced 61.8 g of ethanol/liter, with 86.5% of theoretical yield from raw corn starch.