Browsing by Author "Govender, P."

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    Controlled expression of the dominant flocculation genes FLO1, FLO5, and FLO11 in Saccharomyces cerevisiae
    (2008) Govender, P.; Domingo, J. L.; Bester, M. C.; Pretorius, I. S.; Bauer, Florian
    In many industrial fermentation processes, the Saccharomyces cerevisiae yeast should ideally meet two partially conflicting demands. During fermentation, a high suspended yeast count is required to maintain a satisfactory rate of fermentation, while at completion, efficient settling is desired to enhance product clarification and recovery. In most fermentation industries, currently used starter cultures do not satisfy this ideal, probably because nonflocculent yeast strains were selected to avoid fermentation problems. In this paper, we assess molecular strategies to optimize the flocculation behavior of S. cerevisiae. For this purpose, the chromosomal copies of three dominant flocculation genes, FLO1, FLO5, and FLO11, of the haploid nonflocculent, noninvasive, and non-flor-forming S. cerevisiae FY23 strain were placed under the transcriptional control of the promoters of the ADH2 and HSP30 genes. All six promoter-gene combinations resulted in specific flocculation behaviors in terms of timing and intensity. The strategy resulted in stable expression patterns providing a platform for the direct comparison and assessment of the specific impact of the expression of individual dominant FLO genes with regard to cell wall characteristics, such as hydrophobicity, biofilm formation, and substrate adhesion properties. The data also clearly demonstrate that the flocculation behavior of yeast strains can be tightly controlled and fine-tuned to satisfy specific industrial requirements. Copyright © 2008, American Society for Microbiology. All Rights Reserved.
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    FLO gene-dependent phenotypes in industrial wine yeast strains
    (2010) Govender, P.; Bester, M.; Bauer, Florian
    Most commercial yeast strains are nonflocculent. However, controlled flocculation phenotypes could provide significant benefits to many fermentation-based industries. In nonflocculent laboratory strains, it has been demonstratedthat it is possible to adjust flocculation and adhesion phenotypes to desired specifications by altering expression of the otherwise silent but dominant flocculation (FLO) genes. However, FLO genes are characterized by high allele heterogeneity and are subjected to epigenetic regulation. Extrapolation of data obtained in laboratory strains to industrial strains may therefore not always be applicable. Here, we assess the adhesion phenotypes that are associated with the expression of a chromosomal copy of the FLO1, FLO5, or FLO11 open reading frame in two nonflocculent commercial wine yeast strains, BM45 and VIN13. The chromosomal promoters of these genes were replaced with stationary phase-inducible promoters of the HSP30 and ADH2 genes. Under standard laboratory and wine making conditions, the strategy resulted in expected and stable expression patterns of these genes in both strains. However, the specific impact of the expression of individual FLO genes showed significant differences between the two wine strains and with corresponding phenotypes in laboratory strains. The data suggest that optimization of the flocculation pattern of individual commercial strains will have to be based on a strain-by-strain approach. © 2009 Springer-Verlag.
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    Novel wine-mediated FLO11 flocculation phenotype of commercial Saccharomyces cerevisiae wine yeast strains with modified FLO gene expression
    (2011) Govender, P.; Kroppenstedt, S.; Bauer, Florian
    Depending on the genetic background of Saccharomyces strains, a wide range of phenotypic adhesion identities can be directly attributed to the FLO11-encoded glycoprotein, which includes asexual flocculation, invasive growth and pseudohyphal formation, flor formation and adhesion to biotic and abiotic surfaces. In a previous study, we reported that HSP30-mediated stationary-phase expression of the native chromosomal FLO11 ORF in two nonflocculent commercial Saccharomyces cerevisiae wine yeast strains, BM45 or VIN13 did not generate a flocculent phenotype under either standard laboratory media or synthetic MS300 must fermentation conditions. In the present study, the BM45- and VIN13-derived HSP30p-FLO11 wine yeast transformants were observed to be exclusively and strongly flocculent under authentic red wine-making conditions, thus suggesting that this specific fermentation environment specifically contributes to the development of a flocculent phenotype, which is insensitive to either glucose or mannose. Furthermore, irrespective of the strain involved this phenotype displayed both Ca2+-dependent and Ca2+-independent flocculation characteristics. A distinct advantage of this unique FLO11-based phenotype was highlighted in its ability to dramatically promote faster lees settling rates. Moreover, wines produced by BM45-F11H and VIN13-F11H transformants were significantly less turbid than those produced by their wild-type parental strains. © 2011 University of Stellenbosch. FEMS Microbiology Letters © 2011 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd.
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    Properties of a wine yeast antagonist, saccharomyces cerevisiae T206. a review
    (South African Society for Enology and Viticulture, 2002) Vadasz, A. S.; Franken, D. B.; Govender, B. L.; Jagganath, D. B.; Govender, P.; Ariatti, M.; Pretorius, I. S.; Gupthar, A. S.
    Regardless of the type of killer yeast, winemaking may be adversely affected by a single type of killer yeast. In this review we present the properties of a single K2 strain, Saccharomyces cerevisiae T206, which was isolated from a stuck fermentation in a South African winery. This zymotidal strain has demonstrated its potential as a wine yeast antagonist and may be differentiated from other NCYC killer strains of S. cerevisiae on the basis of CHEF karyotyping and mycoviral RNA separations. Resolution of its genomic DNA into 13 chromosome bands, ranging from 0.2 to 2.2 Mb, has been reported. The resident viral-like particle in strain T206 yields pancreatic RNase-sensitive L and M double-stranded RNA species of 5.1 and 2.0 kb, respectively. The latter can be cured using cycloheximide to produce a K-R- derivative. In microscale vinifications the K2 toxin of strain T206 demonstrates a lethal effect on sensitive mesophilic wine yeasts over a narrow pH range of 3.2 to 3.5, disrupting the cell wall structure and perturbing cytoplasmic membranes. Contrary to known fermentation trends, the challenged fermentations are neither stuck nor protracted as strain T206 competes for substrates and induces over 65 % cell death in enriched Hanepoot grape juice media. Mucoid secretions of mesophilic wine yeasts, induced by nutrient limitation, appear to restrict the K2 killer effect. However, the supplementation of 1 to 100 ppm bovine submaxillary gland mucin in nutrientenriched grape juice media also depresses the K2 killer effect of strain T206 in mesophilic wine yeast starter culture strain, S. cerevisiae VIN7. Preliminary results suggest that mucoid secretions either affect the level of toxin production by strain T206 or block the cognate K2 receptor on the cell wall of challanged yeasts.
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    Yeast flocculation and its biotechnological relevance
    (2010) Bauer, Florian; Govender, P.; Bester, M. C.
    Adhesion properties of microorganisms are crucial for many essential biological processes such as sexual reproduction, tissue or substrate invasion, biofilm formation and others. Most, if not all microbial adhesion phenotypes are controlled by factors such as nutrient availability or the presence of pheromones. One particular form of controlled cellular adhesion that occurs in liquid environments is a process of asexual aggregation of cells which is also referred to as flocculation. This process has been the subject of significant scientific and biotechnological interest because of its relevance for many industrial fermentation processes. Specifically adjusted flocculation properties of industrial microorganisms could indeed lead to significant improvements in the processing of biotechnological fermentation products such as foods, biofuels and industrially produced peptides. This review briefly summarises our current scientific knowledge on the regulation of flocculation-related phenotypes, their importance for different biotechnological industries, and possible future applications for microorganisms with improved flocculation properties. © 2010 Springer-Verlag.