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Yeast Killer Factors: Biology and Relevance in Food Biotechnology

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Yeast Killer Factors: Biology and Relevance in Food Biotechnology Yeast killer factors: Biology and relevance in food biotechnology Maja Starovič Academic internship report Msc Food technology: Food biotechnology and biorefining October 2020 Mentors: Ramón González García, Instituto de Ciencias de la Vid y del Vino prof. dr. Eddy J. Smid, Wageningen University & Research Table of contents 1 Yeast killer toxins ............................................................................................................................. 1 1.1 Saccharomyces cerevisiae killer system .................................................................................. 1 1.1.1 Saccharomyces cerevisiae killer toxins ............................................................................ 1 2 Non- Saccharomyces cerevisiae killer factors.................................................................................. 6 2.1 Non- Saccharomyces cerevisiae killer yeast ............................................................................ 6 2.2 Toxin entry into the target cell and the killing mechanism ..................................................... 7 2.2.1 Killer toxins from Pichia ................................................................................................... 7 1.2.1 Killer toxins from Kluyveromyces lactis ........................................................................... 8 1.2.2 Killer toxins from Candida ............................................................................................... 9 1.2.3 Killer toxins from other non-Saccharomyces yeast ......................................................... 9 3 Toxin immunity ................................................................................................................................ 9 4 Role of killer yeast against common wine spoilage yeast ............................................................. 11 4.1 Spoilage yeast in wine industry ............................................................................................. 11 4.1.1 Brettanomyces bruxellensis ........................................................................................... 11 4.2 Killer yeast used in wine industry .......................................................................................... 12 4.2.1 Torulaspora delbrueckii ................................................................................................. 13 4.2.2 Candida .......................................................................................................................... 14 4.2.3 Pichia ............................................................................................................................. 15 3.2.5 Kluyveromyces spp. .............................................................................................................. 16 5 Killer yeast in other biotechnological applications ....................................................................... 17 6 Influence of environmental conditions on killer toxin production and activity ............................ 18 6.1 Temperature and pH ............................................................................................................. 18 6.2 Other conditions .................................................................................................................... 20 7 Molecular biology of killer yeast ................................................................................................... 20 7.1 Molecular biology techniques applied to killer yeast............................................................ 21 8 Conclusion ..................................................................................................................................... 23 9 Reference list ................................................................................................................................. 25 Abstract In the winemaking, yeast are the microorganisms that lead the fermentation process of grape juice. The starter culture that is inoculated into the grape juice is usually Saccharomyces cerevisiae, however other yeasts are also being applied. Moreover, yeast are not only considered as microorganisms to start a fermentation process, but also as producers of killer toxins. These yeasts are named killer yeast. They have lately received a great deal of attention as natural preservatives, since they proved to kill certain pathogens that cause unwanted changes in the characteristics of wine, other fermented food products, agricultural products and they were even investigated for medical purposes. In the wine industry, SO2 is currently mostly used preservative that efficiently kills nearly all of the pathogens. Since this agent is potentially toxic, researchers are trying to find proper killer yeasts that can be included in a certain winemaking step in order to prevent or kill unwanted microorganisms and thus avoid major economic losses. The killer and immunity mechanism has been mostly investigated for S. cerevisiae killer toxins (K1,K2 and K28). The killer phenotype of these toxins is determined by a medium sized double stranded satellite RNA virus (dsRNA). dsRNAs are identified as M- viruses (M1, M2 and M28) that encode K1, K2 and K28 toxins, respectively. They are encapsidated in virus-like particles (VLPs). M virus encodes the toxin, while L-A virus is responsible for replication, encapsidation and maintenance of both mycoviruses. 1 Yeast killer toxins Yeast that produce killer toxins (known as killer factors or killer proteins) are recognized as killer yeast (El-Banna, El Sahn, & Shehata, 2011; Garcia,’ Esteve-Zarzoso, & Arroyo, 2016). In agriculture and food industries the interest is to apply yeast killer toxins as bioprotective agents which prevent the growth of competing microorganisms. To use them as antimicrobials, killer yeast can be included as a fermentation starter or as purified killer toxins (Chessa et al., 2017). They are mostly being used in the winemaking process, as a partial substitute to SO2. This compound is a commonly used chemical antimicrobial agent which has shown to have carcinogenic and cytotoxic effects to humans and animals. For this reason, winemakers started to select their S. cerevisiae or non-S. cerevisiae yeast starter cultures based on their killing phenotype (Novotna, Flegelova, & Janderova, 2004; Carboni et al. 2020). First killer character was discovered in S. cerevisiae, in 1960s (Garcia, Esteve-Zarzoso, & Arroyo, 2016). Shortly after, they found out that also other yeast species secrete killer toxins. The toxins were found in Zygosaccharomyces bailli, Hanseniaspora uvarum, Ustilago maydis, Candida, Cryptococcus, Debaryomyces, Hansenula, Kluyveromyces, Metschnikowia, Pichia, Torulopsis, Williopsis, Zygosaccharomyces, Aureobasidium, Zygowilliopsis and Mrakia (Liu et al. 2013; Schmitt & Breinig, 2006). In case where there are different microbes present in the same niche, they start to compete. Some strains or species, known as killer yeast, have therefore developed a competitive advantage of possessing dsRNA viruses which encode killer toxins. These cells than gain advantage by killing sensitive cells. Killer yeast producing the toxins are immune to their own toxin and the reason for it is understood only in some cases (Belda et al., 2017). 1.1 Saccharomyces cerevisiae killer system Yeast killer toxins are either small basic proteins or larger multimeric protein complexes (glycoproteins) that kill sensitive cells (yeast, fungi or bacteria) via two-step mode of action and without cell-cell contact (Schmitt & Breinig, 2002; El-Banna, El Sahn, & Shehata, 2011; Liu et al., 2013; Garcia, Esteve-Zarzoso, & Arroyo, 2016; Schaffrath, Meinhardt, & Klassen, 2018). They are mediated by specific cell wall receptors of a target cell and immune to their own toxin, while being susceptible to the toxins excreted by other yeast species (Magliani et al., 1997; Schmitt & Breinig, 2006). 1.1.1 Saccharomyces cerevisiae killer toxins In S. cerevisiae there are three major killer viruses (ScV-M1, ScV-M2 and ScV-M28) and each of them encodes a specific killer toxin (K1, K2 and K28, respectively) (Schmitt & Breinig, 2006). These toxins are classed in K1, K2, K28 and Klus groups based on their lack of cross 1 immunity, their molecular mode of action and their killing profiles (Schmitt & Breinig, 2002; Ramirez et al. 2015). K1 and K28 toxin belong to A/B family of toxins due to their entry into the cell by endocytosis and retrograde trafficking for toxicity. Shiga, Cholera and Ricin toxins also belong to this family. It is common for A/B toxins to contain one or more β subunits responsible for entry into the cell and intracellular targeting (Carrol et al. 2009). 1.1.1.1 L-A mycoviruses and satellite M dsRNAs The killer phenotype of toxins secreted by S. cerevisiae is determined by a medium sized double stranded RNA virus (dsRNA) belonging to Totiviridae family (Schmitt et al. 2006; Liu et al. 2013). These satellite dsRNAs are identified as M-viruses (M1, M2 and M28) that encode K1, K2 and K28 toxins, respectively. They are encapsidated in virus-like particles (VLPs) (Magliani et al. 1997). In certain yeast, linear dsDNA plasmids carry the toxin genes (Kluyveromyces lactis, P. acaciae, P. inositovora, D. robertsiae, Trichosporon pullulans, etc.) and in some others they stay in the chromosomal DNA (Williopsis saturnus, Pichia anomala, Pichia kluyveri, Pichia membranifaciens, etc..). (Liu et al. 2013; Belda et al. 2017) Depending on the killer type, the killer toxin-encoding genes
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