Targeted Mutations Produce Divergent Characteristics in Pedigreed Sake Yeast Strains

Japanese sake making using wild yeasts isolated from natural environments

Saccharomyces cerevisiae is one of the most important microorganisms for the food industry, including Japanese sake, beer, wine, bread, and other products. For sake making, Kyokai sake yeast strains are considered one of the best sake yeast strains because these strains possess fermentation properties that are suitable for the quality of sake required. In recent years, the momentum for the development of unique sake, which is distinct from conventional sake, has grown, and there is now a demand to develop unique sake yeasts that have different sake making properties than Kyokai sake yeast strains. In this minireview, we focus on "wild yeasts," which inhabit natural environments, and introduce basic research on the wild yeasts for sake making, such as their genetic and sake fermentation aspects. Finally, we also discuss the molecular breeding of wild yeast strains for sake fermentation and the possibility for sake making using wild yeasts.

Sake yeast symbiosis with lactic acid bacteria and alcoholic fermentation

The yeast Saccharomyces cerevisiae plays a pivotal role in the production of fermented foods by converting sugars in ingredients into ethanol through alcoholic fermentation. However, how accurate is our understanding of its biological significance? Although yeast is essential to produce alcoholic beverages and bioethanol, yeast does not yield ethanol for humankind. Yeast obtains energy in the form of ATP for its own vital processes through alcoholic fermentation, which generates ethanol as a byproduct. The production of ethanol may have more significance for yeast, since many other organisms do not produce ethanol, a highly toxic substance, to obtain energy. The key to address this issue has not been found using conventional microbiology, where yeasts are isolated and cultured in pure form. This review focuses on a possible novel role of yeast alcohol fermentation, which is revealed through our recent studies of microbial interactions.

Application of unimodal probability distribution models for morphological phenotyping of budding yeast

Morphological phenotyping of the budding yeast Saccharomyces cerevisiae has helped to greatly clarify the functions of genes and increase our understanding of cellular functional networks. It is necessary to understand cell morphology and perform quantitative morphological analysis (QMA) but assigning precise values to morphological phenotypes has been challenging. We recently developed the Unimodal Morphological Data image analysis pipeline for this purpose. All true values can be estimated theoretically by applying an appropriate probability distribution if the distribution of experimental values follows a unimodal pattern. This reliable pipeline allows several downstream analyses, including detection of subtle morphological differences, selection of mutant strains with similar morphology, clustering based on morphology, and study of morphological diversity. In addition to basic research, morphological analyses of yeast cells can also be used in applied research to monitor breeding and fermentation processes and control the fermentation activity of yeast cells.

CAR1 as a new selective marker for genetic engineering of wine yeasts

A common problem in engineering industrial yeasts, and wine yeasts in particular, is the lack or scarcity of selective markers for introducing desired genetic changes. Almost all such markers, which are usually auxotrophic mutations, would reduce the growth characteristics of yeast strains. However, a potentially useful marker could be the CAR1 gene encoding arginase, the deletion of which reduces the accumulation of the carcinogen ethyl carbamate in wine, making such a deletion beneficial for wine production and maintainable in wine yeast strains. Here we demonstrate the use of the CAR1 gene as a selective marker. First, we observe that complete deletion of CAR1 in a triploid wine strain of Saccharomyces cerevisiae causes strong growth inhibition on a medium containing arginine as the only nitrogen source. Then, we show that strains with CAR1 deletion can be reliably transformed using CAR1 as a plasmid marker. Thus, the CAR1 gene can be used as a convenient selective marker in genetic engineering of wine yeasts, in particular using CRISPR/Cas9 technology.