Yeast strains are the common dwellers of most of nutrient rich media/sources such as fruits, tree bark, soils, etc., [Alfenore et al, 2002]. They form one of the important class of microorganisms that are more complex than bacteria. Unlike other fungi, yeasts are ovoid single cells that are about 8 µm long and 5 µm in diameter. Their doubling times are 1-3hunder optimal growing conditions [Morris et al, 1992]. According to published news reports the global market for yeast products has reached nearly $7.6 billion in 2017and it is increasing rapidly and expected to grow to nearly $10.7 billion by 2022 [Globe news wire, 2018]. Yeasts are used for baking, brewing and various other industrial applications. Such applications include manufacturing of shoyu, miso and production of various fermentation products such as enzymes, vitamins, capsular polysaccharides, carotenoids, polyhydric alcohols, lipids, glycolipids, citric acid, etc [Turker, 2014] and as eukaryotic system to produce novel compounds. Given the importance of the yeasts and yeast by products described above, extensive research has been undertaken to identify, catalog and preserve yeast strains worldwide [James et al, 1995].
The process of identification of yeasts involves sequence analysis of ribosomal RNA genes that are conserved. The ribosomal RNA genes coding for both 18 s and 26 s RNA have been extensively analyzed and the analysis has proven that it is not only important in establishing them as useful molecular markers for studying evolutionary relationships between organisms but also useful tools for molecular characterization of yeasts [Ciardo et al, 2006]. Early studies related to characterization of yeasts and their classifications have shown a widespread pattern of disparity between phenotypes and genotypes. For the purpose of clarity and to systematically classify yeasts, analysis of genes coding for 18S rRNA [James et al, 1995], internal transcribed spacer (ITS) of18s rRNA [Ciardo et al, 2006] and the DNA sequences for domains 1 and 2 (D1/D2) of 26 s rRNA [Kurtzman et al, 2015] have proven to be optimal.
Many efforts have been made to isolate and characterize yeasts from various climates of Nepal with applications in baking and brewing [Karki et al, 2017], however their molecular characterization and systematic evaluation of their application, especially, in brewing is lacking. An important parameter in selecting brewing yeast is its tolerance to salt and ethanol [Kodama et al, 2013], because these are known to damage, the lipid layers and thus destabilize the cultures [Stanley et al, 2013]. Our laboratory has been working towards characterizing various yeast isolates from Nepal, and assesses their ability to tolerate salt and ethanol, and the effect of yeast to electric voltage supply especially on ethanol production [Joshi et al, 2019]. Further, we are also interested in isolating yeasts that utilize both glucose and xylose for alcohol fermentation from xylose containing substrate like lignocellulosic biomass. In the present study, 12 yeast isolates collected from various sources have been characterized by (i) nucleotide sequencing of domains 1 and 2 (D1/D2) of 26 s rRNA genes, (ii) their ability to tolerate salt and ethanol as well as utilization of glucose and xylose as substrates for ethanol production, and (iii) effect of voltage supply on alcohol dehydrogenase and pyruvate decarboxylase expression.