Revisiting the Taxonomic Synonyms and Populations of Saccharomyces cerevisiae—Phylogeny, Phenotypes, Ecology and Domestication

Saccharomyces cerevisiae—the most emblematic and industrially relevant yeast—has a long list of taxonomical synonyms. Formerly considered as distinct species, some of the synonyms represent variants with important industrial implications, like Saccharomyces boulardii or Saccharomyces diastaticus, but with an unclear status, especially among the fermentation industry, the biotechnology community and biologists not informed on taxonomic matters. Here, we use genomics to investigate a group of 45 reference strains (type strains) of former Saccharomyces species that are currently regarded as conspecific with S. cerevisiae. We show that these variants are distributed across the phylogenetic spectrum of domesticated lineages of S. cerevisiae, with emphasis on the most relevant technological groups, but absent in wild lineages. We analyzed the phylogeny of a representative and well-balanced dataset of S. cerevisiae genomes that deepened our current ecological and biogeographic assessment of wild populations and allowed the distinction, among wild populations, of those associated with low- or high-sugar natural environments. Some wild lineages from China were merged with wild lineages from other regions in Asia and in the New World, thus giving more resolution to the current model of expansion from Asia to the rest of the world. We reassessed several key domestication markers among the different domesticated populations. In some cases, we could trace their origin to wild reservoirs, while in other cases gene inactivation associated with domestication was also found in wild populations, thus suggesting that natural adaptation to sugar-rich environments predated domestication.

Yeast selection for non-alcoholic and low-alcoholic beverages based on wort

A series of wort fermentations with eight yeast strains were carried out in laboratory conditions. The strains used were: Saccharomyces cerevisiae (2 strains), Saccharomyces diastaticus (3 strains), Saccharomyces carlsbergensis (1 strain), Saccharomyces lactis (1 strain), Saccharomyces sake gekkeikan (1 strain). Selection of yeast strains has been performed in order to study the possibilities for their aplication to obtain fermentable non-alcoholic and low-alcoholic beverages based on wort. Three yeast strains (two of Saccharomyces cerevisiae and one Saccharomyces diastaticus), were selected based on their good growth in the used medium and the pleasant organoleptic profile formed as a result of the fermentation carried out. The accumulated alcohol values varied between 0.05 and 0.22 % (w/w).

Fermentation of Microalgae Biomass through Mild Acid Pretreatment for Bioethanol Production

Conversion of microalgae biomass to bioethanol is actively being researched to establish a cost effective and sustainable production technology. The main challenge is to break down the carbohydrates content in the biomass to obtain fermentable sugar for subsequent fermentation process. This study focuses on the effectiveness of the usage phosphoric acid pretreatment and capability of Saccharomyces diastaticus as the fermentation microbe to produce ethanol. Scenedesmus dimorphus microalgae biomass was used as the feedstock due to its high carbohydrate content. Mild acid hydrolysis at various conditions were carried out on biomass and the hydrolysates were subjected to fermentation. The optimum condition of acid pre-treatment using phosphoric acid was determined in this study. Based on the results, bioethanol yield of 94% was obtained at optimum condition of 2.5% v/v phosphoric acid at temperature of 120 °C for 30 min. This study proved that combination of phosphoric acid pre-treatment process with Saccharomyces diastaticus yeast provides a practicable method for the production of bioethanol from microalgae.

Recruitment of the Swi/Snf Complex by Ste12-Tec1 Promotes Flo8-Mss11-Mediated Activation of STA1 Expression

ABSTRACT In the yeast Saccharomyces diastaticus, expression of the STA1 gene, which encodes an extracellular glucoamylase, is activated by the specific DNA-binding activators Flo8, Mss11, Ste12, and Tec1 and the Swi/Snf chromatin-remodeling complex. Here we show that Flo8 interacts physically and functionally with Mss11. Flo8 and Mss11 bind cooperatively to the inverted repeat sequence TTTGC-n-GCAAA (n = 97) in UAS1-2 of the STA1 promoter. In addition, Flo8 and Mss11 bind indirectly to UAS2-1 of the STA1 promoter by interacting with Ste12 and Tec1, which bind to the filamentation and invasion response element (FRE) in UAS2-1. Furthermore, our findings indicate that the Ste12, Tec1, Flo8, and Mss11 activators and the Swi/Snf complex bind sequentially to the STA1 promoter, as follows: Ste12 and Tec1 bind first to the FRE, whereby they recruit the Swi/Snf complex to the STA1 promoter. Next, the Swi/Snf complex enhances Flo8 and Mss11 binding to UAS1-2. In the final step, Flo8 and Mss11 directly promote association of RNA polymerase II with the STA1 promoter to activate STA1 expression. In the absence of glucose, the levels of Flo8 and Tec1 are greatly increased, whereas the abundances of two repressors, Nrg1 and Sfl1, are reduced, suggesting that the balance of transcriptional regulators may be important for determining activation or repression of STA1 expression.

Glucose Repression of STA1 Expression Is Mediated by the Nrg1 and Sfl1 Repressors and the Srb8-11 Complex

ABSTRACT In the yeast Saccharomyces diastaticus, expression of the STA1 gene, which encodes an extracellular glucoamylase, is negatively regulated by glucose. Here we demonstrate that glucose-dependent repression of STA1 expression is imposed by both Sfl1 and Nrg1, which serve as direct transcriptional repressors. We show that Nrg1 acts only on UAS1, and Sfl1 acts only on UAS2, in the STA1 promoter. When bound to its specific site, Sfl1 (but not Nrg1) prevents the binding to UAS2 of two transcriptional activators, Ste12 and Tec1, required for STA1 expression. We also found that Sfl1 contributes to STA1 repression by binding to the promoter and inhibiting the expression of FLO8, a gene that encodes a third transcriptional activator involved in STA1 expression. In addition, we show that the levels of Nrg1 and Sfl1 increase in glucose-grown cells, suggesting that the effects of glucose are mediated, at least in part, through an increase in the abundance of these repressors. NRG1 and SFL1 expression requires the Srb8-11 complex, and correspondingly, the Srb8-11 complex is also necessary for STA1 repression. However, our evidence indicates that the Srb8-11 complex does not associate with either the SFL1 or the NRG1 promoter and thus plays an indirect role in activating NRG1 and SFL1 expression.