Yeast strains co-expressing exogenous glucoamylases, the method for obtaining said yeast strains and the use thereof to produce bioethanol

A method for obtaining a Saccharomyces cerevisiae yeast strain, said method comprising steps of:

• a) genetically modifying a Saccharomyces cerevisiae yeast to co-express a gene encoding a glucoamylase of fungal origin selected from the group consisting of a glucoamylase of Aspergillus niger and a glucoamylase of Saccharomycopsis fibulifera;
• and a gene encoding the glucoamylase of Saccharomyces cerevisiae var. diastaticus, wherein the gene encoding the glucoamylase of Aspergillus niger comprises the nucleic acid sequence set forth in SEQ ID NO:
• 1, the gene encoding the glucoamylase of Saccharomycopsis fibulifera comprises the nucleic acid sequence set forth in SEQ ID NO:
• 17, and the gene encoding the glucoamylase of Saccharomyces cerevisiae var. diastaticus comprises the nucleic acid sequence set forth in SEQ ID NO:
• 3,
• wherein said Saccharomyces cerevisiae yeast strain comprises between 2 and 10 copies of the gene encoding the glucoamylase of fungal origin;
• and between 2 and 10 copies of the gene encoding the glucoamylase of Saccharomyces cerevisiae var. diastaticus, and wherein said Saccharomyces cerevisiae yeast strain hydrolyzes liquefied starch extracted from a biomass while at the same time ferments the sugars resulting from hydrolyzed liquefied starch;
• b) culturing and fermenting the Saccharomyces cerevisiae yeast strain obtained in step a) on a dextrin medium;
• c) selecting the Saccharomyces cerevisiae yeast strains having fermentation kinetics at least equal to or greater than the fermentation kinetics, under the same conditions, with the Saccharomyces cerevisiae yeast strain deposited, on Jul. 9, 2015, at the CNCM under number I-4999.