Improvement of Maltose Fermentation Efficiency: Constitutive Expression of MAL Genes in Brewing Yeasts

One of the major limiting factors in high-gravity brewing is a fermentation delay caused by glucose repression of maltose assimilation. The maltose assimilation is controlled by a MAL locus, which consists of three genes with maltase or α-glucosidase (MALS), maltose transporter (MALT), and a positive regulator (MALR). Transcription of the MALS and MALT genes is induced by maltose and repressed by glucose. To improve maltose assimilation efficiency, we attempted constitutive expression of the MAL genes with high-copy-number plasmids in a brewing yeast strain of Saccharomyces cerevisiae. Each of the MAL genes was connected to a constitutive promoter of the yeast glyceraldehyde 3-phosphate dehydrogenase gene and successfully introduced into a brewing yeast strain, BH-84. The Northern analysis and the enzyme activities of both maltase and maltose transporter in the transformants confirmed the constitutive expression of each gene in the presence of glucose. In the brewing trials of the high gravity of 24° P, the constitutive expression of the MALT gene was effective for the improvement of maltose fermentation efficiency, whereas expression of MALS or MALR genes had no effect on maltose fermentability. The transformants that constitutively expressed MALT gene achieved significantly higher fermentation rates.