Differences among AGT1-encoded α-glucoside transporters and their ability to transport maltotriose in Saccharomyces yeasts
Improved fermentation of starch and its dextrin products would benefit the brewing and whiskey industries. Most strains of Saccharomyces ferment glucose and maltose and partially ferment maltotriose, but are unable to utilise the larger dextrin products of starch. This utilisation pattern is partly attributed to the ability of yeast cells to transport the aforementioned mono-, di- and trisaccharides into the cytosol. The maltotriose transporting efficiency varies between different Saccharomyces strains. In this study, several Saccharomyces strains, including whiskey strains, were screened for growth on maltotriose. The AGT1 genes, which encode a maltose transporter that show affinity for maltotriose uptake, were isolated from the strains that grew strongest in media with maltotriose as sole carbon source. The isolated AGT1 alleles were sequenced and their chromosomal locations determined in the strains from which they were cloned. Nucleotide and deduced amino acid sequences of the isolated genes shared 95% and 98% identity, respectively. The efficiency of maltotriose transport was determined by expressing the AGT1 variants in an identical genetic background. The Km values obtained for all the permeases were very similar (≈ 3), but the permease with improved performance for maltotriose transport showed an approximately 30% higher Vmax value than for the others. The data obtained suggest that the genetic variation among the AGT1-encoded transporters is reason for the variation in maltotriose transport efficiency among different Saccharomyces strains. This study offers prospects for the development of yeast strains with improved maltose and maltotriose uptake capabilities that, in turn, could increase the overall fermentation efficiencies in the beer and whiskey industries.