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Appl. Environ. Microbiol. doi:10.1128/AEM.02570-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Molecular analysis of maltotriose active transport and fermentation by Saccharomyces cerevisiae: a determinant role for the AGT1 permease

Departamento de Bioquímica, Centro de Ciências Biológicas, Universidade Federal de Santa Catarina, Florianópolis, SC 88040-900, Brazil, Programa de Pós-graduação Interunidades em Biotecnologia, USP-IPT-I. BUTANTAN, São Paulo, Brazil, Programa de Pós-graduação em Biotecnologia, Centro de Ciências Biológicas, Universidade Federal de Santa Catarina, Florianópolis, Brazil, and Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, Brazil

^* To whom correspondence should be addressed. Email: bstambuk{at}mbox1.ufsc.br, boris.stambuk@pq.cnpq.br.

	Abstract

Maltotriose incomplete and/or sluggish fermentation causes both quality and economic problems in the ale brewing industry. Although it has been proposed that the sugar uptake must be responsible for these undesirable phenotypes, there have been conflicting reports on whether all the -glucoside transporters known in S. cerevisiae (MALx1, AGT1, and MPH2/3) allow efficient maltotriose utilization by yeast cells. We characterized the kinetics of yeast cell growth, sugar consumption and ethanol production during maltose or maltotriose utilization by several S. cerevisiae yeast strains (both MAL^C constitutive and MAL inducible), and by their isogenic counterparts specifically deleted in the AGT1 gene. Our results clearly showed that yeast strains carrying functional MALx1 permeases in their plasma membrane, encoded by the MAL21, MAL31 and/or MAL41 genes, were unable to utilize maltotriose. While both high- and low-affinity transport activities were responsible for maltose uptake from the medium, in the case of maltotriose the only low-affinity (K_m 36±2 mM) transport activity was mediated by the AGT1 permease. In conclusion the AGT1 transporter is required for efficient maltotriose fermentation by S. cerevisiae yeasts, highlighting the importance of this permease for breeding and/or selection programs aimed at improving sluggish maltotriose fermentations.