Multiple alpha-glucoside transporter genes in brewer's yeast

Maltose and maltotriose are the two most abundant fermentable sugars in bre wer's wort, and the rate of uptake of these sugars by brewer's yeast can ha ve a major impact on fermentation performance. In spite of this, no informa tion is currently available on the genetics of maltose and maltotriose upta ke in brewing strains of yeast. In this work, we studied 30 brewing strains of yeast (5 ale strains and 25 lager strains) with the aim of examining th e alleles of maltose and maltotriose transporter genes contained by them. T o do this, we hybridized gene probes to chromosome blots. Studies performed with laboratory strains have shown that maltose utilization is conferred b y any one of five unlinked but highly homologous MAL loci (MAL1 to MAL4 and MAL6). Gene 1 at each locus encodes a maltose transporter. All of the stra ins of brewer's yeast examined except two were found to contain MAL11 and M AL31 sequences, and only one of these strains lacked MAL41. MAL21 was not p resent in the live ale strains and 12 of the lager strains. MAL61 was not f ound in any of the yeast strains. In three of the lager strains, there was evidence that MAL transporter gene sequences occurred on chromosomes other than those known to carry MAL loci. Sequences corresponding to the AGT1 gen e, which encodes a transporter of several alpha-glucosides, including malto se and maltotriose, were detected in all but one of the yeast strains. Homo logues of AGT1 were identified in three of the lager strains, and two of th ese homologues were mapped, one to chromosome II and the other to chromosom e XI. AGT1 appears to be a member of a family of closely related genes, whi ch may have arisen in brewer's yeast in response to selective pressure.