Expression Patterns of Genes and Enzymes Involved in Sugar Catabolism in IndustrialSaccharomyces cerevisiaeStrains Displaying Novel Fermentation Characteristics

Abstract

The utilisation of maltose or sucrose by a selection of nine brewing, baking or laboratory strains of Saccharomyces cerevisiae was either repressible, constitutive or absent. Overall fermentation rate showed a good correlation with maximum specific maltose transport rate (R 2 = 0.79), but a poor correlation with maximum maltase activity (R 2 = 0.34), implying that transport rather than hydrolysis of maltose was a rate-limiting step determining fermentation performance. The genetic basis for differences seen between the strains in terms of fermentation kinetics was investigated. All strains were found to possess genomic sequences detectable with probes for the MAL11 (AGT1), MAL31 permease genes and the SUC2 invertase gene, however, the loci present and the pattern of expression of these and other MAL genes varied widely. From comparisons of sugar utilisation and gene expression patterns, constitutive maltose utilisation in strain NCYC 1681 was best explained by AGT1 expression. Such expression of AGT1 was not, however, mirrored by changes in the pattern of expression of the transcriptional activator(s) (MALx3). Patterns of sucrose utilisation were poorly predicted by the kinetics of SUC2 gene expression, indicating that other SUC loci may be of greater importance.

Molecular Analysis of Maltotriose Active Transport and Fermentation by Saccharomyces cerevisiae Reveals a Determinant Role for the AGT1 Permease

“…Due to its relevance for beer fermentation, maltotriose utilization has been studied mostly with the two classes of yeast strains used for brewing, S. cerevisiae ale strains and S. pastorianus lager strains. These studies have revealed that maltotriose utilization by ale strains is significantly slower than that by lager yeasts and that, consequently, residual maltotriose is more common at the end of ale fermentations (22,23,44).Previous genetic and biochemical studies focusing mainly on the maltose fermentation system revealed a series of five unlinked telomere-associated MAL loci: MAL1 through MAL4 and MAL6. Each locus contains at least one copy of three different genes (MALx1, where x stands for one of the five loci) encoding a maltose permease responsible for the active uptake of maltose, a gene (MALx2) for an intracellular maltase or ␣-glucosidase that hydrolyzes the sugar into glucose molecules, and a gene (MALx3) encoding a positive regulatory protein that induces the transcription of the two previous genes in the presence of maltose (25).…”

“…Due to its relevance for beer fermentation, maltotriose utilization has been studied mostly with the two classes of yeast strains used for brewing, S. cerevisiae ale strains and S. pastorianus lager strains. These studies have revealed that maltotriose utilization by ale strains is significantly slower than that by lager yeasts and that, consequently, residual maltotriose is more common at the end of ale fermentations (22,23,44).…”

“…Of these sugars, glucose is preferentially and rapidly utilized by the yeast cells, but process efficiency also requires the complete fermentation of both maltose and maltotriose. After glucose exhaustion, maltose is easily fermented by the majority of S. cerevisiae strains, and not only is maltotriose the least preferred sugar for uptake by these yeast cells, but many yeasts may not use it at all (23,37,44). The difficulty that some strains have in consuming maltotriose leads to one of the problems experienced by many breweries, namely, a high content of residual sugar in the finished beer, low ethanol yields, and atypical beer flavor profiles.…”

Development of a “Stress Model” Fermentation System for Fuel Ethanol Yeast Strains

“…Hazell and Attfield 10 established that yeast strains which displayed rapid maltose utilization possessed considerably higher levels of maltose utilizing enzymes when compared to strains which utilized maltose slowly. More recently, it was established that the capacity for maltose transport, rather than the hydrolysis of maltose, is the main factor which limits the rate of maltose fermentation 15,17 . However, the apparent increased consumption of maltose by J006 cannot be explained solely by increased maltose permease (or other maltose utilizing enzymes) since the expression of the MAL genes required for maltose fermentation is repressed in the presence of glucose 11,14,24 .…”

“…4). Previous studies have established that maltose uptake and maltotriose uptake are competitive, with maltose being the preferred substrate 7 , and that maltose is a potential repressor of maltotriose utilization, allowing consumption to begin only after the majority of the maltose has been depleted 15 . The more rapid disappearance of the maltotriose from the mash with J006 could signify that the maltotriose utilizing enzymes of this strain were not completely repressed by maltose in the stress model fermentation.…”