Genetic evidence that high noninduced maltase and maltose permease activities, governed by MALx3-encoded transcriptional regulators, determine efficiency of gas production by baker's yeast in unsugared dough

Abstract

Strain selection and improvement in the baker's yeast industry have aimed to increase the speed of maltose fermentation in order to increase the leavening activity of industrial baking yeast. We identified two groups of baker's strains of Saccharomyces cerevisiae that can be distinguished by the mode of regulation of maltose utilization. One group (nonlagging strains), characterized by rapid maltose fermentation, had at least 12-fold more maltase and 130-fold-higher maltose permease activities than maltose-lagging strains in the absence of inducing sugar (maltose) and repressing sugar (glucose). Increasing the noninduced maltase activity of a lagging strain 13-fold led to an increase in CO2 production in unsugared dough. This increase in CO2 production also was seen when the maltose permease activity was increased 55-fold. Only when maltase and maltose permease activities were increased in concert was CO2 production by a lagging strain similar to that of a nonlagging strain. The noninduced activities of maltase and maltose permease constitute the largest determinant of whether a strain displays a nonlagging or a lagging phenotype and are dependent upon the MALx3 allele. Previous strategies for strain improvement have targeted glucose derepression of maltase and maltose permease expression. Our results suggest that increasing noninduced maltase and maltose permease levels is an important target for improved maltose metabolism in unsugared dough.

FIG. 1

Northern (RNA) analysis of MALx2, MALx1, and MALx3 mRNA levels in baker’s yeast strains L38 and NL67 and their transformants. Cells were growing exponentially under inducing (maltose), noninducing (galactose or ethanol), and repressing (glucose) conditions. Total RNA was loaded in each lane, and the filters were hybridized with labelled probes (MAL6-2, MAL6-1, MAL6-3, and ACT1). All carbon sources were used at a concentration of 2% (wt/wt). Lanes: 1, L38; 2, L38 + BEJ17; 3, L38 + MALx3–VH1; 4, L38 + MALx3–VH7; 5, NL67; 6, NL67 + BEJ17; 7, NL67 + MALx3–VH1; 8, NL67 + MALx3–VH7.

FIG. 2

Fermentation by nonlagging and lagging strains of S. cerevisiae in unsugared synthetic dough medium. Yeast cells were inoculated into unsugared synthetic dough containing 1% sucrose and 5% maltose. Samples were withdrawn at intervals and centrifuged, and supernatants were assayed for ethanol by gas chromatography. Values shown are means of data derived from two experiments. Assays were carried out in triplicate with standard errors of less than 10%. (A) Cycloheximide was added at 0 min; (B) no cycloheximide was added. □, L38; ○, L67.