A TRAC study of gene expression in wort fermentations reveals novel aspects of the regulation of sugar transport and membrane synthesis

During wort fermentations yeast must efficiently use the major wort sugars, glucose, maltose and maltotriose. The first step is transport across the cell membrane. Transport is determined by the presence or absence of appropriate sugar transporters and also by the membrane's lipid composition. Under laboratory conditions, glucose represses the expression of genes for ?-glucosidases (maltases) and maltose and maltotriose transporters. Further, biosynthesis of sterols and unsaturated fatty acids requires oxygen. Because oxygen is present only at the start of brewery fermentations, yeast membranes are deficient in these lipids at the end of such fermentations. To help understand how brewer's yeasts adjust their physiology to changing sugar composition and oxygen availability during fermentation of brewer's wort, we used the rapid TRAC technology to follow expression of several genes involved in sugar uptake and lipid synthesis. Lager yeasts are hybrids containing components from both S. cerevisiae and S. bayanus genomes. We followed the expression of over 70 S. cerevisiae and S. bayanus genes at frequent intervals (up to every 2 h) through tall tube wort fermentations by industrially cropped lager yeast. Genes for maltose and maltotriose metabolism (MALx1, MALx2 and the S. bayanus MTT1) showed strong co-ordinate increases before glucose began to be used and declined while maltose concentrations were still high. Expression of genes for ergosterol synthesis (needed for correct membrane function) peaked in the first few hours and again late in fermentation when oxygen was absent. Evidently the physiology of lager yeast fermenting wort diverges from that of laboratory strains in laboratory media.