Yeast cells often encounter a mixture of different carbohydrates in industrial processes. However, glucose and sucrose are always consumed first. The presence of these sugars causes repression of gluconeogenesis, the glyoxylate cycle, respiration and the uptake of less-preferred carbohydrates. Glucose and sucrose also trigger unexpected, hormone-like effects, including the activation of cellular growth, the mobilization of storage compounds and the diminution of cellular stress resistance. In an industrial context, these effects lead to several yeast-related problems, such as slow or incomplete fermentation, 'off flavors' and poor maintenance of yeast vitality. Recent studies indicate that the use of mutants with altered responses to carbohydrates can significantly increase productivity. Alternatively, avoiding unnecessary exposure to glucose and sucrose could also improve the performance of industrial yeasts.

The endo-beta-xylanase-encoding gene (xynA) of Bacillus pumilus PLS was isolated from a genomic DNA library and the open reading frame (ORF) was inserted in expression vectors for the yeast Saccharomyces cerevisiae. Plasmid pFN3 harboured the xynA ORF fused to the yeast mating pheromone alpha-factor signal sequence (MFalpha1s) under the control of the alcohol dehydrogenase II gene promotor (ADH2P) and terminator (ADH2T) sequences. In plasmid pFN4, the MFalpha1S-xynA ORF was brought under the control of the phosphoglycerate kinase I gene promotor (PGK1p) and terminator (PGK1T) sequences. Autoselective, recombinant S. cerevisiae [fur1::LEU2] strains bearing pFN3 or pFN4 secreted functional endo-beta-xylanase when grown in complex medium. Enzymatic activities in the culture supernatants reached maximum levels of 8.5 nkat/ml and 4.5 nkat/ml, respectively. The temperature and pH optimum for both the bacterial and the recombinant xylanase were 58 degrees C and pH 6.2.