Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 19, 1995
Publication Date: N/A
Interpretive Summary: In order to try to produce fuel ethanol from agricultural materials at a lower cost, multiple sugars must be used by yeasts. However, most yeasts will preferentially use glucose over all other sugars. The exact mechanisms by which yeasts preferentially utilize glucose in the presence of other sugars are not well understood. In this study, we examined the effects that various sugar mixtures have upon enzymes produced by an unusual yeast that is capable of utilizing several sugars and has industrial potential for production of enzymes and fuel ethanol. We found different enzymes. One of the enzymes was not active in the presence of glucose, while a similar enzyme was. This "system" may be useful in better understanding the factors that regulate sugar utilization in yeasts, thus, allowing insights into ways to design yeast to co-utilize multiple sugars.
Technical Abstract: The fermentation of cellobiose is a rare trait among yeasts. Of the 308 yeast species that aerobically utilize cellobiose, 12 species ferment it, and only 2 species, Candida molischiana and C. wickerhamii, also ferment cellodextrins. C. molischiana produced beta-glucosidase activity on all carbon sources tested, except glucose, mannose and fructose. When these sugars were added to growing cellobiose cultures, the synthesis of beta-glucosidase ceased. However, the total amount of enzyme activity remained constant, indicating that the C. molischiana beta-glucosidase is catabolite repressed, not catabolite inactivated. When grown in medium initially containing glucose plus either xylose, cellobiose, maltose, mannitol or glucitol, C. molischiana preferentially utilized glucose and produced little beta-glucosidase activity until glucose was nearly depleted from the medium. When grown in medium containing cellobiose plus either fructose or mannose, the yeast preferentially utilized the monosaccharides and produced little beta-glucosidase activity. C. molischiana produced beta-glucosidase and co-utilized cellobiose and either xylose, maltose or trehalose mixtures. Glucose and either fructose, mannose or trehalose mixtures were co-utilized; however, no beta-glucosidase activity was detected. Thus, the order of substrate preference groups appeared to be: [glucose, trehalose, fructose, mannose] > [cellobiose, maltose, xylose] > [mannitol, glucitol].