ORIGINAL PAPER J. Dynesen á H. P. Smits á L. Olsson á J. Nielsen Carbon catabolite repression of invertase during batch cultivations of Saccharomyces cerevisiae : the role of glucose, fructose, and mannose Received: 5 May 1998 / Received revision: 3 August 1998 / Accepted: 8 August 1998 Abstract When Saccharomyces cerevisiae are grown on a mixture of glucose and another fermentable sugar such as sucrose, maltose or galactose, the metabolism is dia- uxic, i.e. glucose is metabolized ®rst, whereas the other sugars are metabolized when glucose is exhausted. This phenomenon is a consequence of glucose repression, or more generally, catabolite repression. Besides glucose, the hexoses fructose and mannose are generally also believed to trigger catabolite repression. In this study, batch fermentations of S. cerevisiae in mixtures of su- crose and either glucose, fructose or mannose were performed. It was found that the utilization of sucrose is inhibited by concentrations of either glucose or fructose higher than 5 g/l, and thus that glucose and fructose are equally capable of exerting catabolite repression. How- ever, sucrose was found to be hydrolyzed to glucose and fructose, even when the mannose concentration was as high as 17 g/l, indicating, that mannose is not a re- pressing sugar. It is suggested that the capability to trigger catabolite repression is connected to hexokinase PII, which is involved in the in vivo phosphorylation of glucose and fructose. Introduction Carbon catabolite repression, also known as glucose repression, is a widespread phenomenon in microor- ganisms. As a result of this phenomenon, cells grown on glucose repress the expression of a large number of genes necessary for the utilization of other carbon sources. In the yeast Saccharomyces cerevisiae, glucose repression aects genes coding for enzymes required for the me- tabolism of the sugars sucrose, galactose, and maltose as well as gluconeogenetic and respiratory enzymes (Trumbly 1992; Gancedo 1992; Klein et al. 1998). When S. cerevisiae grows in a medium containing a mixture of glucose and another fermentable sugar such as sucrose, galactose or maltose, the metabolism is therefore dia- uxic, glucose being metabolized ®rst. Both the phosphorylation step, where glucose is converted to glucose 6-phosphate, and the glucose con- sumption rate have been pointed out as key factors for triggering and the degree of glucose repression. In 1991 Rose et al. showed that the triggering of the glucose repression was directly associated with the activity of the enzymes hexokinase PI and PII, but not with that of glucokinase. Hexokinase PI and PII phosphorylate glucose, fructose and mannose upon the entry of these sugars into the cells, whereas glucokinase is speci®c for glucose and mannose. Reduction of hexokinase PII ac- tivity, by promoter deletion, was accompanied by a de- crease in the glucose repression of invertase and maltase, and removal of hexokinase PI in addition to hexokinase PII further decreased the repression. Stable hexokinase PI overproducers, in the absence of hexokinase PII, were nearly as eective for glucose repression as was the wild type. This showed that hexokinase PI activity can lead to glucose repression, but that the triggering mechanism under normal physiological conditions is primarily as- sociated with the activity of hexokinase PII. To in- vestigate which glycolytic reactions are necessary for glucose repression, cells with a residual phosphogluco- isomerase activity of less than 1% were studied by Rose et al. (1991). Phosphoglucoisomerase catalyzes the iso- merization of glucose 6-phosphate to fructose 6-phos- phate. This residual activity (1%) was sucient for glucose repression, implying that no additional glyco- lytic reactions besides the phosphorylation by hexoki- nase PI and PII were necessary for glucose repression. Recent results have shown that the degree of glucose repression correlates with glucose consumption rates, indicating that glucose transport limits the provision of a triggering signal rather than being directly involved in the triggering mechanism (Reifenberger et al. 1997). Appl Microbiol Biotechnol (1998) 50: 579±582 Ó Springer-Verlag 1998 J. Dynesen á H. P. Smits á L. Olsson (&) á J. Nielsen Department of Biotechnology, Center for Process Biotechnology, Technical University of Denmark, DK-2800 Lyngby, Denmark e-mail: lo@ibt.dtu.dk Fax: +45-45-88-41-48