Carbon catabolite repression in yeast.

Control of gene expression is a basic regulatory mechanism of living organisms. In microorganisms, glucose or other rapidly metabolizable carbon sources repress the expression of genes that code for enzymes related to the metabolism of other carbon sources. This phenomenon, known as catabolite repression, allows microorganisms to cope effectively with changes in the carbon sources present in their environment. In the case of Escherichiu coli, a model to explain at the molecular level the mechanism of catabolite repression has been worked out (Ullmann, 1985; Saier, 1989), although some of its elements remain unidentified. Yeasts are also subject to carbon catabolite repression but the underlying mechanism(s) is less understood than it is in E. coli and we do not yet know how glucose exerts its repressive effect. In the present article I will review information gathered in the last few years on a variety of catabolite-repressible systems, try to integrate it and to elaborate a scheme for carbon catabolite repression consistent with the results obtained. Although this review will be centered around Sacchuromyces cerevisiue, reference to other yeast species will be made when information is available. The degree of repression caused by glucose depends strongly on the enzyme affected and the strain used. As shown in Table 1, it can vary from about 800-fold for invertase to less than 10-fold for aconitase, cytochrome c oxidase or isocitrate dehydrogenase. In most cases, the decrease in enzyme levels caused by glucose is paralleled by a decrease in the concentration of the corresponding mRNA. This could be due to an effect of glucose either on the rate of transcription, or on the stability of the corresponding mRNA or on both. Direct measurements of transcription rates have been performed in only a few cases. For the genes CYCI, encoding iso-1 cytochrome c, and MAL6S, encoding maltase, it was found that derepressed cells synthesized the corresponding mRNAs 6 times and 15 times faster, respectively, than repressed cells (Zitomer et al., 1979; Federoff et al., 1983a). It should be noted that, in the case of maltase, an induction by maltose is superimposed on the process of catabolite repression, but even in the presence of maltose, the addition of glucose to a yeast culture causes an approximately I5-fold decrease in the rate of transcription of the MAL6S gene in