DIFFERENTIAL REQUIREMENT OF THE YEAST SUGAR KINASES FOR SUGAR SENSINGIN ESTABLISHING THE CATABOLITE-REPRESSED STATE

Addition of rapidly fermentable sugars to cells of the yeast Saccharom yces cerevisiae grown on nonfermentable carbon sources causes a variet y of short-term and long-term regulatory effects, leading to an adapta tion to fermentative metabolism. One important feature of this metabol ic switch is the occurrence of extensive transcriptional repression of a large group of genes. We have investigated transcriptional regulati on of the SUC2 gene encoding repressible invertase, and of HXK1, HXK2 and GLK1 encoding the three known yeast hexose kinases during transiti on from derepressed to repressed growth conditions. Comparing yeast st rains that express various combinations of the hexose kinase genes, we have determined the importance of each of these kinases for establish ing the catabolite-repressed state. We show that catabolite repression involves two distinct mechanisms. An initial rapid response is mediat ed through any kinase, including Glk1, which is able to phosphorylate the available sugar. In contrast, long-term repression specifically re quires Hxk2 on glucose and either Hxk1 or Hxk2 on fructose. Both HXK1 and GLK1 are repressed upon addition of glucose or fructose. However, fructose repression of HXK1 is only transient, which is in line with i ts preference for fructose as substrate and its requirement for long-t erm fructose repression. In addition, expression of HXK1 and GLK1 is r egulated through cAMP-dependent protein kinase. These results indicate that sugar sensing and establishment of catabolite repression are con trolled by an interregulatory network, involving all three yeast sugar kinases and the Ras-cAMP pathway.