Glucose-triggered signalling in Saccharomyces cerevisiae: different requirements for sugar phosphorylation between cells grown on glucose and those grown on non-fermentable carbon sources.

Addition of glucose or fructose to cells of the yeast Saccharomyces cerevisiae grown on a nonfermentable carbon source triggers within a few minutes posttranslational activation of trehalase, repression of the CTT1 (catalase) and SSA3 (Hsp70) genes, and induction of the ribosomal protein genes RPL1, RPL25 and RPS33. By using appropriate sugar kinase mutants, it was shown that rapid glucose- or fructose-induced activation of trehalase requires phosphorylation of the sugar. On the other hand, partial induction of RPL1, RPL25 and RPS33 as well as partial repression of CTT1 and SSA3 were observed in the absence of sugar phosphorylation. In glucose-grown nitrogen-starved yeast cells readdition of a nitrogen source triggers activation of trehalase in a glucose- or fructose-dependent way, but with no apparent requirements for phosphorylation of the sugar. Repression of CTT1 and SSA3 under the same conditions was also largely dependent on the presence of the sugar and also in these cases there was a strong effect when the sugar could not be phosphorylated. Nitrogen induction of RPL1, RPL25 and RPS33 was much less dependent on the presence of the sugar, and only phosphorylated sugar caused a further increase in expression. These results show that two glucose-dependent signalling pathways, which can be distinguished on the basis of their requirement for glucose phosphorylation, appear to be involved in activation of trehalase, repression of CTT1 and SSA3 and induction of ribosomal protein genes. They also show that nutrient-induced repression of CTT1 and SSA3 is not a response to improvement of the growth conditions because the addition of nonmetabolizable sugar does not ameliorate the growth conditions. Similarly, the upshift in ribosomal protein synthesis cannot be a response to increased availability of energy or biosynthetic capacity derived from glucose, but it is apparently triggered to a significant extent by specific detection of glucose as such.