Addition of glucose or fructose to cells of the yeast grown on a nonfermentable carbon source triggers within a few minutes post-translational activation of trehalase, repression of the (catalase) and (Hsp70) genes, and induction of the ribosomal protein genes and 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 and as well as partial repression of and were observed in the absence of sugar phosphorylation. In glucose-grown nitrogen-starved yeast cells re-addition of a nitrogen source triggers activation of trehalase in a glucose- or fructose-dependent way, but with no apparent requirement for phosphorylation of the sugar. Repression of and 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 and 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 and SSA3 and induction of ribosomal protein genes. They also show that nutrient-induced repression of and 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.


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