S. cerevisiae has a strong preference for glucose as its carbon source. This is reflected in the preferential conversion of glucose into ethanol, even under aerobic conditions, and in the preferential utilization of glucose when cells are grown in mixtures of glucose and other sugars. In addition, growth on glucose is faster than on other carbon sources and a range of glucose-dependent effects can be observed on properties correlated with growth and stationary phase. During its evolution, S. cerevisiae has gained sophisticated regulatory mechanisms to sense fluctuating levels of glucose, and as a result has many different glucose-sensing and signaling pathways 2. Three of these pathways are particularly well understood. The first is the main glucose repression pathway which controls, for example, respiration and the preferential utilization of glucose 3; the second is the Snf3/Rgt2 pathway, which controls the expression of glucose carriers 4; and the third is the Gpr1/Gpa2 pathway, which controls the synthesis of cAMP and activity of the protein kinase A (PKA) pathway 5 (Figure 1). In addition, many other glucose-induced regulatory effects have been described for which the glucose-sensing mechanism(s) and signaling pathway(s) are unclear. Glucose sensing via these pathways affects specific targets at the post-translational and/or transcriptional level, and a variety of transcription factors have been identified as targets of the glucose-signaling pathways (summarized in Table 1).

The first of the three well-characterized pathways, the main glucose repression pathway (Figure 1a) has been studied extensively for many years and is responsible for the repression - when glucose is available - of genes involved in respiration, gluconeogenesis and the metabolism of alternative carbon sources 36. A central glucose-responsive repressor complex, made up of the Mig1, Ssn6 and Tup1 proteins, represses the transcription of gluconeogenic genes, respiratory genes and genes encoding enzymes involved in the utilization of galactose and maltose (Table 1). In the presence of glucose, the repressor complex is inhibited by a complex comprised of the protein kinase Snf1, the associated regulatory β subunits Sip1, Sip2 or Gal83, and the activating subunit Snf4 78. The Snf1 kinase was recently shown to be activated through phosphorylation of a conserved threonine residue in its activation loop by the Pak1, Tos3 and Elm1 kinases 910. Down-regulation of Snf1 in low-glucose conditions is catalyzed by protein phosphatase type 1 (PP1), encoded by GLC7, which acts in a complex with the regulatory subunit Reg1/Hex2 11. Although several glucose effects on the Snf1 complex have been identified, the precise nature of the glucose-sensing mechanism involved in glucose repression still remains unclear. Glucose phosphorylation by Hxk2 is required for glucose repression, but whether this links to Snf1 function and/or to direct involvement of Hxk2 in transcriptional regulation in the nucleus remains to be elucidated 12.

The second of the well-studied glucose-response pathways, the Snf3/Rgt2 glucose-sensing pathway (Figure 1b), is involved in controlling the expression of glucose transporter genes 4. High glucose concentrations induce the HXT1 gene through Rgt2, while low glucose concentrations induce the HXT1-HXT4 genes through Snf3. S. cerevisiae has seven functional glucose transporter genes (HXT1-HXT7) that are well characterized and many more for which the nature and physiological role is rather unclear 413; the expression of many of these genes depends on the concentration of glucose in the medium. The Snf3 and Rgt2 proteins are glucose-carrier homologs but are apparently unable to carry out active glucose transport; instead, Snf3 and Rgt2 specifically mediate regulation of the active glucose carriers by low and high glucose concentrations, respectively 1415. In the absence of glucose, Mth1 and Std1 stabilize the Rgt1-Ssn6-Tupi repressor-corepressor complex. When glucose is present, the transcription factor Rgt1 is inactivated through SCF-Grr1-mediated inactivation and degradation of Mth1 and Std1, and hyperphosphorylation by an unknown kinase, resulting in dissociation of Rgt1 from the HXT promoters 1617. Remarkably, whereas Rgt1 represses HXT gene expression in the absence of glucose, at high glucose concentrations Rgt1 is converted to an activator that is required for maximal expression of the low-affinity glucose transporter HXT1 (Table 1) 18. The HXT2, HXT4, HXT6 and SNF3 genes are repressed by Mig1 via the main glucose-repression pathway when glucose is abundant 13.

When cells that have been growing on nonfermentable carbon sources have glucose added to the medium, the third well-studied pathway, the Gpr1/Gpa2 glucose-sensing pathway (Figure 1c), is involved in the rapid increase in cAMP levels that is mediated by the adenylate cyclase Cyr1; the activity of Cyr1 in yeast depends on the Ras proteins. Activation of the Gpr1/Gpa2 pathway requires both extracellular glucose detection by the Gpr1-Gpa2 G-protein-coupled receptor system and intracellular glucose phosphorylation by one of the sugar kinases Hxk1, Hxk2 or Glk1 2. Subsequent activation of PKA affects the transcriptional regulation of genes involved in sugar metabolism, stress resistance and growth 5. PKA mediates the repression of stress-resistance genes through the inactivation of the transcription factors Msn2 and Msn4. When activated, Msn2 and Msn4 are translocated from the cytoplasm to the nucleus 19, where they act through stress response elements (STREs) in the promoters of responsive genes (Table 1) 20. The related post-diauxic shift element (Table 1) is affected in a similar way by the Gis1 transcription factor, which acts downstream in the Gpr1/Gpa2-PKA pathway 21.

Apart from these three well-characterized glucose-sensing pathways, other glucose-induced regulatory phenomena have been identified for which the signaling pathway(s) remain unclear. Examples include the glucose-induced upshift of ribosomal protein genes and CLN3 expression; ribosomal gene expression seems to involve, to some extent, the PKA pathway 22, while induction of the CLN3 gene requires the Azf1 transcription factor (Table 1) 23, but does not require active growth, is not blocked by mutations in the Ras/cAMP pathway or in the RGT2, SNF3, HXK2 or MIG1 genes and apparently depends on active glucose metabolism. The Yak1 kinase is also regulated by an unknown glucose-sensing pathway 24; when glucose is exhausted, Yak1 accumulates in the nucleus and phosphorylates Pop2, a subunit of the Ccr4-Not complex that regulates the transcription of many genes involved in control of growth, metabolism and proper entry into stationary phase.

In addition to these complex transcriptional regulation pathways, glucose also controls mRNA stability, translation rate, protein turnover and enzyme activity by a range of different mechanisms. Most of these signaling pathways and mechanisms have been studied using classical methods, taking one or a few genes at a time, and glucose signaling in yeast was already recognised as being 'very complex' before the advent of genomic approaches.

The introduction of genome-wide gene-expression analysis has extended the scope of glucose regulation to much larger parts of the genome than were previously implicated 25. The recent work by the group of Al Brown 1 has added another dimension to glucose regulation by revealing different levels of genomic sensitivity to glucose in yeast. They have shown that very low (0.01%), low (0.1%) and high (1.0%) glucose concentrations elicit strikingly different responses in the genome-wide expression pattern. They confirmed the extreme sensitivity of yeast to glucose by showing that genes involved in carbon metabolism are strongly upregulated in the presence of only 0.01% glucose. In addition, the strong differential expression profiles of the different hexose transporter genes (HXT1-HXT7) was evident from the transcript-profiling data 1. Remarkably, the expression of not only gluconeogenic genes, but also genes involved in the glyoxylate and tricarboxylic acid cycles, is very sensitive to these minute amounts of glucose. This shows that even at very low concentrations of glucose, yeast cells start to adapt to fermentative growth, although respiration is still active.

Another novel aspect of the recent work from the Brown group is the influence of different glucose signals on ribosome biogenesis 1. On the one hand, the microarray data confirmed that ribosomal gene expression is induced in response to high, but not low, glucose signals. On the other hand, mRNA stability assays revealed that glucose, even at concentrations as low as 0.01%, transiently stabilizes the mRNAs for ribosomal proteins. Ribosomal protein biosynthesis requires a major investment of cellular resources because ribosomal protein mRNAs are abundantly present in fast-growing cells and yet have short half-lives. The stabilization of the mRNAs for ribosomal proteins by a low-glucose signal, therefore, appears to be an 'economical' means by which to maintain proliferative capacity in a nutrient-limiting regime without synthesis of new ribosomal proteins. Typically for glucose regulation, this temporary stabilization of ribosomal protein mRNAs is not controlled by just one signaling pathway; rather, the mRNAs for different ribosomal subunits appear to be regulated by distinct glucose-signaling mechanisms.

The genes involved in gluconeogenesis are strongly repressed at glucose concentrations as low as 0.01%. This response to very low glucose concentrations is mediated by two regulatory phenomena: transcriptional repression and accelerated mRNA degradation 26. Yin et al. 26 also showed that the degradation of gluconeogenic mRNA is dependent on the high-affinity glucose sensor Snf3, the phosphorylation of internalized sugars by hexokinases, the protein phosphatase-1 subunit Reg1 and the serine/threonine protein kinase Ume5.

In conclusion, genome-wide transcription analyses will broaden our understanding of the global response of a yeast cell to fluctuating nutrient levels. The glucose response in yeast is an excellent illustration of the complexity of transcriptional regulation by nutrients. Because of this complexity, microarray experiments will be very useful in identifying groups of genes regulated through the different signaling pathways. In addition, this technique will be indispensable for identifying the scope of transcriptional regulation and the extent of overlap between the different signaling pathways, in particular as a function of nutrient concentration. The remarkable sensitivity of the yeast transcriptome to very low glucose concentrations has revealed a new field of application for microarrays: identification of cellular responses to minute changes in the environmental conditions. Microarray analysis appears to provide unprecedented power to identify even the smallest cellular responses.