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Glucose can provide a primary source of energy and metabolic intermediates for eukaryotic microorganisms. The yeast 'Saccharomyces cerevisiae' has been considered the paradigm for eukaryotic cells. In yeast, extracellular glucose is transported by a range of high and low affinity transporters, and transport is regulated in response to carbon status. Nutrient sensing and signaling via pathways involving hexokinases, glucose transporter-like proteins that lack transport activity and G-protein-coupled receptors have been characterised in yeast. Glucose is the preferred energy source; the genes that are required for the use of alternative carbon sources are not transcribed when glucose is available, and the key components of this transcriptional repression mechanim, including the Mig1p repressor protein, the Ssn6p-Tup1p corepressor complex, and the Snflp kinase, have been extensively studied. Studies of multi cellular filamentous fungi are only at an embryonic stage, but it is becoming clear that the situation is considerably more complex in filamentous fungi than in yeast, probably due to the restricted metabolic capacity of yeast due to strong selection for anaerobic fermentation of sugars to ethanol, rather than aerobic metabolism via the Krebs cycle as in filamentous fungi. A large number of sugar transporters have been predicted from the filamentous fungal genome projects, but functional data exist for only a small fraction of these proteins in Ascomycetes ('Aspergillus niger', 'Aspergillus nidulans', 'Neurospora crassa', 'Trichoderma harzianum', and 'Tuber borchii') and Basidiomycetes ('Amanita muscaria' and 'Uromyces fabae'). |
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