Transcriptional responses to glucose at different glycolytic rates in Saccharomyces cerevisiae

The addition of glucose to Saccharomyces cerevisiae cells causes reprogramming of gene expression. Glucose is sensed by membrane receptors as well as (so far elusive) intracellular sensing mechanisms. The availability of four yeast strains that display different hexose uptake capacities allowed us to study glucose-induced effects at different glycolytic rates. Rapid glucose responses were observed in all strains able to take up glucose, consistent with intracellular sensing. The degree of long-term responses, however, clearly correlated with the glycolytic rate: glucose-stimulated expression of genes encoding enzymes of the lower part of glycolysis showed an almost linear correlation with the glycolytic rate, while expression levels of genes encoding gluconeogenic enzymes and invertase (SUC2) showed an inverse correlation. Glucose control of SUC2 expression is mediated by the Snf1-Mig1 pathway. Mig1 dephosphorylation upon glucose addition is known to lead to repression of target genes. Mig1 was initially dephosphorylated upon glucose addition in all strains able to take up glucose, but remained dephosphorylated only at high glycolytic rates. Remarkably, transient Mig1-dephosphorylation was accompanied by the repression of SUC2 expression at high glycolytic rates, but stimulated SUC2 expression at low glycolytic rates. This suggests that Mig1-mediated repression can be overruled by factors mediating induction via a low glucose signal. At low and moderate glycolytic rates, Mig1 was partly dephosphorylated both in the presence of phosphorylated, active Snf1, and unphosphorylated, inactive Snf1, indicating that Mig1 was actively phosphorylated and dephosphorylated simultaneously, suggesting independent control of both processes. Taken together, it appears that glucose addition affects the expression of SUC2 as well as Mig1 activity by both Snf1-dependent and -independent mechanisms that can now be dissected and resolved as early and late/sustained responses

Biochemical characterisation of a glucoamylase from Aspergillus niger produced by solid-state fermentation

In this work, glucoamylase was produced by Aspergillus niger in solid-state fermentation. The enzyme was partially purified by ammonium sulphate precipitation and ion exchange and gel filtration chromatographies. Its molecular mass was estimated as 118.17 kDa by electrophoresis. The partially purified enzyme had an optimum pH range of 4.5-5.0 and an optimum temperature of 60 °C, with average activity 152.85 U mL-1. Thermal and pH stability assays with the crude extract showed that more than 60 % of the activity remained at pH 4.6 and 60 °C, even after an exposition to these conditions longer than 24 h. Yet, after purification, the enzyme was stable at these for at least 4 h, which indicated that its purification for use in starch saccharification was inadvisable. K M and Vmax were 0.34 mg mL-1 and 160.22 U mL-1, respectively