An in-depth characterization of the Aspergillus niger glucoamylase (glaA) 
promoter performance was carried out on defined medium employing multi-well 
high-throughput screening as well as controlled batch and fed-batch bioreactor 
culture techniques with GFP as a fluorescent reporter protein. A variety of 
metabolizable carbon substrates and non-metabolizable analogs were screened with 
regard to their effect on the glaA expression system. The results clearly 
demonstrate that only starch and its hydrolytic products, including glucose, act 
as inducers. However, induction of the glaA expression system through the 
monosaccharide glucose is significantly lower compared to starch and the higher 
molecular weight starch degradation products. All other 26 carbon substrates 
tested do not induce, or even, as in the case of the easily metabolizable 
monosaccharide xylose, repress glaA-promoter controlled gene expression in the 
presence of the inducing disaccharide maltose with an increase of repression 
strength by increasing xylose concentrations. The complex effect of glucose on 
glaA-promoter controlled expression was also analyzed using non-metabolizable 
glucose analogs, namely 5-thio-glucose and 2-deoxyglucose, which were identified 
as novel and potent inducers of the glaA expression system. The results show 
that the induction strength depends on the inducer concentration with a maximum 
at defined concentrations and lower induction or even repression at 
concentrations above. Moreover, controlled fed-batch cultivations using a high 
maltose feed rate with concomitant extracellular accumulation of glucose 
resulted in lower levels of the reporter protein compared to cultures with a 
low-maltose feed rate without extracellular glucose accumulation, thus 
supporting the conclusion that increasing the glucose concentration beyond a 
critical point reduces the induction strength or may even cause repression. This 
way, the speed of polymer hydrolysis, glucose uptake and intracellular breakdown 
can be fine-tuned for optimal fungal growth and the metabolic burden for 
glucoamylase synthesis can be limited adequately in response to nutrient 
availability.