Recombinant glucoamylase production by Aspergillus niger B1 in chemostat and pH auxostat cultures

The production of glucoamylase (GAM) by Aspergillus niger B1, a genetic tra nsformant containing an additional 20 copies of the homologous glucoamylase gene (glaA) was studied in nutrient (maltodextrin)-limited chemostat and n utrient-excess pH auxostat cultures. In these culture systems the specific production rate of GAM increased with dilution rate and reached a maximum ( up to 15.0 mg GAM [g biomass](-1) h(-1)) when A. niger B1 was grown at its maximum specific growth rate in pH auxostat culture, indicating that GAM is a growth-associated product. The appearance of spontaneous morphological m utants was observed in all continuous flow cultures grown at pH 5.4, with a light brown mutant always displacing the parental strain. However, no morp hological mutants were observed in cultures grown at pH 4.0. Further, when A. niger B1 was grown in pH auxostat culture, the specific production rate of GAM was 31% higher at pH 4.0 than at pH 5.4. Southern blot analyses show ed that some morphological mutants (including the light brown mutant) isola ted from a pH auxostat culture had lost copies of the glaA genes. (C) 1998 Academic Press.