Effect of branch frequency in Aspergillus oryzae on protein secretion and culture viscosity

Highly branched mutants of two strains of Aspergillus oryzae (IFO4177, whic h produces alpha-amylase, and a transformant of IFO4177 [AMG#13], which pro duces heterologous glucoamylase in addition to alpha-amylase) were generate d by UV or nitrous acid mutagenesis. Four mutants of the parental strain (I FO4177), which were 10 to 50% more branched than the parental strain, were studied in stirred batch culture and no differences were observed in either the amount or the rate of enzyme production. Five mutants of the transform ed parental strain (AMG#13), which were 20 to 58% more branched than the pa rental strain, were studied in either batch, fed-batch or continuous cultur e. In batch culture, three of the mutants produced more glucoamylase than t he transformed parental strain, although only two mutants produced more glu coamylase and alpha-amylase combined. No increase in enzyme production was observed in either chemostat or fed-batch culture. Cultures of highly branched mutants were less viscous than those of the par ental and transformed parental strains. A linear relationship was found bet ween the degree of branching (measured as hyphal growth unit length) and cu lture viscosity (measured as the torque exerted on the rheometer impeller) for these strains. DOT-controlled fed-batch cultures (in which the medium f eed rate was determined by the DOT) were thus inoculated with either the tr ansformed parent or highly branched mutants of the transformed parent to de termine whether the reduced viscosity would improve aeration and give highe r enzyme yields. The average rate of medium addition was higher for the two highly branched mutants (ca. 8.3 g medium h(-1)) than for the parental str ain (5.7 g medium h(-1)). Specific enzyme production in the DOT controlled fed-batch cultures was similar for all three strains (approx. 0.24 g alpha- amylase and glucoamylase [g of biomass](-1)), but one of the highly branche d mutants made more total enzyme (24.3 +/- 0.2 g alpha-amylase and glucoamy lase) than the parental strain (21.7 +/- 0.4 g oc-amylase and glucoamylase) . (C) 1999 John Wiley & Sons, Inc.