The effect of organic nitrogen sources on recombinant glucoamylase production by Aspergillus niger in chemostat culture

Aspergillus niger B1, a recombinant strain carrying 20 extra copies of the native glucoamylase gene, was grown in glucose-limited chemostat cultures s upplemented with various organic nitrogen sources (dilution rate 0.12 +/- 0 .01 h(-1), pH 5.4). In cultures supplemented with L-alanine, L-methionine, casamino acids, or peptone, specific glucoamylase (GAM) production rapidly decreased to less than 20% of the initial level. Reducing the pH of the cul ture to 4.0 resulted in stable GAM production for up to 400 h. Morphologica l mutants (a light brown and a dark brown mutant) appeared in each fermenta tion and generally displaced B1, Light brown mutants had higher selection c oefficients relative to B1 than dark brown mutants and became the dominant strain in all fermentations except those maintained at pH 4.0. Several muta nts isolated from these cultures had reduced ability to produce GAM in batc h culture, although few had lost copies of the glaA gene. Some mutants had methylated DNA. (C) 2000 Academic Press.