Molecular basis of glucoamylase overproduction by a mutagenised industrialstrain of Aspergillus niger

We have compared a mutagenized strain of Aspergillus niger (S1), used indus trially for glucoamylase production, and a related low glucoamylase-produci ng strain (S2) with a laboratory strain of A. niger (AB4.1). Our aim was to assess the properties of S1 in relation to the laboratory strain and to ac count at the molecular level for the basis of its glucoamylase overproducti on. Both S1 and S2 have similar multiple copies of the glucoamylase-encodin g gene (glaA) but only SI has enhanced glaA transcript and glucoamylase lev els compared to AB4.1 that has a single copy of the glaA gene. Glucoamylase production by S1 and AB4.1 was repressed by xylose and induced by starch b ut, in S2, remained unaffected by carbon source. S1 also secreted elevated levels of alpha-amylase relative to both S2 and AB4.1 but the production of alpha-glucosidase was low in all three strains. The gene encoding aspergil lopepsin (pepA), an abundant secreted aspartyl protease, was present as a s ingle copy in all strains but no aspergillopepsin could be detected by West ern blotting in either S1 or S2 culture supernatants. We conclude that A. n iger strain improvement by mutagenesis and screening for glucoamylase overp roduction has led to glaA gene multiplication and an expression defect in t he pepA gene. (C) 2000 Elsevier Science Inc. All rights reserved.