Glucoamylase overexpression in Aspergillus niger: molecular genetic analysis of strains containing multiple copies of the glaA gene.

A strategy, based on the usage of the amdS selection marker and a cosmid vector containing four copies of the glucoamylase gene (glaA), was developed to obtain glucoamylase (GLA)-overproducing A. niger strains. With this strategy, fungal strains carrying up to 200 copies of the glaA gene could be isolated at a relatively high frequency. In each transformant analysed, integration occurred in a single chromosome. A significant increase in the extracellular GLA production was observed in most of the transformants carrying multiple copies of the glaA gene. Further analysis showed that the amount of GLA that is produced was not proportional to the number of glaA copies in these transformants. However, the level of GLA production clearly correlated with the amount of glaA mRNA produced in these transformants. From these results it is concluded that GLA production is limited at the level of transcription.