Use of Aspergillus overproducing mutants, cured for integrated plasmid, to overproduce heterologous proteins.

Aspergillus niger var. awamori was previously transformed with a vector designed to express a fused glucoamylase-prochymosin gene and bearing the Neurospora crassa pyr4 gene as a selectable marker. Mutant strains that overproduced the glucoamylase-prochymosin fusion protein were derived from one of the transformants. Despite the fact that the expression vector was integrated into the genome of these strains it was possible to obtain strains from which the vector sequences had been removed. This was performed by selection against the pyr4 gene present on the expression vector using 5-fluoroorotic acid. The cured strains were retransformed in order to investigate production of heterologous proteins using other expression vectors. In addition to the glucoamylase-prochymosin fusion protein, the mutant Aspergillus strains also overproduced Rhizomucor miehei aspartic proteinase but not preprochymosin produced as a non-fusion protein. The ability to select for loss of integrated plasmid from Aspergillus transformants may prove to be important for a variety of applications.