Production of the homologous pectin lyase B protein in six genetically defined protease-deficient Aspergillus niger mutant strains.

An expression cassette has been transformed into six protease-deficient (prt) mutant strains of Aspergillus niger. Transformants were tested for improved production of the proteolytically susceptible PELB tester protein. In four complementation groups (prtA, B, D and F) distinct improvement of PELB yield was observed. These in vivo experiments in single prt mutants confirmed earlier in vitro PELB degradation data and demonstrated how the use of protease-deficient mutants can significantly improve protein production in A. niger. The strong effects of several prt alleles on the stability of the PELB tester protein have initiated a more detailed genetical and molecular characterization of the prt mutations. Mapping of the cloned protease genes pepA [I], B [II], C [IV], D [I], E [IV] and F [IV] indicated that none of the prt mutations represent alleles of the presently cloned protease (pep) genes from A. niger. Analysis of the expression of the pep genes in prt strains demonstrated that the strongly reduced protease activities observed in several prt mutants are not reflected by reduced transcription levels for a number of extracellular proteases. These results indicate that the mode of action of the prt genes constitute an interesting group of new genetic functions which severely affect protease production, and as such improve protein production, in A. niger.