STRAIN IMPROVEMENT OF CHYMOSIN-PRODUCING STRAINS OF ASPERGILLUS-NIGERVAR AWAMORI USING PARASEXUAL RECOMBINATION

Parasexual recombination was used to obtain improved chymosin-producin g strains and to perform genetic analysis on existing strains. Chlorat e resistance was used to select for a variety of spontaneous nitrate a ssimilation pathway mutations in strains previously improved for chymo sin production using classical strain improved methods including mutat ion and screening, and selection for 2-deoxyglucose resistance (dgr). Diploids of these improved strains were generated via parasexual recom bination and were isolated on selective media by complementation of ni trate assimilation mutations. A preliminary genetic analysis of diploi d and haploid segregants indicated that the dgr trait, resulting in ov erexpression of chymosin, was recessive. Also, mutations in two differ ent dgr genes resulted in an increased level of chymosin production. W hen these mutations were combined via parasexual recombination, the re sulting haploid segregants produced about 15% more chymosin than eithe r pal ental strain. CHEF gel electrophoresis was used to determine the chromosomal location of the integrated chymosin DNA sequences, and to verify diploidy in one case where the chromosome composition of two h aploid parents differed.