Protoplast fusion in Aspergillus niger strains accumulating citric acid.

Auxotrophic strains of Aspergillus niger were obtained from citric-acid-producing strains of the fungus after irradiation with UV light. Protoplasts were isolated from young hyphae of the auxotrophic strains after treatment with snail enzyme and than treated with polyethylene glycol (30%, W/V), in a Ca2+ (10 mmol/L) solution. The pH value of the suspension was adjusted to 9.0. The frequency of the heterokaryons (related to the number of protoplasts reverting after PEG treatment) was 0.67%. Prototrophic heterozygous spores were isolated from a heterokaryon with the frequency of 1.2 x 10(-6). Citric acid production in the best heterozygous strains was about 15% higher than that of the high-production parent strain.