Intraspecific fusion of Aspergillus niger strains producing citric acid using heat-inactivated and living protoplasts

Protoplasts from a prototrophic Aspergillus niger strain were first inactivated at 55 degrees C for 12 min with a regeneration frequency of 35 x 10(-6), and then fused with living protoplasts from an auxotrophic strain. The fusion frequency was 1.1 x 10(-5). Some fusants segregated sectors of prototrophic recombinants. Citric acid production in submerged cultivation of these recombinants was examined. More recombinants were obtained by further treating the fusants with (+)-camphor, a diploidization inducer.