EFFECTS OF LIGHT-CERCOSPORIN AND ALTERED-CERCOSPORIN PHENOTYPES ON GENE-EXPRESSION IN CERCOSPORA-KIKUCHII

Cercospora kikuchii is a fungal pathogen of soybean that produces a photosensitizing and phytotoxic polyketide, cercosporin, in culture and in planta. We have studied the influence of growth stage, light, and growth medium on cercosporin accumulation in a wild-type isolate and three mutant strains with altered toxin phenotypes. After an initial logarithmic growth phase, the wild-type isolate accumulated high levels of cercosporin on either complete medium or potato dextrose broth, but only when cultured in the light. Dark-grown cultures of the wild-type and light-grown cultures of two uv-induced mutant derivatives accumulated 100-fold lower cercosporin levels. A third mutant strain accumulated wild-type cercosporin levels, but only when cultured in the light in potato dextrose broth. Two-dimensional gel electrophoresis of both extracted proteins and in vitro translation products from wild-type cultures revealed the presence of polypeptides and poly A + RNAs whose accumulation was positively regulated by light. Comparison of translated polypeptide patterns from wild-type and mutant cultures also demonstrated differential accumulation of translatable poly A + RNAs in cercosporin-producing and nonproducing cultures.