CHARACTERIZATION OF EXPERIMENTALLY-INDUCED, NONAFLATOXIGENIC VARIANT STRAINS OF ASPERGILLUS-PARASITICUS

Six previously isolated, nonaflatoxigenic variants of Aspergillus para siticus, designated sec mutants, were characterized morphologically by electron microscopy, biochemically by biotransformation studies,vith an aflatoxin precursor, and genetically by Northern (RNA) hybridizatio n analysis of aflatoxin biosynthetic gene transcripts, Scanning electr on micrographs clearly demonstrated that compared with the parental se c(+) forms, the variant sec forms had an abundance of vegetative mycel ia, orders of magnitude reduced number of conidiophores and conidia, a nd abnormal metulae, Conidiospores were detected in sec cultures only at higher magnifications (x500), in contrast to the sec(+) (wild type) strain, in which abundant conidiospores (masking the vegetative mycel ia) were observed even at lower magnifications (x300), All sec(+) form s, but none of the sec forms, showed bioconversion of sterigmatocystin to aflatoxins, Northern blots probed with pathway genes demonstrated lack of expression of both the aflatoxin biosynthetic pathway structur al (nor-1 and omtA) and regulatory (aflR) genes in the sec forms; PCR and Southern hybridization analysis confirmed the presence of the gene s in the sec genomes. Thus, the loss of aflatoxigenic capabilities in the sec form is correlated with alterations in the conidial morphology of the fungus, suggesting that the regulation of aflatoxin synthesis and conidiogenesis may be interlinked.