Molecular characterization of the afl-1 locus in Aspergillus flavus.

An unusual mutation at the afl-1 locus, affecting aflatoxin biosynthesis in Aspergillus flavus 649, was investigated. The inability of strain 649 to produce aflatoxin was found to be the result of a large (greater than 60 kb) deletion that included a cluster of aflatoxin biosynthesis genes. Diploids formed by parasexual crosses between strain 649 and the aflatoxigenic strain 86 did not produce aflatoxin, indicating the dominant nature of the afl-1 mutation in strain 649. In metabolite feeding experiments, the diploids did not convert three intermediates in the aflatoxin pathway to aflatoxin. Northern (RNA blot) analysis of the diploids grown in medium conducive for aflatoxin production indicated that the aflatoxin pathway genes nor1, ver1, and omt1 were not expressed; however, there was low-level expression of the regulatory gene aflR. Pulsed-field electrophoresis gels indicated a larger (6 Mb) chromosome in strain 649 than the apparently homologous (4.9 Mb) chromosome in strain 86. The larger chromosome in strain 649 suggests that a rearrangement occurred in addition to the deletion. From these data, we proposed that a trans-sensing mechanism in diploids is responsible for the dominant phenotype associated with the afl-1 locus in strain 649. Such a mechanism is known in Drosophila melanogaster but has not been described for fungi.