Pre-termination in aflR of Aspergillus sojae inhibits aflatoxin biosynthesis

The aflR gene product is the main transcriptional regulator of aflatoxin bi osynthesis in Aspergillus parasiticus and Aspergillus flavus. Although A. s ojae strains do not produce aflatoxins, they do have an aflR homologue. Whe n compared with the aflR of A. parasiticus, the A. sojae gene contains two mutations: an HAHA motif and a premature stop codon. To investigate the fun ctionality of the A. sojae aflR gene product, we used a GAL4 one-hybrid sys tem in yeast. The transcription-activating activity of AflR from A. sojae w as 15% of that from A. pamsiticus. The introduction of an additional aflR f rom A. sojae into an A. parasiticus strain did not affect aflatoxin product ivity. A hybrid aflR comprising the amino-terminal region of A. sojae aflR and the carboxy-terminal region of A. parasiticus aflR suppressed the effec t associated with pre-termination of the A. sojae AflR. We conclude that th e premature stop codon of the A. sojae aflR is the key to its functionality and leads to prevention of aflatoxin biosynthesis through loss of the tran scription of aflatoxin biosynthesis-related genes.