Abstract
Our group found that some strains, but not all, of A, oryzae and A. sojae have homologs of ver-1, a gene essential for aflatoxin biosynthesis. We compared the nucleotide sequences of ver-1 homologs in A, oryzae, A. sojae, A. flavus and A. parasiticus, and that the homologs in A. oryzae and A. sojae exhibited an extremely high degree of sequence identity with that of A. flavus and A. parasiticus. Transcripts of the homologs in A. oryzae and A. sojae are not detected. Moreover, we examined the expression of the aflR-homolog genes in A, oryzae strains having this gene, which encodes a regulatory protein for other aflatoxin synthesis genes, and demonstrated that no transcripts of the aflR-homolog genes in the examined strains of A. oryzae were detected under the aflatoxin producing condition. Considering the above observations together with the information from other research groups, non-aflatoxigenic A. oryzae and A. sojae basically have some homologs of aflatoxin synthesis genes. Thus, they are possibly domesticated, differentiated from aflatoxin producer evolutionally. Nevertheless, their transcripts are not found in any of the strains and some homologs are not found in the examined strains. Therefore these species do not produce aflatoxin because the synthesis genes are neither transcribed nor existent, probably due to mutations during repetitive use as domesticated strains in manufacturing fermented foods. Further characterization of the regulatory mechanism for aflatoxin production might resolve why koji molds having the homologs do not produce aflatoxin.
