Atypical Aspergillus parasiticus isolates from pistachio with aflR gene nucleotide insertion identical to Aspergillus sojae

Aflatoxins are the most toxic and carcinogenic secondary metabolites produced primarily by the filamentous fungi Aspergillus flavus and Aspergillus parasiticus. The toxins cause devastating economic losses because of strict regulations on distribution of contaminated products. Aspergillus sojae are highly valued as koji molds in the traditional preparation of fermented foods, such as miso and shoyu.  Koji mold species are generally perceived of as being non-toxigenic and is generally recognized as safe (GRAS).  Even though A. sojae resembles A. parasiticus, the difference between the species was not well defined. Geographical distribution of A. sojae indicates that the fungus is most common in East Asian food preparation. 

Fungal isolates were collected from a California pistachio orchard and named as PWE strains. All the strains were identified to be A. sojae and A. parasiticus using b-tubulin gene sequences. However these isolates produced aflatoxins B1, B2, G1 and G2.  Aflatoxin (AF) biosynthesis genes are clustered on a 82-kb genome region in A. parasiticus. Regulation of AF biosynthesis genes is mediated by a complex network of interactions involving global regulators and pathway-specific transcription factors. The aflR gene encode a transcription activator interacting with  aflJ.  Comparison of aflR DNA sequences of PWE, A. parasiticus and A. sojae, we discovered that aflatoxigenic PWE strains had the six base insertion (CTCATG) similar to domesticated A. sojae but a pre-termination codon TGA at nucleotide positions1153-1155  was absent.  Because these strains produced four types of aflatoxins, we named these new isolates as atypical A. parasiticus. Concordance analysis of multi locus DNA sequences indicated PWE strains were most likely to be a link between A. parasiticus and A. sojae.