Regulated expression of the nor-1 and ver-1 genes associated with aflatoxin biosynthesis.

RNA transcript accumulation for the ver-1 and nor-1 genes, which are associated with aflatoxin biosynthesis in the fungus Aspergillus parasiticus, was measured before and during aflatoxin production in liquid shake culture. Transcripts were not detected until near the end of trophophase (growth phase) and could still be observed well into stationary phase during batch fermentation in an aflatoxin-supporting growth medium. Maximum accumulation of both transcripts occurred just prior to the onset of stationary phase. Aflatoxin B1 was first detected approximately 8 h after the appearance of the ver-1 and nor-1 transcripts. In contrast, maximum transcript accumulation for the pyrG gene (encoding orotidine monophosphate decarboxylase), which is involved in primary metabolism, was observed at the onset of trophophase when the ver-1 and nor-1 transcripts could not be detected. Accumulation of the ver-1 and nor-1 transcripts was also studied following a nutritional shift from a non-aflatoxin-supporting medium (peptone mineral salts [PMS]) to a glucose-containing medium (glucose mineral salts [GMS]), which does support aflatoxin biosynthesis. Transcripts from ver-1 and nor-1 could not be detected on PMS medium but did accumulate approximately 4 to 7 h following transfer to GMS medium. Additionally, aflatoxins were not detected in PMS medium but were observed to accumulate within 24 h after the shift from PMS to GMS. These data suggest that aflatoxin biosynthesis is in part regulated by the accumulation of the ver-1 and nor-1 transcripts.