A new metabolite which could be converted to aflatoxin (AF) B2 was detected during cofermentation analysis of two nonaflatoxigenic strains (SRRC 2043 and SRRC 163) of Aspergillus parasiticus. SRRC 2043, which accumulates the xanthone O-methylsterigmatocystin (OMST), a late precursor in the AFB, pathway, was observed to accumulate another chemically related compound (HOMST; molecular weight, 356); SRRC 163 is blocked early in the pathway and accumulates averantin. During cofermentation of the two strains, levels of OMST and HOMST were observed to be greatly reduced in the culture, with simultaneous production of AFB1, AFB2, and AFG1. Intact cells of SRRC 163 were able to convert pure OMST or its precursor, sterigmatocystin, to AFBI and AFG, without AFB2 accumulation; the same cells converted isolated HOMST to AFB2 with no AFB, or AFG1 production. The results indicate that AFB2 is produced from a separate branch in the AF biosynthetic pathway than are AFBI and AFG1; AFB2 arises from HOMST, and AFB, and AFG, arise from
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