Genetic organization and function of the aflatoxin B1 biosynthetic genes

Abstract

Aflatoxins are secondary metabolites produced by Aspergillus flavus and Aspergillus parasiticus. Most of the genes involved in the biosynthesis of aflatoxin are contained within a single cluster in the genome of these filamentous fungi. Studies directed toward understanding the molecular biology of aflatoxin biosynthesis have led to a number of important discoveries. A pair of fatty acid synthase genes were identified that are involved uniquely in aflatoxin biosynthesis. Two genes were also characterized that represent new families of cytochrome P450 monooxygenases. Gene expression is coordinated during aflatoxin production and is under the control of a positive regulatory gene belonging to a family of fungal transcriptional activators associated with various metabolic pathways in fungi. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.

Molecular biology of mycotoxin biosynthesis

“…Molecular genetics con¢rmed these initial reports, as the development of e¤cient DNA transformation systems for both A. £avus and A. parasiticus allowed the elucidation of genes corresponding to the enzyme activities of the AF/ST pathway [10,11,18]. Two gene transfer techniques, gene complementation which involves the restoration of gene function in AF blocked mutants and gene disruption where`knock-out' strains are formed, have been invaluable in assigning the function of the isolated AF/ST genes.…”

“…£avus transformed with a series of overlapping cosmids containing the AF cluster accumulated averufanin (AVNN) and AVF and was relieved by complementation with the avf1 locus. Sequence analysis of the avf1 locus revealed two genes, a£B and a£W, which encode products with similar amino acid identity to stcB and stcW respectively in A. nidulans but their functions are as yet unknown [18].…”

Transcript of a homolog of aflR, a regulatory gene for aflatoxin synthesis in Aspergillus parasiticus, was not detected in Aspergillus oryzae strains

“…Evidence (K. Kusumoto, Y. Nogata and H. Ohta, submitted, also see reference [8]) suggests that several strains of A. oryzae and A. sojae have the entire gene cluster homologous to that of a£atoxin biosynthesis in A. parasiticus, while they do not produce a£atoxin. Klich et al [8] demonstrated that two of three strains of A. sojae express homologs of a£R and fas-1A (a gene encoding fatty acid synthetase for initiation of a£atoxin biosynthesis [3]), but not the other ¢ve homolog genes (norA, nor-1, pksA, ver-1, and omt-1). The reason for this might be that expression of these ¢ve homolog genes requires factors other than a£R in a£atoxin production.…”

“…A: The RNA was prepared from the fungal mycelia and was treated with (lanes with odd numbers) or without (even numbers) reverse transcriptase, and used as template for PCR with primers AFLF1 and AFLR1, followed by electrophoresis with 1.5% agarose gel. M, molecular mass marker (100-bp ladder); lanes 1 and 2, A. oryzae IFO 4202 cultivated under a£atoxin-producing condition (the other A. oryzae strains were also cultivated under the same condition); lanes 3 The samples are the same as A, besides CMDF1 and CMDR1 as primers. DNA fragments of 800 bp (size marker), 650 bp (PCR product for the a£R cDNA and DNA) and 270 bp (that for the cmdA cDNA) are indicated with bars.…”

“…They are produced by ¢lamentous fungi, Aspergillus £avus and Aspergillus parasiticus, which belong taxonomically to Aspergillus Section Flavi [2]. A£atoxin biosynthesis has been studied extensively in terms of enzymology and molecular biology: several genes encoding enzymes involved in a£atoxin biosynthesis and a gene (a£R) responsible for regulation of the above genes have been characterized (reviewed by Woloshuk and Prieto [3]). The a£R product is known to regulate the structural genes positively at the level of transcription [3].…”