Versicolorin synthesis by Aspergillus parasiticus: Regulation by temperature and zinc

Characterization of the polyketide synthase gene (pksL1) required for aflatoxin biosynthesis in Aspergillus parasiticus

Aflatoxins are potent toxic and carcinogenic compounds, produced by Aspergillus parasiticus and A. flavus as secondary metabolites. In this research, a polyketide synthase gene (pksL1), the key gene for aflatoxin biosynthesis initiation in A. parasiticus, has been functionally identified and molecularly characterized. PCR-derived DNA probes were used to find the pksL1 gene from subtracted, aflatoxin-related clones. Gene knockout experiments generated four pksL1 disruptants which lost both the ability to produce aflatoxins B1, B2, and G1 and the ability to accumulate norsolorinic acid and all other intermediates of the aflatoxin biosynthetic pathway. A pksL1 DNA probe detected a 6.6-kb poly(A)+ RNA transcript in Northern (RNA) hybridizations. This transcript, associated with aflatoxin production, exhibited a regulated expression that was influenced by growth phase, medium composition, and culture temperature. DNA sequencing of pksL1 revealed an open reading frame for a polypeptide (PKSL1) of 2,109 amino acids. Sequence analysis further recognized four functional domains in PKSL1, acyl carrier protein, beta-ketoacyl-acyl carrier protein synthase, acyltransferase, and thioesterase, all of which are usually present in polyketide synthases and fatty acid synthases. On the basis of these results, we propose that pksL1 encodes the polyketide synthase which synthesizes the backbone polyketide and initiates aflatoxin biosynthesis. In addition, the transcript of pksL1 exhibited heterogeneity at the polyadenylation site similar to that of plant genes.

Molecular cloning of genes related to aflatoxin biosynthesis by differential screening

A differential hybridization strategy was used to clone genes associated with aflatoxin biosynthesis. A genomic library, formed between nuclear DNA and the pUC19 plasmid, was screened with three different cDNA probes by the colony hybridization procedure. Nineteen clones were selected; all were positively correlated with and presumably enriched with genes associated with aflatoxin production. Some of these clones were further characterized by using them as probes in Northern (RNA blot) hybridizations. Five clones hybridized strongly with some polyadenylated RNAs formed during the transition to or during idiophase when aflatoxin was produced. However, little or no corresponding hybridization occurred with polyadenylated RNAs formed in early and mid-log growth phase. Two of the clones were further used as probes to hybridize with polyadenylated RNAs formed under aflatoxin-permissive and nonpermissive temperatures. Hybridization occurred with RNA species formed under the permissive temperature only.

Variation in regulation of aflatoxin biosynthesis among isolates of Aspergillus flavus

Two new phenotypes of Aspergillus flavus which exhibit novel patterns of aflatoxin production have been identified and characterized. In one of the new variants of A. flavus, aflatoxin is made in the absence of carbohydrate and concomitantly with growth, without a lag period. A second variant did not produce aflatoxin in the presence or absence of carbohydrate. Chemical mutagenesis of this nonaflatoxigenic strain resulted in mutant strains which produced aflatoxin on carbohydrate-containing media. The aflatoxin production pattern observed in these mutants resembled the typical production scheme, with a lag period through log phase growth.

Nitrate induces enzymes of the mannitol cycle and suppresses versicolorin synthesis in Aspergillus parasiticus

Addition of sodium nitrate to growing cultures of Aspergillus parasiticus (ATCC 36537) induces the synthesis of enzymes involved in nitrate assimilation (NO3- reductase), of enzymes in the pentose pathway (glucose-6-phosphate dehydrogenase), and of enzymes in the mannitol cycle (mannitol- and mannitol-1-phosphate dehydrogenases). Addition of NO3- also causes a dose-dependent suppression of synthesis of the polyketide secondary metabolite, versicolorin A. We suggest that in the presence of NO3- plus peptone, the cytoplasmic NADPH/NADP ratio may be elevated, resulting in increased conversion of malonyl coenzyme A to fatty acid rather than to polyketide.