Culture conditions control expression of the genes for aflatoxin and sterigmatocystin biosynthesis in Aspergillus parasiticus and A. nidulans

Abstract

High temperature and nitrate supported gene expression for sterigmatocystin biosynthesis in Aspergillus nidulans; ammonium did not. Homologous genes for aflatoxin biosynthesis in A. parasiticus showed the opposite transcript expression pattern, suggesting that the two mycotoxins are regulated differently. The aflR gene is postulated to require additional genetic elements to effect its own activation by the different culture conditions. A patulin polyketide synthase (PKS) gene was found to be regulated differently than the aflatoxin PKS. Thus, the biosyntheses of structurally similar compounds in these two fungi appear to be regulated very differently.

FIG. 1

Influence of culture temperature on expression of genes for AF and ST biosynthesis. Supplemented minimal medium was used with 3.7 g of NH₄Cl per liter for A. parasiticus and 6 g of NaNO₃ per liter for A. nidulans. Each of the above probes was from the relevant cloned gene in A. nidulans and A. parasiticus. The A. parasiticus probes were a 4.5-kb KpnI fragment for pksA and a cloned β-tub gene (6a) as an internal control. The A. nidulans probes were a 5.4-kb KpnI fragment for stcA (3, 13) and a 5.6-kb KpnI fragment for aflR, stcE, and stcD (3).

FIG. 2

Influence of nitrogen sources in the culture medium on transcription of genes for AF and ST biosynthesis. The medium background was supplemented minimal medium. The sole nitrogen source was either 3.7 g of NH₄Cl per liter for (NH₄)⁺, 6 g of NaNO₃ per liter for (NO₃)⁻, or 20 g of peptone per liter. When used in combination, each was reduced to half strength. The media for A. nidulans were appropriately supplemented. The respective temperatures employed were 27°C for A. parasiticus and 33°C for A. nidulans. A PCR-amplified 0.8-kb fragment was used for nor-1 (10); other probes were the same as described for Fig. 1.

FIG. 3

Influence of nitrogen sources in the culture media (GMS and minimal medium [MM]) on expression of the two PKS genes in A. parasiticus. Nitrogen sources were the same as described for Fig. 2. The culture temperature was 27°C. The A. parasiticus pksA probe was as described for Fig. 1. The probe for A. parasiticus pksP1 was a 2.6-kb fragment from the cloned pksP1 gene (6a).