External pH constitutes one of the most important environmental factors that control growth, metabolism and differentiation in microorganisms, including fungi. We have analyzed the effect of external pH on sterigmatocystin biosynthesis in Aspergillus nidulans. It was observed in repeated experiments that alkaline pH, in opposition to acid pH, increased sterigmatocystin production and the transcript levels of aflR, the master gene that regulates expression of the sterigmatocystin cluster in A. nidulans. It is known that pH effects in fungi operate mostly through the Pal/Pac signaling pathway, originally described in Aspergillus nidulans. Accordingly, we studied the role of this signaling pathway in ST biosynthesis. It was observed that aflR transcript levels were increased in the "alkalinity mimicking" mutant pacCc14 and were minimal in the "acidity mimicking" mutant palA1. No sterigmatocystin was produced by palA1 or pacC- mutants at neither acid or alkaline pH of incubation. Finally, fluG and flbA, genes known to regulate both conidiation and sterigmatocystin synthesis upstream in the regulatory cascade, were up-regulated at alkaline pH.
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