Mechanism of Sterigmatocystin Biosynthesis Regulation by pH in Aspergillus nidulans

Abstract

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 pacC c 14 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.

Defining the transcriptional responses ofAspergillus nidulansto cation/alkaline pH stress and the role of the transcription factor SltA

“…In the wild-type strain, the number of downregulated genes at pH 8 was twice that of upregulated genes (861 vs 387 genes), suggesting that adaptation to medium alkalinisation after shift from an acid culture involves induction but mainly repression of specific sets of genes. Analyses of the lists of significantly up- and downregulated genes (Supplementary Table S3; see also a specific bioinformatic analysis of genes with the Top100 log2FC values in Supplementary Tables S4 and S5) confirmed the previously described upregulation of “alkaline-expressed genes” such as pacC, aflR and enaA [2, 5, 38, 39], and the downregulation of “acid-expressed genes” such as palF [40] (see below). These results support the reliability of the expression values obtained by RNA sequencing.…”

“…The differences in the transcriptional responses to alkaline pH and sodium stress were also supported by the analysis of those genes coding for proteins involved in secondary metabolite (SM) production (Figure 5A). Panel D shows the deregulation of mainly different subsets of SM genes, such as the upregulation of penicillin and sterigmatocystin cluster genes at pH 8 [39, 44], or the downregulation of pkdA , aspyridone and pkh clusters also at pH 8, with only the dba cluster being downregulated also under sodium stress [45–48]. Overall, results strongly suggest that the transcriptional responses to sodium and alkaline pH stress differ significantly.…”

Defining the transcriptional responses of Aspergillus nidulans to cation/alkaline pH stress and the role of the transcription factor SltA

“…In the wild-type strain, there were twice as many downregulated genes at pH 8 compared to upregulated genes (861 vs 387 genes), suggesting that adaptation to medium alkalinization after a shift from an acid culture involves the induction but mainly the repression of specific sets of genes. Analyses of the lists of significantly up-and downregulated genes (Table S3; see also a specific bioinformatic analysis of genes with the Top100 log 2 FC values in Tables S4 and S5) confirmed the previously described upregulation of 'alkaline-expressed genes' such as pacC/An2855, aflR/An7820 and enaA/An6642 [2,5,41,42], and the downregulation of 'acid-expressed genes' such as palF/An1844 (see below) [43]. These results support the reliability of the expression values obtained by RNA sequencing.…”

Conserved Regulatory Mechanisms Controlling Aflatoxin and Sterigmatocystin Biosynthesis

“…PacC-dependent penicillin production and expression of ipnA, gene encoding the isopenicillin N synthetase, increased under alkaline conditions when compared to acidic conditions. In contrast with the reports mentioned above, a recent study by Delgado-Virgen and Guzman-de-Pena (2009) showed increased ST production in alkaline pH with respect to the levels produced at acidic pH. According to this report, aflR transcript levels are elevated in the "alkalinity mimicking" pacCc14 mutant.…”