Regulatory mutations affecting sulfur metabolism induce environmental stress response in Aspergillus nidulans

• Stress response genes are induced in the Aspergillus nidulans cysB, sconB and sconC mutants.
• The scon strains are resistant to exogenous stress agents but cysB is sensitive.
• Stress response leads to up-regulation of carbohydrate metabolism genes.
• Sugar and polyol levels in conidia of mutants are also changed.
• Numerous genes encoding membrane proteins are down-regulated.

Mutations in the cysB, sconB and sconC genes affect sulfur metabolism in Aspergillus nidulans in different ways. The cysB mutation blocks synthesis of cysteine by the main pathway and leads to a shortage of this amino acid. The sconB and sconC mutations affect subunits of the SCF ubiquitin ligase complex, which inactivates the MetR transcription factor in the presence of an excess of cysteine. In effect, both cysB and scon mutations lead to permanent derepression of MetR-dependent genes. We compared transcriptomes of these three mutants with that of a wild type strain finding altered expression of a few hundred genes belonging to various functional categories. Besides those involved in sulfur metabolism, many up-regulated genes are related to stress responses including heat shock and osmotic stress. However, only the scon strains are more resistant to exogenous stress agents than the wild type strain while cysB is more sensitive. The two-component signal transduction system is a functional category, which is most enriched among genes up-regulated in the cysB, sconB and sconC mutants. A large group of up-regulated genes are involved in carbohydrate and energy metabolism, including genes coding for enzymes of trehalose and glycerol synthesis. The altered expression of these genes is accompanied by changes in sugar and polyol accumulation in conidia of the mutants. Genes encoding enzymes of the glyoxylate bypass and the GABA shunt are also up-regulated along with genes coding for enzymes of alcohol fermentation. Among the down-regulated genes the most numerous are those encoding membrane proteins and enzymes involved in secondary metabolism, including the penicillin biosynthesis cluster.