Carbon starvation and carbon limitation stress responses in Aspergillus nidulans
Genome-wild transcriptional changes caused by carbon stress in Aspergillus nidulans cultures were analyzed. Carbon starvation and carbon limitation stresses were induced by transferring exponentially phase hyphae, pre-grown on glucose, into carbon source free and lactose containing media, respectively. Up-regulation of genes involved in autolytic cell wall degradation was characteristic for carbon starved cultures and genes encoding amino acid catabolic enzymes were more active in starving than in lactose-fed cultures. In contrast, up-regulations of galX and galR encoding regulators of D-galactose degradation as well as lacpA and lacpB lactose permeases were characteristic for cultures growing on lactose. Genes of lactose and hemicellulose utilizing enzymes (including members of the D-galactose oxidoreductive pathway) as well as genes involved in ribosome biogenesis and amino acid biosynthesis were more active on lactose than under carbon starvation conditions. Despite of these differences, the two stresses caused surprisingly similar changes in the transcriptome because more than 80 % of carbon stress genes responded to the two treatments in a similar manner. As an example, 80 out of the 90 carbon stress induced extracellular enzyme genes showed up-regulation in both stresses. Not surprisingly, the secretomes of 2 d cultures were also very similar.
Our results support the view that lactose as a ß-galactoside is utilized by A. nidulans via pathways involved in hemicellulose degradation. Although A. nidulans can grow well on this disaccharide, transferring mycelia from glucose to lactose means a real stress which induces a very similar stress response to that detected under carbon starvation.