The transcriptomic fingerprint of glucoamylase over-expression in <it>Aspergillus niger</it>

• Main Authors: Kwon Min Jin, Jørgensen Thomas R, Nitsche Benjamin M, Arentshorst Mark, Park Joohae, Ram Arthur FJ, Meyer Vera
• Format: Article
• Language: English
• Published: BMC 2012-12-01
• Series: BMC Genomics
• Subjects: Aspergillus niger; Protein expression; Secretion; HacA; Unfolded protein response; Endoplasmic reticulum; Glucoamylase; Transcriptome
• Online Access: http://www.biomedcentral.com/1471-2164/13/701

<p>Abstract</p> <p>Background</p> <p>Filamentous fungi such as <it>Aspergillus niger</it> are well known for their exceptionally high capacity for secretion of proteins, organic acids, and secondary metabolites and they are therefore used in biotechnology as versatile microbial production platforms. However, system-wide insights into their metabolic and secretory capacities are sparse and rational strain improvement approaches are therefore limited. In order to gain a genome-wide view on the transcriptional regulation of the protein secretory pathway of <it>A. niger</it>, we investigated the transcriptome of <it>A. niger</it> when it was forced to overexpression the <it>glaA</it> gene (encoding glucoamylase, GlaA) and secrete GlaA to high level.</p> <p>Results</p> <p>An <it>A. niger</it> wild-type strain and a GlaA over-expressing strain, containing multiple copies of the <it>glaA</it> gene, were cultivated under maltose-limited chemostat conditions (specific growth rate 0.1 h^-1). Elevated <it>glaA</it> mRNA and extracellular GlaA levels in the over-expressing strain were accompanied by elevated transcript levels from 772 genes and lowered transcript levels from 815 genes when compared to the wild-type strain. Using GO term enrichment analysis, four higher-order categories were identified in the up-regulated gene set: i) endoplasmic reticulum (ER) membrane translocation, ii) protein glycosylation, iii) vesicle transport, and iv) ion homeostasis. Among these, about 130 genes had predicted functions for the passage of proteins through the ER and those genes included target genes of the HacA transcription factor that mediates the unfolded protein response (UPR), e.g. <it>bipA, clxA, prpA, tigA</it> and <it>pdiA</it>. In order to identify those genes that are important for high-level secretion of proteins by <it>A. niger</it>, we compared the transcriptome of the GlaA overexpression strain of <it>A. niger</it> with six other relevant transcriptomes of <it>A. niger</it>. Overall, 40 genes were found to have either elevated (from 36 genes) or lowered (from 4 genes) transcript levels under all conditions that were examined, thus defining the core set of genes important for ensuring high protein traffic through the secretory pathway.</p> <p>Conclusion</p> <p>We have defined the <it>A. niger</it> genes that respond to elevated secretion of GlaA and, furthermore, we have defined a core set of genes that appear to be involved more generally in the intensified traffic of proteins through the secretory pathway of <it>A. niger</it>. The consistent up-regulation of a gene encoding the acetyl-coenzyme A transporter suggests a possible role for transient acetylation to ensure correct folding of secreted proteins.</p>