Cloning stress-induced genes from aspergillus niger using polymerase chain reaction-augmented subtractive hybridization

Genomic analysis of the secretion stress response in the enzyme-producing cell factory Aspergillus niger

Background

Filamentous fungi such as Aspergillus niger have a high capacity secretory system and are therefore widely exploited for the industrial production of native and heterologous proteins. However, in most cases the yields of non-fungal proteins are significantly lower than those obtained for fungal proteins. One well-studied bottleneck appears to be the result of mis-folding of heterologous proteins in the ER during early stages of secretion, with related stress responses in the host, including the unfolded protein response (UPR). This study aims at uncovering transcriptional and translational responses occurring in A. niger exposed to secretion stress.

Results

A genome-wide transcriptional analysis of protein secretion-related stress responses was determined using Affymetrix DNA GeneChips and independent verification for selected genes. Endoplasmic reticulum (ER)-associated stress was induced either by chemical treatment of the wild-type cells with dithiothreitol (DTT) or tunicamycin, or by expressing a human protein, tissue plasminogen activator (t-PA). All of these treatments triggered the UPR, as shown by the expression levels of several well-known UPR target genes. The predicted proteins encoded by most of the up-regulated genes function as part of the secretory system including chaperones, foldases, glycosylation enzymes, vesicle transport proteins, and ER-associated degradation proteins. Several genes were down-regulated under stress conditions and these included several genes that encode secreted enzymes. Moreover, translational regulation under ER stress was investigated by polysomal fractionation. This analysis confirmed the post-transcriptional control of hacA expression and highlighted that differential translation also occurs during ER stress, in particular for some genes encoding secreted proteins or proteins involved in ribosomal biogenesis and assembly.

Conclusion

This is first genome-wide analysis of both transcriptional and translational events following protein secretion stress. Insight has been gained into the molecular basis of protein secretion and secretion-related stress in an effective protein-secreting fungus, and provides an opportunity to identify target genes for manipulation in strain improvement strategies.

UPR-independent dithiothreitol stress-induced genes in Aspergillus niger

A subtraction library was prepared from cultures of Aspergillus niger that had or had not been exposed to dithiothreitol (DTT), in order to identify genes involved in the unfolded protein response (UPR) or in the response to reductive stress. A large fraction of the clones in the library (40%) encoded two putative methyltransferases (MTs) whose function has yet to be determined. Other stress-responsive genes included a homologue of the Mn2+-containing superoxide dismutase gene (sodB) and a number of genes predicted to code for products that function in protein turnover and in intra- and extracellular transport of molecules. Transcriptional microarray analysis was carried out with a group of 15 genes, comprising 11 from the cDNA library, two genes linked to the putative MT genes but not represented in the library, and two UPR control genes (bipA and pdiA). Eleven of the 15 genes were inducible with DTT. This was either reflected by the presence of transcripts in cells subjected to DTT stress compared to absence under control conditions, or by an induction ratio of between 1.4 and 8.0 in cases where transcripts were already detectable under control conditions. The MT genes were among the four most highly induced. None of the genes, apart from bipA and pdiA, showed significant induction in response to other stresses that are known to induce the UPR in fungi. We conclude that DTT alone does not provide for specific induction of UPR genes and that other stress conditions must also be examined.

Endoplasmic reticulum stress leads to the selective transcriptional downregulation of the glucoamylase gene in Aspergillus niger

We describe a new endoplasmic reticulum (ER)-associated stress response in the filamentous fungus Aspergillus niger. The inhibition of protein folding within the ER leads to cellular responses known collectively as the unfolded protein response (UPR) and we show that the selective transcriptional downregulation of the gene encoding glucoamylase, a major secreted protein, but not two non-secreted proteins, is an additional consequence of ER stress. The transcriptional downregulation effect is shown by nuclear run-on studies to be at the level of transcription, rather than mRNA stability, and is found to be mediated through the promoter of glaA in a region more than 1 kb upstream of the translational start. The inhibition of protein folding in the ER can be induced in a variety of ways. We examined the effects of dithiothreitol (DTT), a reducing agent that causes the formation of unfolded proteins. Although a general downregulation of transcription was seen with DTT treatment, we show that selective downregulation was observed with the glaA gene compared with genes encoding the non-secreted proteins gamma-actin and glyceraldehyde 3'-phosphate dehydrogenase. The DTT-treated fungal cells also showed evidence for the induction of the UPR because expression of bipA and pdiA, encoding an ER-resident chaperone and foldase, respectively, are upregulated and splicing of hacA, the gene encoding the transcription factor responsible for induction of the UPR, occurs allowing the production of an active HacA protein. As a preliminary attempt to investigate if the transcriptional downregulation effect was mediated through HacA (i.e. part of the UPR), we examined ER stress induced through antisense technology to lower the level of PDI in the ER of A. niger. Although the transcription of glaA was attenuated in that strain of A. niger, UPR was not evident, suggesting that the transcriptional downregulation mechanism is controlled differently from the UPR.

Filamentous fungi as microbial cell factories for food use

The knowledge base that will underpin the more efficient use of filamentous fungi as cell factories in food has increased during the past year in the areas of gene regulation, protein secretion, safety and synthesis of ingredients such as long-chain polyunsaturated fatty acids.