Endoplasmic reticulum stress leads to the selective transcriptional down-regulation of the glucoamylase gene in Aspergillus niger

We describe a new endoplasmic reticulum-associated stress response in the filamentous fungus Aspergillus niger. The inhibition of protein folding within the endoplasmic reticulum (ER) leads to cellular responses known collectively as the unfolded protein response (UPR) and we show that the selective transcriptional down-regulation of the gene encoding glucoamylase, a secreted protein, but not non-secreted proteins, is an additional consequence of ER stress. The inhibition of protein folding in the ER can be induced in a variety of ways. We have examined the effects of dithiothreitol (DTT), a reducing agent that causes the formation of unfolded proteins and have used antisense technology to lower the level of protein disulfide isomerase (PDI) in the ER of A. niger. We show that both approaches cause the down-regulation of transcription in genes encoding secreted glucoamylase and also aspergillopepsin but not genes encoding the non-secreted proteins -actin and glyceraldehyde 3' phosphate dehydrogenase. The DTT-treated fungal cells also show evidence for the induction of the UPR because expression of bipA and pdiA, encoding an ER-resident chaperone and foldase respectively, are up-regulated and splicing of hacA, the gene encoding the transcription factor responsible for induction of the UPR, occurs, allowing the production of an active protein. This response is not evident in the pdiA antisense strains, suggesting that the transcriptional down-regulation mechanism is controlled differently to the UPR. An analysis of the promoter of the glucoamylase gene using truncated glaA promoters to drive the -glucuronidase reporter gene has shown that the down regulation effect is attenuated with a promoter length of 1 kb but that the glaA promoter of 2kb exhibited the effect, suggesting that the motif(s) which mediate the response are situated within the region 1-2kb from the ATG.