The ability of many filamentous fungi, such as Aspergillus niger, to secret a high level of homologous proteins has led to their consideration as hosts for the production of heterologous proteins. However, the levels of some secreted heterologous proteins have often been low. Although many strategies have been developed to improve the level of secreted heterologous proteins, further studies into the remaining bottlenecks are required. One common strategy used to improve secreted protein production from filamentous fungi is to express the target protein under the control of a highly-induced native promoter. One major bottleneck in the secretion of heterologous proteins is caused by the folding of proteins within the lumen of the endoplasmic reticulum (ER). Recent studies have shown that expressing some heterologous proteins could subject A. niger to ER-stress. In this study, A. niger was subjected to different environmental conditions and ER stress responses were examined under each of these environmental conditions to further investigate the regulation of the gene encoding glucoamylase (glaA). Treating A. niger with dithiothreitol (OTT), a reducing agent that causes the formation of unfolded proteins, caused the down-regulation of transcription of the glaA but not the gene encoding the non-secreted protein y-actin. The OTT-treated fungal cells also showed evidence of induction of the UPR because expression of bipA was up-regulated and splicing of hacA, the gene encoding the transcription factor responsible for induction of the Unfolded Protein Response (UPR), occurs allowing the production of an active HacA protein. This is the first study to show clearly by nuclear run-on studies that the transcriptional down-regulation effect occurs at the level of transcription, rather than mRNA stability, and is found to be mediated through the promoter of the glaA gene (PglaA) in a region more than 1.192 kb upstream of the translational start. As a preliminary attempt to investigate if the transcriptional downregulation effect was mediated through HacA (i.e. part of the UPR), the ER stress was induced through antisense technology to lower the level of POI 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 down-regulation mechanism is controlled differently from the UPR. Furthermore, activation of the ER-Associated Degradation (ERAO) mechanism in OTT-treated A. niger cultures was demonstrated by detecting transcriptional up-regulation of the putative gene encoding the RpnG, a homologue of the yeast Rpn7p subunit of the 26S proteasome.

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In yeast, aging appears to be marked by a progressive impairment in cellular mechanisms, resulting in irreversible changes in physiology and morphology. To date, very little has been reported about the biochemical changes that occur in yeast as a function of individual cell aging. To investigate this further, six generations of a brewing yeast strain of Saccharomyces cerevisiae (NCYC 1239) were separated according to cellular age using continuous phased culturing and biotin-streptavidin magnetic cell sorting. / To obtain cells with no bud scars (virgin cells), a concentrated yeast slurry was layered onto sucrose density gradients and centrifuged. The uppermost band from the gradients was collected and cells were biotinylated with biotinamidocaproate- N-hydroxysuccinimide ester, that covalently binds to lysine residues on the yeast cell wall. For continuous phased culturing, biotinylated cells were added to a carbon-limited nutrient medium and growth was synchronized using the doubling time of the cells. Harvested cells were incubated with streptavidin superparamagnetic beads and sorted with a strong permanent magnet. In total, approximately 75% of the biotinylated cells were recovered. Viability testing was conducted using vital staining and plate counts, with >98% viability reported with the vital stain and 37% viability with the agar plates. / In conclusion, continuous phased culture, together with magnetic cell sorting has the potential to become a powerful tool for the study of age-related biochemical changes in yeast. Further studies will focus on ensuring the reproducibility of the method and using the recovered cells to study biochemical changes occurring during yeasts' replicative lifespan.

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Bibliography: last 35 leaves / v, 140, ca. 70 leaves, 29 leaves of plates : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1984

Thesis (M.S.)--Cleveland State University, 2008. / Abstracts. Title from PDF t.p. (viewed on Jan. 29, 2009). Includes bibliographical references (p. 33-36). Available online via the OhioLINK ETD Center. Also available in print.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 107-116).

Thesis (Ph. D.)--Ohio State University, 2003. / Document formatted into pages; contains 119 p. Includes bibliographical references. Abstract available online via OhioLINK's ETD Center; full text release delayed at author's request until 2005 Aug. 19.

Zugl.: Habil'schrift Bern.