Organic acid production in Aspergillus niger and other filamentous fungi

Abstract

Propositions1. Medium acidification is primarily a cause, rather than an effect, of Aspergillus niger citrate secretion.(this thesis) 2. A holistic systems biology approach should consider the organism in the context of its environment.(this thesis) 3. Metabolism has more than two spacial dimensions and should be studied as a system rather than a map.4. Cancer cells behave as a microorganism in a foreign environment.5. Referrals to authors rather than researchers highlights the skewed perception of scientific worth.6. Reviews can both progress and regress a field.7. Recent history suggests that "common sense" is an oxymoron.8. True understanding, and thus friendship, arises from a common mindset rather than a common nationality.Propositions belonging to the thesis, entitled

Global Transcriptional Response of Aspergillus niger to Blocked Active Citrate Export through Deletion of the Exporter Gene

“…Based on RNAseq data, comparing the growth and the citrate accumulation phase of the fermentation, Yin et al [ 42 ], reported a list of transporter genes up-regulated during the citrate production phase. This shortlist was proposed to contain candidate citrate transporters, but did not include the citT gene first discovered by Odoni et al [ 19 , 20 ].…”

“…The CitT exporter protein belongs to Major Facilitator Superfamily (MFS) [ 19 , 20 ]. From the 336 putative MFS encoding genes contained in the genome of A. niger N402, 78 and 87 of them were significantly up- and down-regulated in citrate condition compared to glucose (as reference), respectively ( Supplementary Data S1 in sheet N402_MFS online).…”

“…Recently, a gene facilitating citrate secretion ( citT , alternative name cexA ) of A. niger was discovered and partially characterized [ 19 , 20 , 21 ]. The RNAseq analysis we did in this study indicated that citT expression levels can vary at least a hundred-fold ( Table 1 ).…”

“…Using a threshold log2 (fold change) (log2(FC)) value of ≥0.50 or ≤ −0.50 and padj value below 0.05, 2257 of the 11187 genes were significantly up-regulated while 2217 genes were down-regulated (Figure 2 and Supplementary Data S1 in sheet N402_citrate_vs_glucose online). The CitT exporter protein belongs to Major Facilitator Superfamily (MFS) [19,20]. From the 336 putative MFS encoding genes contained in the genome of A. niger N402, 78 and 87 of them were significantly up-and down-regulated in citrate condition compared to glucose (as reference), respectively (Supplementary Data S1 in sheet N402_MFS online).…”

“…Recently, there have been reports on the discovery of a citrate transporter gene of A. niger ( citT, alternative name cexA ), responsible for active export of citric acid over the plasma membrane [ 19 , 20 , 21 ]. The heterologous expression of the citT gene resulted in citric acid secretion in yeast [ 20 , 21 ], whereas its overexpression in A. niger itself strongly boosted the secretion of citric acid, yielding for 5-times higher than the parental strain [ 21 ].…”