Identification of a Transcription Factor Controlling pH-Dependent Organic Acid Response in Aspergillus niger

Abstract

Acid formation in Aspergillus niger is known to be subjected to tight regulation, and the acid production profiles are fine-tuned to respond to the ambient pH. Based on transcriptome data, putative trans-acting pH responding transcription factors were listed and through knock out studies, mutants exhibiting an oxalate overproducing phenotype were identified. The yield of oxalate was increased up to 158% compared to the wild type and the corresponding transcription factor was therefore entitled Oxalic Acid repression Factor, OafA. Detailed physiological characterization of one of the ΔoafA mutants, compared to the wild type, showed that both strains produced substantial amounts of gluconic acid, but the mutant strain was more efficient in re-uptake of gluconic acid and converting it to oxalic acid, particularly at high pH (pH 5.0). Transcriptional profiles showed that 241 genes were differentially expressed due to the deletion of oafA and this supported the argument of OafA being a trans-acting transcription factor. Furthermore, expression of two phosphoketolases was down-regulated in the ΔoafA mutant, one of which has not previously been described in fungi. It was argued that the observed oxalate overproducing phenotype was a consequence of the efficient re-uptake of gluconic acid and thereby a higher flux through glycolysis. This results in a lower flux through the pentose phosphate pathway, demonstrated by the down-regulation of the phosphoketolases. Finally, the physiological data, in terms of the specific oxygen consumption, indicated a connection between the oxidative phosphorylation and oxalate production and this was further substantiated through transcription analysis.

Effect of Sucrose Concentration on Oxalic Acid Biosynthesis by Aspergillus Niger

“…In all the experiments with gluconic acid present, after reaching its maximum, the concentration of the acid decreased because it was probably reused as a carbon source for oxalic acid production. The same process course and conclusion were noted by other authors [Strasser et al 1994, Cameselle et al 1998, Musiał et al 2005, Poulsen et al 2012, Walaszczyk et al 2014]. In most cases described in the literature, the nr 588, 2017 final gluconic acid concentration was higher than oxalic [Strasser et al 1994, Cameselle et al 1998, Musiał et al 2005.…”

“…Only in the cultures with higher initial substrate amount, 175 and 200 g·dm -3 , citric acid concentration grew constantly to the end of the process. Production of gluconic acid by A. niger is a process of oxidation of available glucose into unavailable to other organisms compound that could be used later as a carbon source for oxalic acid synthesis in the case of lack of easier assailable sources [Andersen et al 2009, Poulsen et al 2012. Formation of gluconic acid is indicated as the main disadvantage of usage of sucrose and glucose as carbon sources in bioprocesses with A. niger [Strasser et al 1994, Cameselle et al 1998, Walaszczyk et al 2014.…”

“…This last process is the most efficient at pH 5.0-5.5, in which many bacteria have their optimum of growth. The formed gluconic acid is later used by A. niger as a carbon source [Andersen et al 2009, Poulsen et al 2012.…”

Genetic and Genomic Manipulations in Aspergillus niger

“…In addition to providing insights into gene expression and gene function, examination of transcriptomes and proteomes can be used to improve structural and functional annotation of genes (Andersen et al 2011 ;Tsang et al 2009 ;Wright et al 2009 ), response to changes in ambient pH (Andersen et al 2009 ), the role of the transcription factor HacA CA in the response to secretion stress (Carvalho et al 2012 ), and conidial germination (Novodvorska et al 2013 ;van Leeuwen et al 2013 (Gruben et al 2012 ), and the oxalic acid repression factor OafA (Poulsen et al 2012 ).…”

Integrated Approaches for Assessment of Cellular Performance in Industrially Relevant Filamentous Fungi

“…Highly reproducible, replicate bioreactor experiments that provide quality biomass can now be performed with filamentous fungi, thus tackling one of the major challenges in the field. [9][10][11][12][13] It is therefore not a coincidence that the terms Aspergillus and systems biology are now appearing together with greater frequency in the literature, in 83 articles published since 2007, where the first genomes were published. Although the gap between yeast and filamentous fungi remains, it is clear that systems biology approaches in filamentous fungi are increasing exponentially.…”