Aspergillus niger is the major industrial citrate producer worldwide. Export as well as
uptake of citric acid are believed to occur by active, proton-dependent, symport systems. Both
are major bottlenecks for industrial citrate production. Therefore, we assessed the consequences
of deleting the citT gene encoding the A. niger citrate exporter, effectively blocking active citrate
export. We followed the consumption of glucose and citrate as carbon sources, monitored the
secretion of organic acids and carried out a thorough transcriptome pathway enrichment analysis.
Under controlled cultivation conditions that normally promote citrate secretion, the knock-out strain
secreted negligible amounts of citrate. Blocking active citrate export in this way led to a reduced
glucose uptake and a reduced expression of high-affinity glucose transporter genes, mstG and mstH.
The glyoxylate shunt was strongly activated and an increased expression of the OAH gene was
observed, resulting in a more than two-fold higher concentration of oxalate in the medium. Deletion
of citT did not affect citrate uptake suggesting that citrate export and citrate uptake are uncoupled
from the system.
