UPTAKE AND EXPORT OF CITRIC-ACID BY ASPERGILLUS-NIGER IS RECIPROCALLYREGULATED BY MANGANESE IONS

The uptake as well as the export of citric acid by Aspergillus niger o ccur by active, Delta pH-driven, H+-symport dependent systems. They ar e inhibited by nonmetabolizable tricarboxylic acid analogues and phtha lic acid, and by several other mono-, di- and tribasic organic acids. However, citrate export could only be demonstrated in a mycelium culti vated under manganese-deficient growth conditions, whereas the uptake of citrate from the medium was only detectable upon precultivation of A. niger in a medium supplemented with Mn2+ ions. In addition, the upt ake of citrate was dependent on the presence of Mn2+ ions in the assay , and inhibited by EDTA. This requirement for Mn2+ could also be parti ally fulfilled by Mg2+, Fe2+ or Zn2+, whereas Cu2+ ions inhibited citr ate transport. The observed divergent effects of manganese ions on cit rate uptake and citrate export may be a major reason for the well docu mented requirement for manganese deficiency of citric acid accumulatio n.