Phenotypes of gene disruptants in relation to a putative mitochondrial malate–citrate shuttle protein in citric acid-producing <i>Aspergillus niger</i>

The mitochondrial citrate transport protein (CTP) functions as a malate–citrate shuttle catalyzing the exchange of citrate plus a proton for malate between mitochondria and cytosol across the inner mitochondrial membrane in higher eukaryotic organisms. In this study, for functional analysis, we cloned the gene encoding putative CTP (<i>ctpA</i>) of citric acid-producing <i>Aspergillus niger</i> WU-2223L. The gene <i>ctpA</i> encodes a polypeptide consisting 296 amino acids conserved active residues required for citrate transport function. Only in early-log phase, the <i>ctpA</i> disruptant DCTPA-1 showed growth delay, and the amount of citric acid produced by strain DCTPA-1 was smaller than that by parental strain WU-2223L. These results indicate that the CTPA affects growth and thereby citric acid metabolism of <i>A. niger</i> changes, especially in early-log phase, but not citric acid-producing period. This is the first report showing that disruption of <i>ctpA</i> causes changes of phenotypes in relation to citric acid production in <i>A. niger</i>. Disruption of the gene encoding the citrate transport protein (CTPA, i.e., malate–citrate shuttle protein) affects the phenotype of <i>Aspergillus niger</i>.