Decrease of citric acid produced by <i>Aspergihllus niger</i> through disruption of the gene encoding a putative mitochondrial citrate-oxoglutarate shuttle protein

The transporter that exports citric acid (CA) generated in mitochondria to the cytosol is an important component of the CA production machinery of <i>Aspergillus niger</i>. In this report, we cloned and identified the gene <i>cocA</i>, encoding a 33.7-kDa putative mitochondrial citrate-oxoglutarate shuttle protein of the CA hyper-producer <i>A. niger</i> WU-2223L. The amount of CA produced by a representative <i>cocA</i> disruptant (35 g/L) was significantly lower than that produced by strain WU-2223L (63 g/L) after culture for 12 days under CA production conditions, and the phenotype of the <i>cocA</i> disruptant differed in part from that of strain WU-2223L. A <i>cocA</i> disruptant complemented with <i>cocA</i> exhibited the same phenotypes as those of strain WU-2223L. This report is the first to show that <i>cocA</i> and its protein product clearly contribute to substantial CA production by <i>A. niger</i>, and provides a significant insight into microbial organic acid production by fermentation. <b>Abbreviations</b>: CA: citric acid; CD medium: Czapek-Dox medium; CS: citrate synthase; CTP: citrate transport protein; HR: homologous recombination; MCF: mitochondrial carrier family; RT-PCR: reverse-transcription PCR; TCA: tricarboxylic acid The gene <i>cocA</i> encoding a putative mitochondrial citrate-oxoglutarate shuttle protein greatly contributes to hyper-production of citric acid by <i>Aspergillus niger</i>.