Abstract
Despite a long and successful history of citrate production in Aspergillus niger, the molecular mechanism of citrate accumulation is only partially understood. In this study, we used comparative genomics and transcriptome analysis of citrate-producing strains—namely, A. niger H915-1 (citrate titer: 157 g L−1), A1 (117 g L−1), and L2 (76 g L−1)—to gain a genome-wide view of the mechanism of citrate accumulation. Compared with A. niger A1 and L2, A. niger H915-1 contained 92 mutated genes, including a succinate-semialdehyde dehydrogenase in the γ-aminobutyric acid shunt pathway and an aconitase family protein involved in citrate synthesis. Furthermore, transcriptome analysis of A. niger H915-1 revealed that the transcription levels of 479 genes changed between the cell growth stage (6 h) and the citrate synthesis stage (12 h, 24 h, 36 h, and 48 h). In the glycolysis pathway, triosephosphate isomerase was up-regulated, whereas pyruvate kinase was down-regulated. Two cytosol ATP-citrate lyases, which take part in the cycle of citrate synthesis, were up-regulated, and may coordinate with the alternative oxidases in the alternative respiratory pathway for energy balance. Finally, deletion of the oxaloacetate acetylhydrolase gene in H915-1 eliminated oxalate formation but neither influence on pH decrease nor difference in citrate production were observed.
Highlights
Despite a long and successful history of citrate production in Aspergillus niger, the molecular mechanism of citrate accumulation is only partially understood
We found that during citrate fermentation, the mycelia of different strains aggregated in various forms (Fig. 1b), which indicated that citrate production was influenced by the morphology of the microcolonies[6]
When aeration was maintained at 3.5 vvm, the citrate production of A. niger H915-1 reached 145 g L−1 in 72 h (Fig. 2)
Summary
Despite a long and successful history of citrate production in Aspergillus niger, the molecular mechanism of citrate accumulation is only partially understood. We used comparative genomics and transcriptome analysis of citrate-producing strains—namely, A. niger H915-1 (citrate titer: 157 g L−1), A1 (117 g L−1), and L2 (76 g L−1)—to gain a genome-wide view of the mechanism of citrate accumulation. Deletion of the oxaloacetate acetylhydrolase gene in H915-1 eliminated oxalate formation but neither influence on pH decrease nor difference in citrate production were observed. The molecular mechanism of citrate accumulation remains only partially understood[5,6]. Transcriptome analysis during citrate fermentation by A. niger H915-1 was conducted to explore how the regulation of enzyme expression facilitates citrate accumulation. The results of this study deepen the understanding of citrate accumulation by A. niger and provide potential engineering targets for further improvement in citrate production
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