Citric Acid Production by A. niger

The strains used in this study are listed in Table S1. Escherichia coli DH5α (Transgene, Beijing, China) was used for plasmid construction and cultured at 37 °C in Luria–Bertani broth containing ampicillin (100 μg/mL). The citric acid-producing strain A. niger D353.8 (kusA::hph, pyrG::hph, hyg^R) was stored in the lab [30 (link)]. A. niger strains were cultivated on defined minimal medium (MM), as reported previously [31 (link)], or on complete medium (CM) consisting of MM supplemented with 0.5% yeast extract and 0.1% casamino acids. Then, 1.5% agar was supplemented for plates. When necessary, 10 mM uracil was supplemented in the media for the pyrG mutants.