Pushing product formation to its limit: Metabolic engineering of Corynebacterium glutamicum for l-leucine overproduction

Using metabolic engineering, an efficient l-leucine production strain of Corynebacterium glutamicum was developed. In the wild type of C. glutamicum, the leuA-encoded 2-isopropylmalate synthase (IPMS) is inhibited by low l-leucine concentrations with a Ki of 0.4 mM. We identified a feedback-resistant IMPS variant, which carries two amino acid exchanges (R529H, G532D). The corresponding leuAfbr gene devoid of the attenuator region and under control of a strong promoter was integrated in one, two or three copies into the genome and combined with additional genomic modifications aimed at increasing l-leucine production. These modifications involved (i) deletion of the gene encoding the repressor LtbR to increase expression of leuBCD, (ii) deletion of the gene encoding the transcriptional regulator IolR to increase glucose uptake, (iii) reduction of citrate synthase activity to increase precursor supply, and (iv) introduction of a gene encoding a feedback-resistant acetohydroxyacid synthase. The production performance of the resulting strains was characterized in bioreactor cultivations. Under fed-batch conditions, the best producer strain accumulated l-leucine to levels exceeding the solubility limit of about 24 g/l. The molar product yield was 0.30 mol l-leucine per mol glucose and the volumetric productivity was 4.3 mmol l−1 h−1. These values were obtained in a defined minimal medium with a prototrophic and plasmid-free strain, making this process highly interesting for industrial application.