Enhanced production of gamma-aminobutyrate (GABA) in recombinant Corynebacterium glutamicum by expressing glutamate decarboxylase active in expanded pH range

Background: Gamma-aminobutylate (GABA) is an important chemical in pharmacetucal field and chemical industry. GABA has mostly been produced in lactic acid bacteria by adding L-glutamate to the culture medium since L-glutamate can be converted into GABA by inherent L-glutamate decarboxylase. Recently, GABA has gained much attention for the application as a major building block for the synthesis of 2-pyrrolidone and biodegradable polyamide nylon 4, which opens its application area in the industrial biotechnology. Therefore, Corynebacterium glutamicum, the major L-glutamate producing microorganism, has been engineered to achieve direct fermentative production of GABA from glucose, but their productivity was rather low. Results: Recombinant C. glutamicum strains were developed for enhanced production of GABA from glucose by expressing Escherichia coli glutamate decarboxylase (GAD) mutant, which is active in expanded pH range. Synthetic P_H36, P_I16, and P_L26 promoters, which have different promoter strengths in C. glutamicum, were examined for the expression of E. coli GAD mutant. C. glutamicum expressing E. coli GAD mutant under the strong P_H36 promoter could produce GABA to the concentration of 5.89 ± 0.35 g/L in GP1 medium at pH 7.0, which is 17-fold higher than that obtained by C. glutamicum expressing wild-type E. coli GAD in the same condition (0.34 ± 0.26 g/L). Fed-bath culture of C. glutamicum expressing E. coli GAD mutant in GP1 medium containing 50 μg/L of biotin at pH 6, culture condition of which was optimized in flask cultures, resulted in the highest GABA concentration of 38.6 ± 0.85 g/L with the productivity of 0.536 g/L/h. Conclusion: Recombinant C. glutamicum strains developed in this study should be useful for the direct fermentative production of GABA from glucose, which allows us to achieve enhanced production of GABA suitable for its application area in the industrial biotechnology.