Improved fermentative gamma-aminobutyric acid production from glucose by the inactivation of respiratory chain components NDH-I and Cytbo₃ in Escherichia coli

Abstract

Gamma-aminobutyric acid (GABA), which is synthesized from l-glutamic acid via glutamate decarboxylase (Gad), is used as food, supplements, and biodegradable plastics. Our previous study demonstrated an Escherichia coli mutant (ΔΔ) strain, lacking type I NADH dehydrogenase (NDH-I) and cytochrome bo₃ oxidase (Cytbo₃), produced 7 g/L glutamic acid on MS1 glucose-minimal medium. In this study, the ΔΔ strain was used for improving GABA production. A plasmid (pMBL19-gadB′) expressing a mutated E. coli GadB (Glu89Gln/Δ452-466), retaining activity at neutral pH, was introduced into the ΔΔ strain and its parent strain (W1485). The ΔΔ strain carrying pMBL19-gadB′ exhibited a twofold increase in GABA production compared to the W1485 strain carrying pMBL19-gadB′. Deleting the C-terminal (Δ471–511) of GadC antiporter in the ΔΔ strain further improved GABA yield to 1.5 g/L when cultured in MS1 glucose-minimal medium. On the other hand, a large amount of glutamic acid produced by the ΔΔ strain was not fully converted to GABA, likely due to the inhibition of GadB activity by the accumulation of acetic acid. Although there is room for improvement, these results indicate the efficacy of the ΔNDH-IΔCytbo₃ double mutation in augmenting GABA production.