Methanol-based γ-aminobutyric acid (GABA) production by genetically engineered Bacillus methanolicus strains

Abstract

The use of methanol as a carbon source for biotechnological processes has recently attracted great interest due to relatively low price, high abundance, high purity of methanol, and the fact that it is a non-food raw material. In this study, methanol-based production of γ-aminobutyric acid (GABA), which is a component of drugs and functional foods and is used as monomer for production of the biodegradable plastic polyamide 4, was established using recombinant Bacillus methanolicus strains. This was achieved by heterologous overexpression of glutamate decarboxylase genes from Sulfobacillus thermosulfidooxidans (gadSt) or Escherichia coli (gadB) in methylotrophic B. methanolicus MGA3. Strains expressing either gadSt or gadB accumulated between 0.03 and 0.4 g/L of GABA in shake flask experiments. Initially, controlled methanol fed-batch fermentations yielded low GABA concentration (0.1 g/L). However, employing a two-phase production strategy with an initial high-cell-density fermentation phase for growth and l-glutamate accumulation followed by pH reduction from 6.5 to 4.6 after 27 h for enzymatic conversion of glutamate to GABA led to 90-fold increased GABA accumulation to a final titer of 9 g/L. To the best of our knowledge, this study represents the first demonstration of methanol-based production of GABA.