The role of GAD pathway for regulation of GABA accumulation and C/N balance in Synechocystis sp. PCC6803

Synechocystis sp. PCC6803 was engineered by interrupting the α-ketoglutarate decarboxylase (KGD) and overexpressing the glutamate decarboxylase (GAD) pathways to create ∆Kgd, GADox, and GADox/∆Kgd mutants with the aim to enhance GABA accumulation. Supplementation of 20 mM glutamate or 0.5 mM spermidine increased the GABA contents by several folds in engineered strains. Supplementation of glutamate not only increased the GABA accumulation of cells but also improved the growth rates of engineered strains, signifying that the GAD pathway plays a crucial role in maintaining the intracellular glutamate homeostasis in Synechocystis. The high GABA level was detected in GADox/∆Kgd grown in 0.1% glucose-supplemented medium, ~ 240-folds higher than that in WT grown without glucose. Overall results suggested that the optimization of carbon and nitrogen sources was effective in enhancing the GABA accumulation in Synechocystis overexpressing the GAD and lacking the functional KGD pathway. This work provides an insight into the strategy to improve GABA production in cyanobacteria and possibly in plants as well.