Glutamate decarboxylase activity and gamma-aminobutyric acid content in Synechocystis sp PCC 6803 under osmotic stress and different carbon sources

Glutamate decarboxylase (GAD), a pyridoxal 5'-phosphate-dependent enzyme, catalyzes the conversion of glutamate to gamma-aminobutyric acid (GABA) through irreversible decarboxylation reaction. In the present study, GAD activity and GABA content in Synechocystis sp. PCC 6803 were investigated in response to osmotic stress and various carbon sources. The highest GAD activity was detected in late log phase cells subjected to 24 h of osmotic stress by 50 mM NaCl and 100 mM sorbitol with a 2- and 5.5-fold increase in the enzyme activity, respectively. GABA content was found to be enhanced by osmotic stress induced by both sorbitol and NaCl. GAD activity was increased by fourfold when late log phase cells were grown mixotrophically for 24 h in medium containing 0.1 % glucose as carbon source, whereas a nearly fivefold increase in GABA content was observed under the same condition. The increase in GAD activity by osmotic stress induced by NaCl and sorbitol up to 50 and 100 mM, respectively, and glucose was also corroborated by the increase in the levels of gad transcripts. Our results suggest that GAD activity, GABA content, and transcription of the gene-encoding GAD in Synechocystis cells are affected by alternative carbon sources as well as osmotic stress.