Photosynthetic production of glutamine through metabolic analysis-based engineering of Picosynechococcus sp. PCC 7002

Glutamine is an important nitrogen donor in the biosynthesis of nucleotides and several other amino acids. Proliferating cells consume high amounts of glutamine, and cell culture media contain glutamine as the most abundant amino acid. Glutamine is industrially manufactured through bacterial fermentation, which requires external supplementation with sugars as the carbon source. Using the cyanobacterium Picosynechococcus sp. PCC 7002, this study aimed to develop a method for the photosynthetic production of glutamine using CO₂ as the sole carbon source. The introduction of glutamate dehydrogenase from Corynebacterium glutamicum and glutamine synthase from Saccharomyces cerevisiae increased the concentration of extracellularly released glutamine. Metabolome analysis revealed decreased intracellular citrate levels in glutamine-producing cells. To enhance citrate replenishment, metabolic engineering approaches, including l-lactate assimilation and glycogen deficiency, were examined. The introduction of pyruvate carboxylase and citrate synthase from C. glutamicum significantly increased glutamine production. After optimizing light intensity and CO₂ concentration, the recombinant strain produced 1168.5 μM (170.76 mg L^-1) glutamine. This study establishes metabolic engineering approaches for converting CO₂ into glutamine and demonstrates that cyanobacteria are promising photosynthetic producers of glutamine.