Native putA Overexpression in Synechocystis sp. PCC 6803 Significantly Enhances Polyhydroxybutyrate Production, Further Augmented by the adc1 Knockout Under Prolonged Nitrogen Deprivation.

This study highlights a new avenue to improve polyhydroxybutyrate (PHB) productivity by optimizing genes related to arginine catabolism, which influences nitrogen metabolism in cyanobacteria based on the carbon/nitrogen metabolism balance. In the Synechocystis sp. PCC 6803 wild type (WT) and its adc1 mutant (Δadc1), the native putA gene, responsible for the oxidation of proline to glutamate, was overexpressed to create the OXPutA and OXPutA/Δadc1 strains, respectively. PHB accumulation was considerably higher in OXPutA and OXPutA/Δadc1 under the nitrogen-deprived condition than in strains that overexpressed the proC gene, involved in proline synthesis. The increased transcript level of glgX, associated with glycogen degradation, confirmed that glycogen served as the primary carbon source for PHB synthesis under nitrogen stress without any carbon source addition. Furthermore, proline and glutamate level changes helped cells deal with nitrogen stress and considerably improve intracellular carbon/nitrogen metabolism. As indicated by elevated levels of proA and argD transcripts as well as chlorophyll a accumulation, this impact was most noticeable in strains that overexpressed putA, which was crucial for the synthesis of glutamate, a precursor for important metabolic pathways that respond to nitrogen stress. Therefore, our metabolic model presents PHB-producing strains as promising candidates for biomaterial biotechnology applications in medical and agricultural fields.