Regulation of l-aspartate oxidase contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803

Cyanobacteria have been promoted as a biomass resource that can contribute to carbon neutrality. Synechocystis sp. PCC 6803 is a model cyanobacterium that is widely used in various studies. NADP+ and NAD+ are electron receptors involved in energy metabolism. The NADP+/NAD+ ratio in Synechocystis sp. PCC 6803 is markedly higher than that in the heterotrophic bacterium Escherichia coli. In Synechocystis sp. PCC 6803, NADP+ primarily functions as an electron receptor during the light reaction of photosynthesis, and NADP+ biosynthesis is essential for photoautotrophic growth. Generally, the regulatory enzyme of NADP+ biosynthesis is NAD kinase, which catalyzes the phosphorylation of NAD+. However, a previous study suggested that the regulation of another enzyme contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803 under photoautotrophic conditions. l-Aspartate oxidase is the first enzyme in NAD(P)+ biosynthesis. In this study, we biochemically characterized Synechocystis sp. PCC 6803 l-aspartate oxidase and determined the phenotype of a Synechocystis sp. PCC 6803 mutant overexpressing l-aspartate oxidase. The catalytic efficiency of l-aspartate oxidase from Synechocystis sp. PCC 6803 was lower than that of l-aspartate oxidases and NAD kinases from other organisms. l-Aspartate oxidase activity was affected by different metabolites such as NADP+ and ATP. The l-aspartate oxidase-overexpressing strain grew faster than the wild-type strain under photoautotrophic conditions. The l-aspartate oxidase-overexpressing strain accumulated NADP+ under photoautotrophic conditions. These results indicate that the regulation of l-aspartate oxidase contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803 under photoautotrophic conditions. These findings provide insight into the regulatory mechanism of cyanobacterial NADP+ biosynthesis.