Cyanophycinase is required for heterotrophy in cyanobacteria

Cyanophycin is a biopolymer of arginine (Arg) and aspartate, and it is found in various prokaryotes. Two key enzymes of cyanophycin metabolism are cyanophycin synthase (CphA), producing cyanophycin, and cyanophycinase (CphB), catalyzing the first step of cyanophycin degradation. CphB is a well-conserved enzyme found in the majority of cyanobacteria and ubiquitous amongst those that are known to perform heterotrophy besides their primary photosynthetic lifestyle. Unlike in diazotrophs, where CphB is connected to the mobilization of fixed nitrogen, the importance of this enzyme remains elusive in nondiazotrophs, such as the model cyanobacterium Synechocystis sp. PCC 6803. The Synechocystis ΔcphB deletion strain does not accumulate cyanophycin and shows no photoautotrophic growth defect. However, we show here that ΔcphB is not able to proliferate heterotrophically, although the CphA-less strain exhibits no obvious defect under heterotrophic conditions. Metabolomics profiling revealed that ΔcphB failed to upregulate the biosynthesis of Arg and displayed misregulated carbon and nucleoside metabolisms. These suggest that CphB is needed for the activation of the Arg pathway, which appeared to be crucial for balancing the nitrogen and carbon ratio during the acclimation to heterotrophy. On the other hand, the interaction of CphB with the Arg biosynthetic enzyme, acetylornithine aminotransferase, stimulated the hydrolysis of cyanophycin in an in vitro assay. These data, together with the metabolic profiles of ΔcphB, imply that the catabolism of cyanophycin and the biosynthesis of Arg are mutually coregulated metabolic pathways.