Cyanobacteria are one of the deepest branching bacterial phyla on earth. Today, cyanobacteria occupy almost all illuminated habitats, where they utilize various nitrogen sources. Nitrogen assimilation strictly depends on carbon and nitrogen availability and requires a fine-tuned regulatory network involving the PII signal transduction protein. In the present study I focused on the regulation of the carbon/nitrogen storage polymer cyanophycin by the PII signaling protein. Cyanophycin is a

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... osomal synthesized polyamide consisting of arginine and aspartate. Cyanophycin accumulation depends on the arginine availability. PII controls the rate limiting step of arginine biosynthesis by regulating the key enzyme N-acetylglutamate kinase (NAGK). A PII variant with a single point mutation (I86N) was previously identified as a NAGK super activator in vitro. By introducing PII(I86N) in Synechocystis sp. PCC 6803, we created a strain which strongly overproduces cyanophycin up to 57 % of the cell dry mass. Since cyanophycin is a bio-polymer with high industrial interest, we performed several process optimization studies. During these studies, we observed that Synechocystis sp. PCC 6803 harboring the PII (I86N) variant showed impaired ammonium utilization. By analyzing this behavior, we could clarify that PII regulates ammonium uptake by interacting the with Amt1 ammonium permease. We could further demonstrate that PII mediates the light and ammonium dependent inhibition of nitrate uptake by interacting with the NrtC and NrtD subunits of the nitrate/nitrite transporter NrtABCD. During this study, we could also identify the UrtE subunit of the ABC-type urea transporter UrtABCDE as novel PII target. The interaction of PII with the UrtE subunit regulates the urea uptake in cyanobacteria. The occurrence of cyanophycin in cyanobacteria was known for more than 100 years; however, the biological function remained largely uninvestigated. During localization studies in Synechocystis sp. PCC 6803, we could show that the cyanophycin [...]