The transcriptional regulator RbcR controls ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCO) genes in the cyanobacterium Synechocystis sp. PCC 6803

Summary Oxygenic photosynthesis evolved in cyanobacteria, primary producers of striking ecological importance. Like plants, cyanobacteria use the Calvin–Benson–Bassham cycle for CO 2 fixation, fuelled by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCO). In a competitive reaction this enzyme also fixes O 2 which makes it rather ineffective. To mitigate this problem, cyanobacteria evolved a CO 2 concentrating mechanism (CCM) to pool CO 2 in the vicinity of RuBisCO. However, the regulation of these carbon (C) assimilatory systems is understood only partially. Using the model Synechocystis sp. PCC 6803 we characterized an essential LysR‐type transcriptional regulator encoded by gene sll0998 . Transcript profiling of a knockdown mutant revealed diminished expression of several genes involved in C acquisition, including rbcLXS, sbtA and ccmKL encoding RuBisCO and parts of the CCM, respectively. We demonstrate that the Sll0998 protein binds the rbcL promoter and acts as a RuBisCO regulator (RbcR). We propose ATTA(G/A)‐N 5 ‐(C/T)TAAT as the binding motif consensus. Our data validate RbcR as a regulator of inorganic C assimilation and define the regulon controlled by it. Biological CO 2 fixation can sustain efforts to reduce its atmospheric concentrations and is fundamental for the light‐driven production of chemicals directly from CO 2 . Information about the involved regulatory and physiological processes is crucial to engineer cyanobacterial cell factories.