A thylakoid-located carbonic anhydrase regulates CO ₂ uptake in the cyanobacterium Synechocystis sp. PCC 6803

Carbonic anhydrases (CAs) are involved in CO ₂ uptake and conversion, a fundamental process in photosynthetic organisms. Nevertheless, the mechanism underlying the regulation of CO ₂ uptake and intracellular conversion in cyanobacteria is largely unknown. We report the characterization of a previously unrecognized thylakoid-located CA Slr0051 (EcaB) from the cyanobacterium Synechocystis sp. PCC 6803, which possesses CA activity to regulate CO ₂ uptake. Inactivation of ecaB stimulated CO ₂ hydration in the thylakoids, suppressed by the classical CA inhibitor acetazolamide. Absence of ecaB increased the reduced state of the photosynthetic electron transport system, lowered the rate of photosynthetic O ₂ evolution at high light (HL) and pH, and decreased the cellular affinity for extracellular inorganic carbon. Furthermore, EcaB was upregulated in cells grown at limiting CO ₂ concentration or HL in tandem with CupA. EcaB is mainly located in the thylakoid membranes where it interacts with CupA and CupB involved in CO ₂ uptake by converting it to bicarbonate. We propose that modulation of the EcaB level and activity in response to CO ₂ changes, illumination or pH reversibly regulates its conversion to HCO ₃ by the two CO ₂ -uptake systems (CupA, CupB), dissipating the excess HCO ₃ ⁻ and alleviating photoinhibition, and thereby optimizes photosynthesis, especially under HL and alkaline conditions.