Fig. 5.

A model of the proposed function of CO₂ uptake systems in Synechocystis sp. strain PCC6803. CupA or CupB converts CO₂ into HCO₃^– under alkaline conditions, while the conversion is reversed under acidic conditions. Under light conditions, photosynthetic electron transfer couples to the formation of a transthylakoid membrane proton gradient, subsequently forming a strong alkaline region inside the U-type structure for CupA or CupB activity, which leads to the accumulation of HCO₃^– after CO₂ has diffused into the cytosol. After HCO₃^– enters the carboxysome, it is converted into CO₂ by CA for carbon assimilation by Rubisco. (This figure is available in color at JXB online.)