Regulation of prokaryotic adenylyl cyclases by CO2

The Slr1991 adenylyl cyclase of the model prokaroyte Synechocystis PCC 6803 was stimulated 2-fold at 20 mM total C-i (inorganic carbon) at pH 7.5 through an increase in k(cat). A dose response demonstrated an EC50 of 52.7 mM total C-i at pH 6.5. Slr1991 adenylyl cyclase was activated by CO2, but not by HCO3-. CO2 regulation of adenylyl cyclase was conserved in the CyaB1 adenylyl cyclase of Anabaena PCC 7120. These adenylyl cyclases repres- ent the only identified signalling enzymes directly activated by CO2. The findings prompt an urgent reassessment of the activating carbon species for proposed HCO3--activated adenylyl cyclases.