CcbP, a calcium-binding protein from Anabaena sp. PCC 7120, provides evidence that calcium ions regulate heterocyst differentiation

Although it is known that calcium is a very important messenger involved in many eukaryotic cellular processes, much less is known about calcium's role in bacteria. CcbP, a Ca²⁺-binding protein, was isolated from the heterocystous cyanobacterium Anabaena sp. PCC 7120, and the ccbP gene was cloned and inactivated. In the absence of combined nitrogen, inactivation of ccbP resulted in multiple contiguous heterocysts, whereas overexpression of ccbP suppressed heterocyst formation. Calmodulin, which is not present in Anabaena species, could also suppress heterocyst formation in both Anabaena sp. PCC 7120 and Anabaena variabilis. HetR induction upon nitrogen step-down was slow in the strain overexpressing ccbP. The Ca²⁺ reporter protein obelin was used to show that mature heterocysts had a high intracellular free Ca²⁺ concentration {[Ca²⁺]_i), and immunoblotting showed that CcbP was absent from heterocysts. A regular pattern of cells with higher [Ca²⁺]_i was established during heterocyst differentiation before the appearance of proheterocysts. A rapid increase of [Ca ²⁺]_i could be detected 4 h after the removal of combined nitrogen, and this increase was suppressed by excessive CcbP. These results suggest that Ca²⁺ ions play very important roles in hetR induction and heterocyst differentiation.