Regulation of intracellular free calcium concentration during heterocyst differentiation by HetR and NtcA in Anabaena sp PCC 7120

Calcium ions are important to some prokaryotic cellular processes, such as heterocyst differentiation of cyanobacteria. Intracellular free Ca2+ concentration, [Ca2+](i), increases several fold in hetero-cysts and is regulated by CcbP, a Ca2+-binding protein found in heterocyst-forming cyanobacteria. We demonstrate here that CcbP is degraded by HetR, a serine-type protease that controls heterocyst differentiation. The degradation depends on Ca2+ and appears to be specific because HetR did not digest other tested proteins. CcbP was found to bind two Ca2+ per molecule with K-D values of 200 nM and 12.8 mu M. Degradation of CcbP releases bound Ca2+ that contributes significantly to the increase of [Ca2+](i) during the process of heterocyst differentiation in Anabaena sp. strain PCC 7120. We suggest that degradation of CcbP is a mechanism of positive autoregulation of HetR. The down-regulation of ccbP in differentiating cells and mature heterocysts, which also is critical to the regulation of [Ca2+](i), depends on NtcA. Coexpression of ntcA and a ccbP promoter-controlled gfp in Escherichia coli diminished production of GFP, and the decrease is enhanced by a-ketoglutarate. It was also found that NtcA could bind a fragment of the ccbP promoter containing an NtcA-binding sequence in a a-ketoglutarate-dependent fashion. Therefore, [Ca2+], is regulated by a collaboration of HetR and NtcA in heterocyst differentiation in Anabaena sp. strain PCC 7120.