A novel Ca2+-binding protein influences photosynthetic electron transport in Anabaena sp. PCC 7120

Ca2+ is a potent signalling molecule that regulates many cellular processes. In cyanobacteria, Ca2+ has been linked to cell growth, stress response and photosynthesis, and to the development of specialist heterocystcells in certain nitrogen-fixing species. Despite this, the pathways of calcium signaltransduction in cyanobacteriaare poorly understood, and only a few protein components are known. The current study describes a previously unreported calcium-binding proteinwhich was called the Calcium Sensor EF-hand(CSE), which is conserved in filamentous, nitrogen-fixing cyanobacteria. CSE is shown to bind calcium, which induces a conformational change in the protein structure. Poor growth of a strain of Anabaenasp. PCC 7120 overexpressing CSE was attributed to diminished photosynthetic performance. Transcriptomics, biophysics and proteomics analyses revealed modifications in the light-harvesting phycobilisomeand photosynthetic reaction center-protein complexes, and downregulated respiration.