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Background  Filamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca 2+  has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca 2+  is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca 2+ -binding protein called the Ca 2+  Sensor EF-hand (CSE) is found almost exclusively in filamentous cyanobacteria. Expression of  asr1131  encoding the CSE protein in  Anabaena  sp. PCC 7120 was strongly induced by low CO 2  conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca 2+  in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.    Results  In the current study, a mutant  Anabaena  sp. PCC 7120 strain lacking  asr1131  ( Δcse ) was highly prone to filament fragmentation, leading to a striking phenotype of very short filaments and poor growth under nitrogen-depleted conditions. Transcriptomics analysis under nitrogen-replete conditions revealed that genes involved in heterocyst differentiation and function were downregulated in  Δcse , while heterocyst inhibitors were upregulated, compared to the wild-type.    Conclusions  These results indicate that CSE is required for filament integrity and for proper differentiation and function of heterocysts upon changes in the cellular carbon/nitrogen balance. A role for CSE in transmitting Ca 2+  signals during the first response to changes in metabolic homeostasis is discussed.

The small Ca2+-binding protein CSE links Ca2+ signalling with nitrogen metabolism and filament integrity in Anabaena sp. PCC 7120