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

Julia Walter,Eva-Mari Aro,Francisco Leganés,Peter J Gollan

doi:10.1186/s12866-020-01735-5

Julia Walter, Eva-Mari Aro + Show 2 more

Open Access

https://doi.org/10.1186/s12866-020-01735-5

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Abstract

BackgroundFilamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca2+ has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca2+ is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca2+-binding protein called the Ca2+ 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 CO2 conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca2+ in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.ResultsIn 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.ConclusionsThese 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 Ca2+ signals during the first response to changes in metabolic homeostasis is discussed.

Highlights

Filamentous cyanobacteria represent model organisms for investigating multicellularity
A prominent role of Ca2+ in Anabaena is in the regulation of heterocyst formation [13], which is thought to occur through the activity of the cyanobacterial Ca2+-binding protein (CcbP), which binds Ca2+ via negative surface charges [14, 15]
In addition to the short filaments, Δcse cultures grown in BG11 in 3% CO2 contained a small population of long filaments (Fig. 2b) that resembled WT filaments, including occurrence of heterocyst cells

Summary

Introduction

Filamentous cyanobacteria represent model organisms for investigating multicellularity. The involvement of Ca2+ has been established in cyanobacterial growth [4], regulation of reactive oxygen species (ROS) [5], fine-tuning of the carbon (C) and nitrogen (N) balance [6, 7], heat stress acclimation [8], exopolysaccharide production [9], and fatty acid and hydrocarbon composition [10] According to their morphology, cyanobacteria are classified into unicellular and multicellular (filamentous) species [11]. HetR acts as a transcription factor, which regulates expression of several genes involved in the commitment of a vegetative cell to differentiation into a proheterocyst and maturation into a functional heterocyst This genetic reprogramming includes inhibition of cell division and formation of the heterocyst envelope, comprising a gasimpermeable glycolipid layer and outer polysaccharide layer [17,18,19]. This developmental process occurs in about every tenth cell of a filament under N-deprived conditions, due to the action of heterocyst pattern formation proteins such as the small peptide PatS, which is expressed mainly in heterocysts [20, 21] and diffuses into adjacent cells where it inhibits the activity of HetR [22]

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