Department of Biochemistry, Molecular Plant Biology, University of Turku, Tykistökatu 6A, 6. krs, 20520, Turku, Finland.
Present address: Department of Plant Sciences, Environmental Plant Physiology, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK.
BMC Microbiol. 2020 Mar 11;20(1):57. doi: 10.1186/s12866-020-01735-5.
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 has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca-binding protein called the Ca 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 conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.
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.
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 signals during the first response to changes in metabolic homeostasis is discussed.
丝状蓝藻是研究多细胞性的模式生物。对于许多物种来说，固氮异形胞是由在氮限制下的光合营养细胞形成的。细胞内 Ca 被认为参与了异形胞分化的高度调控过程，但它的作用仍不清楚。Ca 在蓝藻的代谢信号转导中发挥着更广泛的作用，尽管信号转导机制实际上是未知的。一种叫做 Ca 传感器 EF 手（CSE）的 Ca 结合蛋白几乎只存在于丝状蓝藻中。在鱼腥藻 PCC 7120 中，asr1131 编码 CSE 蛋白的表达强烈地受到低 CO 条件的诱导，并在氮下降过程中迅速下调。先前的研究表明 CSE 和 Ca 在调节光合作用活性方面起着作用，以响应碳和氮可用性的变化。
在本研究中，一个缺乏 asr1131（Δcse）的鱼腥藻 PCC 7120 突变株极易发生丝状分裂，导致非常短的丝状和在氮缺乏条件下生长不良的明显表型。在氮充足条件下的转录组学分析表明，与野生型相比，异形胞分化和功能相关的基因在Δcse 中下调，而异形胞抑制剂上调。
这些结果表明，CSE 是丝状完整性所必需的，并且在细胞碳/氮平衡发生变化时，对异形胞的正确分化和功能也是必需的。讨论了 CSE 在代谢稳态变化的第一次反应中传递 Ca 信号的作用。
