Early Determinants of Heterocyst Differentiation in Multicellular Cyanobacteria

某些多细胞蓝藻发展毛状体，其中细胞通过细胞间粘附连接，并通过门控隔膜连接进行通信。这些蓝藻分化多达四种不同的细胞类型和毛状体响应某些刺激作为一个整体。因此，这些蓝藻符合真正的多细胞生物的所有标志。我们的建议针对异形胞分化过程中的最早阶段。异形胞是专门用于固定大气氮使用固氮酶和光合作用不活跃。异形胞和邻近的光合活性营养细胞相互依赖并交换代谢产物和信号，这使得异形胞特异性的N2固定成为使这些蓝藻在贫氮环境中生存的紧急功能。在属于念珠藻目的蓝藻中，成熟的异形胞在从环境中去除结合氮超过~24 h的时间过程后从营养细胞分化，使得细胞分化的过程可用于详细的研究。我们的假设1是，营养细胞在一个灯丝是不均匀的，但预先存在的空间异质性，使某些细胞更有可能进入异形胞分化的途径比别人。进入分化过程的细胞可以在由转录的随机性质导致的过程中确定（假设2）。以前的方法经常筛选突变体库不能在N2上生长。因此，重要的和冗余的基因或影响模式或承诺时间的因素将被错过。因此，我们希望确定基因和分子因子，仍然完全未知或未识别的异形胞分化的作用，迄今我们（假设3）。形成异型囊的蓝细菌的多细胞性是不同细菌的自然组合中的重要元素，然而，只有在环境中设置一组非常特定的参数时才会实现（假设4）。该项目的重点是了解在遗传上易于处理和良好表征的Nostoc 7120模型中异形胞分化过程的最早步骤，从非异形胞蓝藻中所涉及的基因和机制的演变以及异形胞分化对环境细菌群落的影响。我们将应用单细胞转录组学，识别和表征多细胞决策的早期调节因子，确定异形胞分化的临界点，并从实验室分析扩展到对环境种群的影响。

Certain multicellular cyanobacteria develop trichomes in which cells are connected by cell-cell adhesion and communicate through gated septal junctions. These cyanobacteria differentiate up to four different cell types and the trichomes respond to certain stimuli as a whole. Thus, these cyanobacteria fulfill all hallmarks of true multicellular organisms. Our proposal targets the earliest steps in the differentiation process of heterocysts. Heterocysts are specialized for the fixation of atmospheric N2 using the enzyme nitrogenase and are photosynthetically inactive. Heterocysts and the neighboring photosynthetically active vegetative cells crucially depend on each other and exchange metabolites and signals, which makes the heterocyst-specific N2 fixation the emergent function that allows these cyanobacteria to survive in nitrogen-poor environments. In cyanobacteria belonging to the Nostocales, mature heterocysts differentiate from vegetative cells upon removal of combined nitrogen from the environment over a time course of ~24 h, making the process of cell differentiation accessible for detailed research. Our hypothesis 1 is that the vegetative cells in a filament are not uniform but that pre-existing spatial heterogeneities render certain cells more likely to enter the pathway toward heterocyst differentiation than others. Cells that enter the differentiation process may be determined in a process that results from the stochastic nature of transcription (hypothesis 2). Previous approaches often screened mutant libraries for the inability to grow on N2. Thus, essential and redundant genes or factors affecting patterning or time of commitment would have been missed. Therefore, we expect to identify genes and molecular factors that have remained entirely uncharacterized or unrecognized for their role in heterocyst differentiation thus far our (hypothesis 3). The multicellularity of heterocyst-forming cyanobacteria is an important element in natural assemblages of different bacteria, which will, however, only materialize if a very specific set of parameters is set in the environment (hypothesis 4). The project focuses on understanding the earliest steps in the heterocyst differentiation process in the genetically tractable and well-characterized model Nostoc 7120, on the evolution of the involved genes and mechanisms from non-heterocystous cyanobacteria and on the impact heterocyst differentiation has on environmental bacterial communities. We will apply single cell transcriptomics, identify and characterize early regulators of multicellular decision making, identify tipping points for heterocyst differentiation and extend from analyses in the laboratory to the impact on environmental populations.