Nieves-Morión Mercedes, Mullineaux Conrad W, Flores Enrique
Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Científicas and Universidad de Sevilla, Seville, Spain.
School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom
mBio. 2017 Jan 3;8(1):e01756-16. doi: 10.1128/mBio.01756-16.
Heterocyst-forming cyanobacteria grow as filaments in which intercellular molecular exchange takes place. During the differentiation of N-fixing heterocysts, regulators are transferred between cells. In the diazotrophic filament, vegetative cells that fix CO through oxygenic photosynthesis provide the heterocysts with reduced carbon and heterocysts provide the vegetative cells with fixed nitrogen. Intercellular molecular transfer has been traced with fluorescent markers, including calcein, 5-carboxyfluorescein, and the sucrose analogue esculin, which are observed to move down their concentration gradient. In this work, we used fluorescence recovery after photobleaching (FRAP) assays in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 to measure the temperature dependence of intercellular transfer of fluorescent markers. We find that the transfer rate constants are directly proportional to the absolute temperature. This indicates that the "septal junctions" (formerly known as "microplasmodesmata") linking the cells in the filament allow molecular exchange by simple diffusion, without any activated intermediate state. This constitutes a novel mechanism for molecular transfer across the bacterial cytoplasmic membrane, in addition to previously characterized mechanisms for active transport and facilitated diffusion. Cyanobacterial septal junctions are functionally analogous to the gap junctions of metazoans.
Although bacteria are frequently considered just as unicellular organisms, there are bacteria that behave as true multicellular organisms. The heterocyst-forming cyanobacteria grow as filaments in which cells communicate. Intercellular molecular exchange is thought to be mediated by septal junctions. Here, we show that intercellular transfer of fluorescent markers in the cyanobacterial filament has the physical properties of simple diffusion. Thus, cyanobacterial septal junctions are functionally analogous to metazoan gap junctions, although their molecular components appear unrelated. Like metazoan gap junctions, the septal junctions of cyanobacteria allow the rapid intercellular exchange of small molecules, without stringent selectivity. Our finding expands the repertoire of mechanisms for molecular transfer across the plasma membrane in prokaryotes.
形成异形胞的蓝细菌以丝状体形式生长，细胞间会发生分子交换。在固氮异形胞的分化过程中，调控因子在细胞间转移。在固氮丝状体中，通过有氧光合作用固定二氧化碳的营养细胞为异形胞提供还原态碳，而异形胞为营养细胞提供固定态氮。细胞间分子转移已通过荧光标记物进行追踪，包括钙黄绿素、5-羧基荧光素和蔗糖类似物七叶苷，观察到它们沿浓度梯度移动。在这项研究中，我们在模式形成异形胞的蓝细菌鱼腥藻属PCC 7120菌株中使用光漂白后荧光恢复（FRAP）分析来测量荧光标记物细胞间转移的温度依赖性。我们发现转移速率常数与绝对温度成正比。这表明连接丝状体中细胞的“隔壁连接”（以前称为“微胞间连丝”）允许分子通过简单扩散进行交换，而无需任何活化的中间状态。这构成了一种跨细菌细胞质膜进行分子转移的新机制，此外还有先前已表征的主动运输和易化扩散机制。蓝细菌隔壁连接在功能上类似于后生动物的间隙连接。
尽管细菌通常被视为单细胞生物，但有些细菌表现得如同真正的多细胞生物。形成异形胞的蓝细菌以细胞间相互通讯的丝状体形式生长。细胞间分子交换被认为是由隔壁连接介导的。在这里，我们表明蓝细菌丝状体中荧光标记物的细胞间转移具有简单扩散的物理特性。因此，蓝细菌隔壁连接在功能上类似于后生动物的间隙连接，尽管它们的分子成分似乎没有关联。与后生动物间隙连接一样，蓝细菌的隔壁连接允许小分子在细胞间快速交换，且没有严格的选择性。我们的发现扩展了原核生物中跨质膜进行分子转移的机制。
