Pini Francesco, De Nisco Nicole J, Ferri Lorenzo, Penterman Jon, Fioravanti Antonella, Brilli Matteo, Mengoni Alessio, Bazzicalupo Marco, Viollier Patrick H, Walker Graham C, Biondi Emanuele G
Unité de Glycobiologie Structurale et Fonctionnelle, UMR8576 CNRS-Université de Lille, Villeneuve d'Ascq, France.
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
PLoS Genet. 2015 May 15;11(5):e1005232. doi: 10.1371/journal.pgen.1005232. eCollection 2015 May.
In all domains of life, proper regulation of the cell cycle is critical to coordinate genome replication, segregation and cell division. In some groups of bacteria, e.g. Alphaproteobacteria, tight regulation of the cell cycle is also necessary for the morphological and functional differentiation of cells. Sinorhizobium meliloti is an alphaproteobacterium that forms an economically and ecologically important nitrogen-fixing symbiosis with specific legume hosts. During this symbiosis S. meliloti undergoes an elaborate cellular differentiation within host root cells. The differentiation of S. meliloti results in massive amplification of the genome, cell branching and/or elongation, and loss of reproductive capacity. In Caulobacter crescentus, cellular differentiation is tightly linked to the cell cycle via the activity of the master regulator CtrA, and recent research in S. meliloti suggests that CtrA might also be key to cellular differentiation during symbiosis. However, the regulatory circuit driving cell cycle progression in S. meliloti is not well characterized in both the free-living and symbiotic state. Here, we investigated the regulation and function of CtrA in S. meliloti. We demonstrated that depletion of CtrA cause cell elongation, branching and genome amplification, similar to that observed in nitrogen-fixing bacteroids. We also showed that the cell cycle regulated proteolytic degradation of CtrA is essential in S. meliloti, suggesting a possible mechanism of CtrA depletion in differentiated bacteroids. Using a combination of ChIP-Seq and gene expression microarray analysis we found that although S. meliloti CtrA regulates similar processes as C. crescentus CtrA, it does so through different target genes. For example, our data suggest that CtrA does not control the expression of the Fts complex to control the timing of cell division during the cell cycle, but instead it negatively regulates the septum-inhibiting Min system. Our findings provide valuable insight into how highly conserved genetic networks can evolve, possibly to fit the diverse lifestyles of different bacteria.
在生命的所有领域中，细胞周期的适当调控对于协调基因组复制、分离和细胞分裂至关重要。在某些细菌群体中，例如α-变形菌纲，细胞周期的严格调控对于细胞的形态和功能分化也是必要的。苜蓿中华根瘤菌是一种α-变形菌，它与特定豆科宿主形成了在经济和生态上都很重要的固氮共生关系。在这种共生过程中，苜蓿中华根瘤菌在宿主根细胞内经历了复杂的细胞分化。苜蓿中华根瘤菌的分化导致基因组大量扩增、细胞分支和/或伸长，以及生殖能力丧失。在新月柄杆菌中，细胞分化通过主调控因子CtrA的活性与细胞周期紧密相连，最近对苜蓿中华根瘤菌的研究表明，CtrA可能也是共生过程中细胞分化的关键。然而，苜蓿中华根瘤菌在自由生活和共生状态下驱动细胞周期进程的调控回路尚未得到很好的表征。在这里，我们研究了苜蓿中华根瘤菌中CtrA的调控和功能。我们证明，CtrA的缺失会导致细胞伸长、分支和基因组扩增，这与在固氮类菌体中观察到的情况相似。我们还表明，细胞周期调控的CtrA蛋白水解降解在苜蓿中华根瘤菌中至关重要，这表明了分化类菌体中CtrA缺失的一种可能机制。通过结合染色质免疫沉淀测序（ChIP-Seq）和基因表达微阵列分析，我们发现尽管苜蓿中华根瘤菌的CtrA与新月柄杆菌的CtrA调控相似的过程，但它是通过不同的靶基因来实现的。例如，我们的数据表明，CtrA并不控制Fts复合体的表达来控制细胞周期中细胞分裂的时间，而是负向调节抑制隔膜形成的Min系统。我们的研究结果为高度保守的遗传网络如何进化提供了有价值的见解，这种进化可能是为了适应不同细菌的多样生活方式。
