Hood Rachel D, Higgins Sean A, Flamholz Avi, Nichols Robert J, Savage David F
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720;
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720; Department of Chemistry, University of California, Berkeley, CA 94720; Energy Biosciences Institute, University of California, Berkeley, CA 94720
Proc Natl Acad Sci U S A. 2016 Aug 16;113(33):E4867-76. doi: 10.1073/pnas.1524915113. Epub 2016 Aug 2.
The cyanobacterium Synechococcus elongatus relies upon photosynthesis to drive metabolism and growth. During darkness, Synechococcus stops growing, derives energy from its glycogen stores, and greatly decreases rates of macromolecular synthesis via unknown mechanisms. Here, we show that the stringent response, a stress response pathway whose genes are conserved across bacteria and plant plastids, contributes to this dark adaptation. Levels of the stringent response alarmone guanosine 3'-diphosphate 5'-diphosphate (ppGpp) rise after a shift from light to dark, indicating that darkness triggers the same response in cyanobacteria as starvation in heterotrophic bacteria. High levels of ppGpp are sufficient to stop growth and dramatically alter many aspects of cellular physiology, including levels of photosynthetic pigments and polyphosphate, DNA content, and the rate of translation. Cells unable to synthesize ppGpp display pronounced growth defects after exposure to darkness. The stringent response regulates expression of a number of genes in Synechococcus, including ribosomal hibernation promoting factor (hpf), which causes ribosomes to dimerize in the dark and may contribute to decreased translation. Although the metabolism of Synechococcus differentiates it from other model bacterial systems, the logic of the stringent response remains remarkably conserved, while at the same time having adapted to the unique stresses of the photosynthetic lifestyle.
聚球藻依赖光合作用来驱动新陈代谢和生长。在黑暗中，聚球藻停止生长，从其糖原储备中获取能量，并通过未知机制大幅降低大分子合成速率。在这里，我们表明严格反应（一种应激反应途径，其基因在细菌和植物质体中保守）有助于这种暗适应。从光照转变为黑暗后，严格反应信号分子鸟苷3'-二磷酸5'-二磷酸（ppGpp）的水平升高，这表明黑暗在蓝细菌中引发的反应与异养细菌中的饥饿反应相同。高水平的ppGpp足以停止生长并显著改变细胞生理学的许多方面，包括光合色素和多聚磷酸盐的水平、DNA含量以及翻译速率。无法合成ppGpp的细胞在暴露于黑暗后表现出明显的生长缺陷。严格反应调节聚球藻中许多基因的表达，包括核糖体休眠促进因子（hpf），它会导致核糖体在黑暗中形成二聚体，并可能导致翻译减少。尽管聚球藻的新陈代谢使其与其他模式细菌系统有所不同，但严格反应的逻辑仍然非常保守，同时也适应了光合生活方式的独特压力。
