Control of the cyanobacterial day-night metabolism by a biological clock system

在自然环境中，蓝藻受到不断变化的，往往是不可预测的环境条件。然而，有些变化是可以预测的，比如昼夜转换。对于光合生物来说，在这样的光暗循环中生长需要完全逆转它们的新陈代谢。光合作用和CO2固定在白天进行，因为这些过程依赖于光。在夜间，在光照期间合成的糖原储备被降解，因此细胞从光合自养代谢转变为异养代谢。蓝细菌能够使用生物钟来预测光可用性的每日变化，该生物钟与真核生物的已知时钟系统没有同源性。在我们以前的研究中，我们发现，已知的细菌型时钟蛋白的同源物是重要的生长模式蓝藻集胞藻属PCC 6803在光暗循环。我们对时钟突变在光照和黑暗中对中央代谢的影响的研究，为研究单位“蓝藻中的自养异养开关：多个监管层的一致决策”（简称“SCyCode”）的目标做出了巨大贡献。 通过与SCyCode专家在代谢和蛋白质组学分析方面的密切合作，我们发现集胞藻PCC 6803时钟突变体的暗代谢发生了特别强烈的变化。最重要的是，这种生物体中有缺陷的时钟系统导致在黑暗中无法关闭碳固定酶RubisCO的活性，并且缺乏储存化合物聚羟基丁酸酯（一种具有生物技术潜力的生物聚合物）的积累。此外，我们研究了一个替代的时钟系统，我们发现是相关的异养生长。 在第二个资助期内，我们将专注于对不同时钟蛋白在昼夜周期中自养和异养之间切换的作用的机械理解。我们与SCyCode成员合作设计的实验应该确定不同时钟系统的直接目标，解释集胞藻时钟蛋白如何有助于碳固定的调节，以及时钟依赖的蛋白磷酸化和基因表达变化如何允许昼夜节律平衡的蓝藻代谢。这些见解可能是至关重要的潜在的生物技术应用可持续利用的光养微生物。

In natural environments, cyanobacteria are subjected to changing and often unpredictable environmental conditions. Some of the changes are, however, predictable, such as the day-night switch. For photosynthetic organisms, growth in such a light-dark cycle requires a complete reversal of their metabolism. Photosynthesis and CO2 fixation is performed during the day because these processes are light-dependent. At night, the glycogen reserves that were synthesized during the light period are degraded and the cells therefore transition from a photoautotrophic to a heterotrophic metabolism. Cyanobacteria are able to predict the daily changes in light availability using a biological clock which shares no homology to known clock systems from eukaryotic organisms. In our previous studies we revealed that homologs of the known bacterial-type clock proteins are important for the growth of the model cyanobacterium Synechocystis sp. PCC 6803 in light-dark cycles. Our studies of the effects of clock mutations on central metabolism in the light and the dark strongly contributed to the aim of the research unit “The Autotrophy Heterotrophy Switch in Cyanobacteria: Coherent decision-making on multiple regulatory layers” (abbreviated “SCyCode”). In close cooperation with SCyCode experts in metabolic and proteomic analyses, we revealed especially strong changes in the dark metabolism of Synechocystis sp. PCC 6803 clock mutants. Most importantly, a defective clock system in this organism led to a failure to switch off the activity of the carbon fixing enzyme RubisCO in the dark and a deficiency in the accumulation of the storage compound polyhydroxybutyrate, a biopolymer of biotechnological potential. Further, we studied an alternative clock system which we found to be relevant for heterotrophic growth. During the second funding period, we will focus on a mechanistic understanding of the role of the different clock proteins in the switch between autotrophy and heterotrophy in day-night cycles. The experiments which we have designed in cooperation with SCyCode members should identify the direct targets of the different clock systems, explain how Synechocystis clock proteins contribute to the regulation of carbon fixation and how clock-dependent protein phosphorylation and gene expression changes allow a balanced cyanobacterial metabolism in a day-night rhythm. These insights might be of crucial importance for potential biotechnological applications for sustainable exploitation of phototrophic microorganisms.