聚球藻PCC 7942响应盐胁迫合成蔗糖的信号转导途径研究

Many cyanobacterial species synthesize sucrose as compatible solutes under salt stress conditions. It was reported that some two-component transduction system proteins are involved in the signal transduction of this process, however, the complete signal transduction pathway still remains to be unraveled. In Synechococcus elongatus PCC 7942, sucrose is the only compatible solute synthesized under salt stress conditions. The genome size of this cyanobacterium is relatively small compared to others and the enzymes responsible for sucrose synthesis is encoded by the single gene, sps. Therefore it is an ideal model for the study on the mechanism of cyanobacterial sucrose biosynthesis. Our previous work demonstrated the existence of 39 two-component system protein- and 5 Ser/Thr kinase-encoding genes in the genome of S. elongatus PCC 7942. We are going to systematically inactivate these genes, analyze the transcriptional level of sps and the sucrose accumulation of the mutants under salt stresses, and thereby identify the key signal proteins involved. Moreover, the interactions between these signal proteins will be studied, and the complete signal transduction pathway from sensing salt stress signals to the biosynthesis of sucrose is supposed to be unraveled. In order to understand the regulatory mechanism of sps transcription preliminarily, the possible interactions between the identified signal proteins and the putative regulatory region of sps will be studied. This study would significantly expand our knowledge about the mechanism of salt response and acclimation of cyanobacteria, and set a substantial basis for developing cyanobacterial sucrose as a novel sugar source.

很多蓝细菌在盐胁迫下合成蔗糖作为渗透调节物来抵抗逆境。已有研究表明，蓝细菌盐胁迫蔗糖合成过程中，有双元信号转导蛋白参与信号传递过程，但完整的信号传递途径仍需进一步揭示。聚球藻PCC 7942盐胁迫下合成蔗糖为唯一相容物，其基因组较小，且蔗糖合成相关酶只由一个基因sps编码，是研究蓝细菌蔗糖合成的理想模式。本项目前期工作已明确该蓝细菌基因组共有39个双元信号蛋白和5个可能的丝氨酸/苏氨酸激酶信号蛋白编码基因。我们将系统敲除这些基因，分析突变株在盐胁迫下sps的转录和蔗糖积累变化情况，确定参与信号传递的关键蛋白，并分析这些蛋白间的互作关系，从而揭示从感知盐胁迫到作出蔗糖合成应答的完整信号传递途径。同时，还将分析信号蛋白与sps调控区域间的互作关系，初步揭示蔗糖合成基因sps的转录调控模式。本研究对全面了解蓝细菌盐胁迫响应、适应机制，探索建立新型蓝细菌蔗糖资源平台具有重要意义。

很多蓝细菌在盐胁迫下合成蔗糖作为渗透调节物来抵抗逆境，该过程中细胞对盐胁迫的响应及调控机制一直是这一领域的核心问题。通过对聚球藻PCC 7942信号蛋白基因敲除突变株文库的系统构建与评价，我们鉴定到3个蔗糖合成能力发生稳定变化的突变株，表明信号转导确实能够影响聚球藻PCC 7942蔗糖合成的生理过程。对部分突变株（Δ1125、Δ1404）的深入研究表明，相关信号转导途径（Synpcc7942_1125、Synpcc7942_1404）并没有直接调控蔗糖合成关键基因sps的表达，而是通过影响细胞光合作用及糖原代谢等间接影响了蔗糖合成。此外，对sps/SPS转录、翻译、酶活的系统分析表明，聚球藻PCC 7942蔗糖合成的调控主要发生在SPS酶学水平。无机盐能显著激活SPS活性，DNA能有效抑制SPS活性。因此，对无机盐-SPS-核酸相互关系的继续研究将是我们深入理解蓝细菌蔗糖合成调控机制的关键。另外，我们的研究还发现糖原合成并非如我们所想是蓝细菌蔗糖合成的竞争性途径，正好相反，其可能作为“碳库”为盐胁迫下蔗糖合成提供了碳素支持。以上结果对全面了解蓝细菌盐胁迫响应、适应机制，探索建立新型蓝细菌蔗糖资源平台具有重要意义。