Salinity is one of the most important abiotic factors in various natural habitats of microbes. Cyanobacteria are the most widely distributed family of photosynthetic microorganisms in environments with fluctuating salinity. In response to salt stress, many cyanobacteria de novo synthesize compatible solutes to maintain osmotic balance in the cell. However, the regulation of intracellular accumulation of these compounds is still not well understood. The freshwater cyanobacterium Synechococcus elongatus PCC 7942 (Syn7942) exclusively accumulates sucrose as a compatible solute upon salt stress and is thus an ideal model microorganism for studying the metabolism of compatible solute dynamics. Here, we focused on elucidating the regulatory mechanisms involved in salt-induced sucrose accumulation in Syn7942. Using a series of physiological and biochemical experiments, we showed that the ionic effect of salt stress plays an important role in inducing sucrose synthesis, whereby elevated ion concentration directly activates the sucrose-synthesizing enzyme sucrose-phosphate synthase and simultaneously inhibits the sucrose-degrading enzyme invertase, resulting in a rapid sucrose accumulation. Thus, we propose a novel mechanism for cyanobacterial adaption to salt stress and fluctuating salinity, i.e., the ion-induced synergistic modulation of the enzymes synthesizing and degrading compatible solutes. These findings greatly enhance our current understanding of microbial adaptation to salt.IMPORTANCE Most microbes de novo synthesize compatible solutes for adaptation to salt stress or fluctuating salinity environments. However, to date, one of the core questions involved in these physiological processes, i.e., the regulation of salt-induced compatible solute biosynthesis, is still not well understood. Here, this issue was systematically investigated by employing the model freshwater cyanobacterium Synechococcus elongatus PCC 7942. A novel mechanism for cyanobacterial adaption to salt stress and fluctuating salinity, i.e., the ion-induced synergistic modulation of key synthesizing and degrading enzymes of compatible solutes, is proposed. Because the ion-induced activation/inhibition of enzymes is a fast and efficient process, it may represent a common strategy of microbes for adaptation to environments with fluctuating salinity.

译文

盐度是微生物各种自然栖息地中最重要的非生物因子之一。蓝细菌是盐度波动环境中分布最广泛的光合微生物家族。为了应对盐胁迫,许多蓝细菌从头合成了相容的溶质,以维持细胞内的渗透平衡。然而,对这些化合物的细胞内积累的调节仍然没有很好的理解。淡水蓝细菌Synechococcus elongatus PCC 7942 (Syn7942) 在盐胁迫下专门积累蔗糖作为相容溶质,因此是研究相容溶质动力学代谢的理想模型微生物。在这里,我们重点阐明了syn7942中盐诱导蔗糖积累的调控机制。通过一系列生理生化实验表明,盐胁迫的离子效应在诱导蔗糖合成中起着重要作用,离子浓度升高直接激活蔗糖合成酶蔗糖-磷酸合成酶,同时抑制蔗糖降解酶转化酶,导致蔗糖快速积累。因此,我们提出了一种新的机制,用于蓝藻适应盐胁迫和盐度波动,即离子诱导的酶合成和降解相容溶质的协同调节。这些发现极大地增强了我们目前对微生物适应盐的理解。重要性大多数微生物从头合成相容的溶质,以适应盐胁迫或盐度波动的环境。然而,迄今为止,这些生理过程中涉及的核心问题之一,即盐诱导的相容性溶质生物合成的调节,仍然没有得到很好的理解。在此,通过使用模型淡水蓝细菌Synechococcus elongatus PCC 7942系统地研究了此问题。提出了一种新的蓝藻适应盐胁迫和盐度波动的机制,即离子诱导的相容溶质关键合成和降解酶的协同调节。由于离子诱导的酶激活/抑制是一个快速有效的过程,因此它可能代表了微生物适应盐度波动环境的通用策略。

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