How polymer synthesis is mobilized or activated as a biological response of Haloferax mediterranei against hypertonic conditions remains largely unexplored. This study investigated the protein expression of H. mediterranei in response to high salinity by using isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic analysis. The microbes were harvested at end of fermentation at the NaCl salinity of 75 and 250 g L⁻¹. Among the identified 2123 proteins, 170 proteins were differentially expressed. Gene ontology annotation revealed that the highest number of proteins was annotated in biological process category, which was responsible for metabolic process, cellular component and catalytic activity. Differentially expressed proteins were belonged to the class of response to stimulus as well as catalytic activity and binding. Under high salinity conditions, three pathways were established as key responses of PHA and EPS production to hypertonic pressure. Two overexpressed proteins, beta-ketoacyl-ACP reductase and 3-hydroxyacyl-CoA dehydrogenase, enhanced the synthesis of PHAs. The serine-pyruvate transaminase and serine-glyoxylate transaminase were upregulated, thereby increasing the conversion of glucose to PHA. Downregulated levels of sulfate-adenylyl transferase and adenylyl-sulfate kinase could cause diminished EPS synthesis. This study could contribute to better understanding of the proteomic mechanisms of the synthesized polymers in defending against salt stress.

Significance

Haloferax mediterranei, a family member of halophilic archaea, is well known for its fermentative production of poly-β-hydroxyalkanoates (PHAs). PHAs are natural polymers that exhibit great potential in a wide range of applications such as a good alternative to petroleum-based plastics and the biocompatible material. For decades, the functional role of PHAs synthesized by H. mediterranei is deemed to be carbon and energy reservations. The finding proved that differential production of PHA and EPS in H. mediterranei exposed to elevated salinity was caused by differential protein expression. This is the first report on how PHA and EPS synthesized by H. mediterranei is mobilized as the response of increased salinity, contributing to the understanding of halophilic archaea's response to hypertonic stress and the precise control of fermentation production. Despite its advantages as a PHA cell factory, H. mediterranei synthesized EPS simultaneously, thereby lowering the maximum yield of PHA production. Overall, salinity can be used as a vital microbial fermentation parameter to obtain the highest harvest of PHA, as well as the lowest EPS synthesis in industrial fermentation.

中文翻译：

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盐度高的Haloferax mediterranei合成的多羟基链烷酸酯和胞外多糖对盐分升高的代谢途径

高分子合成作为高渗条件对Haloferax mediterranei的生物学反应的动员或激活方式仍在很大程度上尚待探索。这项研究调查了H的蛋白质表达。地中海响应于高盐度通过使用同量异序标签用于相对和绝对定量（iTRAQ的）为基础的蛋白质组分析。发酵结束时，NaCl盐度分别为75和250 g L ^-1时收获微生物。在鉴定出的2123种蛋白质中，有170种蛋白质被差异表达。基因本体论注释显示，在生物过程类别中注释了最多的蛋白质，其负责代谢过程，细胞成分和催化活性。差异表达的蛋白质属于对刺激的反应以及催化活性和结合的类别。在高盐度条件下，建立了三种途径作为PHA和EPS产生对高渗压的关键反应。两种过表达的蛋白质，β-酮酰基-ACP还原酶和3-羟基酰基-CoA脱氢酶增强了PHA的合成。丝氨酸丙酮酸转氨酶和丝氨酸乙醛酸转氨酶被上调，从而增加了葡萄糖向PHA的转化。硫酸-腺苷酸转移酶和腺苷酸硫酸激酶的水平下调可能导致EPS合成减少。这项研究可能有助于更好地理解合成的聚合物抗盐胁迫的蛋白质组学机制。

意义

嗜盐古生菌的家族成员Haloferax mediterranei以发酵生产聚β-羟基链烷酸酯（PHA）而闻名。PHA是天然聚合物，在广泛的应用中显示出巨大的潜力，例如可以很好地替代石油基塑料和生物相容性材料。几十年来，由地中海嗜血杆菌合成的PHA的功能作用一直被认为是碳和能量的保留。这一发现证明，盐度升高的地中海嗜血杆菌中PHA和EPS的差异产生是由于蛋白质差异表达引起的。这是有关H合成PHA和EPS的第一份报告。地中海由于盐分增加而引起的动员，其有助于理解嗜盐古细菌对高渗应激的反应以及对发酵生产的精确控制。尽管它具有作为PHA细胞工厂的优势，但地中海嗜血杆菌同时合成了EPS，从而降低了PHA生产的最大产量。总体而言，盐度可以用作重要的微生物发酵参数，以获得最高的PHA收获量，以及工业发酵中最低的EPS合成量。