l-Serine has wide and increasing applications in industries with fast-growing market demand. Although strategies for achieving and improving l-serine production in Corynebacterium glutamicum (C. glutamicum) have focused on inhibiting its degradation and enhancing its biosynthetic pathway, l-serine yield has remained relatively low. Exporters play an essential role in the fermentative production of amino acids. To achieve higher l-serine yield, l-serine export from the cell should be improved. In C. glutamicum, ThrE, which can export l-threonine and l-serine, is the only identified l-serine exporter so far. In this study, a novel l-serine exporter NCgl0580 was identified and characterized in C. glutamicum ΔSSAAI (SSAAI), and named as SerE (encoded by serE). Deletion of serE in SSAAI led to a 56.5% decrease in l-serine titer, whereas overexpression of serE compensated for the lack of serE with respect to l-serine titer. A fusion protein with SerE and enhanced green fluorescent protein (EGFP) was constructed to confirm that SerE localized at the plasma membrane. The function of SerE was studied by peptide feeding approaches, and the results showed that SerE is a novel exporter for l-serine and l-threonine in C. glutamicum. Subsequently, the interaction of a known l-serine exporter ThrE and SerE was studied, and the results suggested that SerE is more important than ThrE in l-serine export in SSAAI. In addition, probe plasmid and electrophoretic mobility shift assays (EMSA) revealed NCgl0581 as the transcriptional regulator of SerE. Comparative transcriptomics between SSAAI and the NCgl0581 deletion strain showed that NCgl0581 is a positive regulator of NCgl0580. Finally, by overexpressing the novel exporter SerE, combined with l-serine synthetic pathway key enzyme serAΔ197, serC, and serB, the resulting strain presented an l-serine titer of 43.9 g/L with a yield of 0.44 g/g sucrose, which is the highest l-serine titer and yield reported so far in C. glutamicum. This study provides a novel target for l-serine and l-threonine export engineering as well as a new global transcriptional regulator NCgl0581 in C. glutamicum.

中文翻译：

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通过新颖的鉴定出的出口商结合谷氨酸棒状杆菌中的合成途径，高产生产L-丝氨酸。

l-Serine在市场需求快速增长的行业中具有广泛且不断增加的应用。尽管在谷氨酸棒杆菌（C.glutamicum）中实现和改善L-丝氨酸生产的策略集中在抑制其降解和增强其生物合成途径上，但是L-丝氨酸的产量仍然相对较低。出口商在氨基酸的发酵生产中起着至关重要的作用。为了获得更高的l-丝氨酸产量，应当改善细胞中l-丝氨酸的输出。在谷氨酸棒杆菌中，可以输出左旋苏氨酸和左旋丝氨酸的ThrE是迄今为止唯一被确定的左旋丝氨酸出口商。在这项研究中，鉴定了一种新型的l-丝氨酸输出蛋白NCgl0580，并在谷氨酸棒杆菌ΔSSAAI（SSAAI）中进行了表征，并将其命名为SerE（由serE编码）。SSAAI中serE的删除导致l-丝氨酸滴度下降56.5％，serE的过表达弥补了关于l-丝氨酸效价缺乏serE的问题。构建了具有SerE和增强型绿色荧光蛋白（EGFP）的融合蛋白，以确认SerE位于质膜上。通过肽喂养方法研究了SerE的功能，结果表明SerE是谷氨酸棒杆菌中L-丝氨酸和L-苏氨酸的新型出口者。随后，研究了已知的L-丝氨酸出口者ThrE和SerE的相互作用，结果表明，在SSAAI中，SerE比ThrE更重要。另外，探针质粒和电泳迁移率变动分析（EMSA）显示NCgl0581是SerE的转录调节因子。SSAAI和NCgl0581缺失菌株之间的比较转录组学表明，NCgl0581是NCg10580的正调控子。最后，通过过表达新型出口蛋白SerE，并结合l-丝氨酸合成途径关键酶serAΔ197，serC和serB，所得菌株的l-丝氨酸效价为43.9 g / L，收率为0.44 g / g蔗糖，是迄今为止报道的谷氨酸棒杆菌中最高的l-丝氨酸滴度和产量。该研究为谷氨酸棒杆菌中的L-丝氨酸和L-苏氨酸出口工程以及新的全球转录调节因子NCgl0581提供了新的靶标。