Liu Nian, Gagnot Séverine, Denis Yann, Byrne Deborah, Faulds Craig, Fierobe Henri-Pierre, Perret Stéphanie
Aix Marseille Univ, CNRS, LCB, Marseille, France, 31 chemin Joseph Aiguier F-13402, Marseille Cedex 20, Marseille, France.
Aix Marseille Univ, CNRS, IMM, Marseille, France.
Biotechnol Biofuels Bioprod. 2022 Nov 19;15(1):127. doi: 10.1186/s13068-022-02225-8.
Primary degraders of polysaccharides play a key role in anaerobic biotopes, where plant cell wall accumulates, providing extracellular enzymes to release fermentable carbohydrates to fuel themselves and other non-degrader species. Ruminiclostridium cellulolyticum is a model primary degrader growing amongst others on arabinoxylan. It produces large multi-enzymatic complexes called cellulosomes, which efficiently deconstruct arabinoxylan into fermentable monosaccharides.
Complete extracellular arabinoxylan degradation was long thought to be required to fuel the bacterium during this plant cell wall deconstruction stage. We discovered and characterized a second system of "arabinoxylan" degradation in R. cellulolyticum, which challenged this paradigm. This "selfish" system is composed of an ABC transporter dedicated to the import of large and possibly acetylated arabinoxylodextrins, and a set of four glycoside hydrolases and two esterases. These enzymes show complementary action modes on arabinoxylo-dextrins. Two α-L-arabinofuranosidases target the diverse arabinosyl side chains, and two exo-xylanases target the xylo-oligosaccharides backbone either at the reducing or the non-reducing end. Together, with the help of two different esterases removing acetyl decorations, they achieve the depolymerization of arabinoxylo-dextrins in arabinose, xylose and xylobiose. The in vivo study showed that this new system is strongly beneficial for the fitness of the bacterium when grown on arabinoxylan, leading to the conclusion that a part of arabinoxylan degradation is achieved in the cytosol, even if monosaccharides are efficiently provided by the cellulosomes in the extracellular space. These results shed new light on the strategies used by anaerobic primary degrader bacteria to metabolize highly decorated arabinoxylan in competitive environments.
The primary degrader model Ruminiclostridium cellulolyticum has developed a "selfish" strategy consisting of importing into the bacterium, large arabinoxylan-dextrin fractions released from a partial extracellular deconstruction of arabinoxylan, thus complementing its efficient extracellular arabinoxylan degradation system. Genetic studies suggest that this system is important to support fitness and survival in a competitive biotope. These results provide a better understanding of arabinoxylan catabolism in the primary degrader, with biotechnological application for synthetic microbial community engineering for the production of commodity chemicals from lignocellulosic biomass.
多糖的主要降解菌在厌氧生物环境中起着关键作用，在这些环境中植物细胞壁会积累，多糖降解菌通过分泌胞外酶来释放可发酵碳水化合物，为自身及其他非降解菌提供能量。解纤维瘤胃梭菌是一种主要降解菌，可利用阿拉伯木聚糖等生长。它能产生名为纤维小体的大型多酶复合体，可有效地将阿拉伯木聚糖分解为可发酵单糖。
长期以来，人们一直认为在植物细胞壁解构阶段，阿拉伯木聚糖完全在胞外降解才能为细菌提供能量。我们在解纤维瘤胃梭菌中发现并鉴定了第二个“阿拉伯木聚糖”降解系统，这一发现挑战了这一传统观念。这个“自私”的系统由一个负责转运大的、可能乙酰化的阿拉伯木糖寡糖的ABC转运蛋白，以及一组四种糖苷水解酶和两种酯酶组成。这些酶对阿拉伯木糖寡糖具有互补的作用模式。两种α-L-阿拉伯呋喃糖苷酶作用于不同的阿拉伯糖基侧链，两种外切木聚糖酶分别从还原端或非还原端作用于木寡糖主链。在两种不同酯酶去除乙酰修饰的帮助下，它们共同实现了阿拉伯木糖寡糖解聚为阿拉伯糖、木糖和木二糖。体内研究表明，当在阿拉伯木聚糖上生长时，这个新系统对细菌的适应性非常有益，这表明即使纤维小体在胞外空间有效地提供了单糖，一部分阿拉伯木聚糖的降解是在胞质溶胶中完成的。这些结果为厌氧主要降解菌在竞争环境中代谢高度修饰的阿拉伯木聚糖所采用的策略提供了新的见解。
主要降解菌模型解纤维瘤胃梭菌已经发展出一种“自私”策略，即从阿拉伯木聚糖的部分胞外解构中摄取释放出的大的阿拉伯木聚糖-寡糖片段进入细菌，从而补充其高效的胞外阿拉伯木聚糖降解系统。遗传学研究表明，这个系统对于在竞争的生物环境中维持适应性和生存很重要。这些结果有助于更好地理解主要降解菌中阿拉伯木聚糖的分解代谢，对合成微生物群落工程从木质纤维素生物质生产商品化学品具有生物技术应用价值。
