PNAS：美国科学家培育出可同时发酵葡萄糖和木糖的酵母

美国研究人员12月27日报告说，他们利用转基因工程培育出可以较高效率发酵葡萄糖和木糖的酵母，这项研究将来有望应用于生物燃料工业。
 
在生物燃料工业中，为了更经济地生产乙醇等燃料，所用微生物必须有能力发酵简单和复杂的糖，例如植物中的葡萄糖和木糖。然而，大多数微生物并不同时具备发酵这两种糖的能力。
 
酿酒工业中常用的酿酒酵母尽管善于发酵葡萄糖，但却无法发酵植物的另一种重要糖——木糖。很多经过转基因改造的酵母尽管也具备发酵木糖的能力，但效率非常低。来自伊利诺伊大学等机构的研究人员在27日出版的美国《国家科学院院刊》（PNAS）上介绍说，他们利用转基因工程成功扩展了酿酒酵母的发酵能力。
 
研究人员首先将某种对纤维二糖（葡萄糖前体之一）利用率高的真菌中的两种蛋白质通过基因改造植入酵母中。然后把来自某种能发酵木糖的酵母中的3种蛋白质也植入这种酵母。最终培育出的酵母菌能同时发酵葡萄糖和木糖，且木糖发酵效率比目前已知酵母菌要高出至少20%。这种改良酵母的乙醇生产效率远远高于天然菌株。

Engineered Saccharomyces cerevisiae capable of simultaneous cellobiose and xylose fermentation

Abstract
The use of plant biomass for biofuel production will require efficient utilization of the sugars in lignocellulose, primarily glucose and xylose. However, strains of Saccharomyces cerevisiae presently used in bioethanol production ferment glucose but not xylose. Yeasts engineered to ferment xylose do so slowly, and cannot utilize xylose until glucose is completely consumed. To overcome these bottlenecks, we engineered yeasts to coferment mixtures of xylose and cellobiose. In these yeast strains, hydrolysis of cellobiose takes place inside yeast cells through the action of an intracellular β-glucosidase following import by a high-affinity cellodextrin transporter. Intracellular hydrolysis of cellobiose minimizes glucose repression of xylose fermentation allowing coconsumption of cellobiose and xylose. The resulting yeast strains, cofermented cellobiose and xylose simultaneously and exhibited improved ethanol yield when compared to fermentation with either cellobiose or xylose as sole carbon sources. We also observed improved yields and productivities from cofermentation experiments performed with simulated cellulosic hydrolyzates, suggesting this is a promising cofermentation strategy for cellulosic biofuel production. The successful integration of cellobiose and xylose fermentation pathways in yeast is a critical step towards enabling economic biofuel production.

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