运动单胞菌（Zymomonas mobilis）突变菌株ZM401自絮凝分子机制

与酿酒酵母EMP途径相比，运动单胞菌ED途径糖代谢ATP生成减少一半，发酵过程菌体增殖量少，乙醇对糖表观收率高。然而该菌株只利用葡萄糖、果糖和蔗糖，且蔗糖代谢副产物生成量大，不能用于糖质原料乙醇发酵；淀粉质原料水解糖除葡萄糖外还有很多运动单胞菌不能代谢的糖，使其也不能用于淀粉质原料生产乙醇。纤维素组分水解只得到葡萄糖，适宜该菌株糖代谢特点。虽然运动单胞菌不能代谢五碳糖，但五碳糖代谢改造表达异构酶，不存在酿酒酵母氧化还原途径改造辅酶不平衡问题。因此，运动单胞菌在纤维素乙醇生产中更有优势。.运动单胞菌呈游离单细胞和多细胞自絮凝两种形态，自絮凝后连续发酵时可以在发酵罐中自固定化获得高密度，提高乙醇生产强度，而且对纤维质原料预处理产生毒性副产物等环境胁迫有更好耐受性。本项目在鉴明运动单胞菌自絮凝物质为纤维素微丝前提下，解析其自絮凝分子机制，为调控策略开发及改造其他菌株，赋予其自絮凝表型，奠定科学基础。

Compared with the Embden-Meyerhof-Parnas (EMP) pathway in Saccharomyces cerevisiae, Zymomonas mobilis metabolizes sugars through the Entner-Doudoroff (ED) pathway, through which ATP production is reduced 50%. Since ATP is dissipated mainly by cell growth during ethanol fermentation for intracellular energy homeostasis, less biomass is accumulated, and consequently more sugars can be used for ethanol production with observed yield improved. However, Z. mobilis metabolizes glucose, fructose and sucrose only, and substantial amounts of levan are formed as a byproduct when sucrose is fed, which inevitably compromises its ethanol yield, making the species not suitable for ethanol production from sugar-based feedstock. On the other hand, in addition to glucose there are still other sugars in the hydrolysate of starch such as maltose and maltotriose, which can be metabolized by S. cerevisiae, but cannot be fermented by Z. mobilis. As a result, Z. mobilis cannot be used for ethanol production from grains and other starch-based feedstock. ..Lignocellulosic biomass is a sustainable feedstock for fuel ethanol production, and only glucose is released from cellulose hydrolysis, making the narrow substrate spectrum with Z. mobilis not challenging. Moreover, compared to the oxido-reductase pathway applied to S. cerevisiae when it is engineered with pentose utilization for cellulosic ethanol production, strategy for engineering Z. mobilis with pentose metabolism is the heterologous expression of isomerase without cofactor imbalance and intermediate accumulation. Therefore, Z. mobilis is more suitable for being engineered to produce cellulosic ethanol...Z. mobilis presents two significantly different morphologies: unicellular cells and flocs developed through the self-flocculation of the bacterial cells. When microbial cells are self-flocculated, they can be immobilized within bioreactors without consumption of any supporting materials for high density under continuous fermentation conditions to improve productivities of the bioreactors. Most importantly, enhanced tolerance to stresses such as inhibition from toxic byproducts released during the pretreatment of lignocellulosic biomass has been observed with the self-flocculating cells, either S. cerevisiae or Z. mobilis. Preliminary research has identified that cellulose fibrils synthesized by the self-flocculating mutant of Z. mobilis 401 are chemical basis for the unique phenotype. In this project, mechanism underlying cellulose synthesis and the development of cellulose fibrils in ZM401 will be investigated with an objective to developing strategies for controlling the self-flocculation of Z. mobilis properly for more efficient production of cellulosic ethanol as well as engineering other bacteria with the self-flocculating phenotype.