Russell J B, Wilson D B
Agricultural Research Service, USDA, Cornell University, Ithaca, NY 14853, USA.
J Dairy Sci. 1996 Aug;79(8):1503-9. doi: 10.3168/jds.S0022-0302(96)76510-4.
Ruminant animals depend on cellulolytic ruminal bacteria to digest cellulose, but these bacteria cannot resist the low ruminal pH that modern feeding practices can create. Because the cellulolytic bacteria cannot grow on cellobiose at low pH, pH sensitivity is a general aspect of growth and not just a limitation of the cellulases per se. Acid-resistant ruminal bacteria have evolved the capacity to let their intracellular pH decrease, maintain a small pH gradient across the cell membrane, and prevent an intracellular accumulation of VFA anions. Cellulolytic bacteria cannot grow with a low intracellular pH, and an increase in pH gradient leads to anion toxicity. Prevotella ruminicola cannot digest native cellulose, but it grows at low pH and degrades the cellulose derivative, carboxymethylcellulose. The Prevotella ruminicola carboxymethylcellulase cannot bind to cellulose, but a recombinant enzyme having the Prevotella ruminicola catalytic domain and a binding domain from Thermomonspora fusca was able to bind and had cellulase activity that was at least 10-fold higher. Based on these results, gene reconstruction offers a means of converting Prevotella ruminicola into a ruminal bacterium that can digest cellulose at low pH.
反刍动物依靠瘤胃中的纤维素分解菌来消化纤维素，但这些细菌无法抵御现代饲养方式可能导致的瘤胃低pH值环境。由于纤维素分解菌在低pH值下无法利用纤维二糖生长，所以pH敏感性是生长的一个普遍特征，而不仅仅是纤维素酶本身的局限性。耐酸性瘤胃细菌已经进化出一种能力，即让其细胞内pH值降低，维持细胞膜两侧较小的pH梯度，并防止挥发性脂肪酸阴离子在细胞内积累。纤维素分解菌在低细胞内pH值下无法生长，而pH梯度的增加会导致阴离子毒性。反刍月形单胞菌不能消化天然纤维素，但它能在低pH值下生长并降解纤维素衍生物羧甲基纤维素。反刍月形单胞菌的羧甲基纤维素酶不能与纤维素结合，但一种具有反刍月形单胞菌催化结构域和来自栖热单孢菌的结合结构域的重组酶能够结合且具有至少高10倍的纤维素酶活性。基于这些结果，基因重组提供了一种将反刍月形单胞菌转化为能在低pH值下消化纤维素的瘤胃细菌的方法。
