Carlson R W, Price N P, Stacey G
Complex Carbohydrate Research Center, University of Georgia, Athens 30602.
Mol Plant Microbe Interact. 1994 Nov-Dec;7(6):684-95. doi: 10.1094/mpmi-7-0684.
While a great deal has been learned concerning the biosynthesis of Nod factors, there is much that remains to be determined. The functions of many Nod proteins involved in adding the host-specific modifications to the Nod factors remain to be unequivocally identified. Some of the genes required for these modifications have not yet been isolated, e.g., those involved in carbamylation, or addition of D-Ara. Additionally the cellular location of most of the Nod proteins and, concomitantly, the modifications they determine are not known. The actual in vivo substrates for the NodABC proteins have not been identified, and the enzyme activities of purified NodA and NodC have not been demonstrated. The synthesis and export of the Nod factors most probably involves some type of carrier/anchor which remains unidentified. Analysis of GlcNAc metabolites from various mutants, e.g., nodA-, nodB-, or nodC- mutants, should facilitate the identification of the in vivo substrates involved in the synthesis of the "common" Nod factor and, thereby, lead to a greater understanding of Nod factor biosynthesis and transport. Finally, comparison of Nod factor biosynthesis to other examples of polysaccharide or glycolipid biosynthetic pathways suggest that several key enzymes remain to be identified. It is hoped that this discussion will be helpful in designing strategies for the detection and isolation of such novel enzymes.
虽然关于根瘤菌因子的生物合成已经有了很多了解，但仍有许多有待确定的地方。许多参与对根瘤菌因子进行宿主特异性修饰的根瘤菌蛋白的功能仍有待明确鉴定。这些修饰所需的一些基因尚未分离出来，例如参与氨甲酰化或添加D - 阿拉伯糖的基因。此外，大多数根瘤菌蛋白的细胞定位以及它们所决定的修饰尚不清楚。尚未确定NodABC蛋白在体内的实际底物，并且纯化的NodA和NodC的酶活性也未得到证实。根瘤菌因子的合成和输出很可能涉及某种尚未确定的载体/锚定物。对来自各种突变体（如nodA - 、nodB - 或nodC - 突变体）的GlcNAc代谢物的分析，应该有助于鉴定参与“普通”根瘤菌因子合成的体内底物，从而加深对根瘤菌因子生物合成和运输的理解。最后，将根瘤菌因子生物合成与多糖或糖脂生物合成途径的其他例子进行比较表明，仍有几种关键酶有待鉴定。希望这一讨论将有助于设计检测和分离此类新酶的策略。
