Institute of Biological Chemistry, Washington State University, Pullman 99164-6340, USA.
Mol Plant Microbe Interact. 2012 Mar;25(3):355-62. doi: 10.1094/MPMI-09-11-0249.
To contribute nitrogen for plant growth and establish an effective symbiosis with alfalfa, Sinorhizobium meliloti Rm1021 needs normal operation of the GlnD protein, a bifunctional uridylyltransferase/uridylyl-cleavage enzyme that measures cellular nitrogen status and initiates a nitrogen stress response (NSR). However, the only two known targets of GlnD modification in Rm1021, the PII proteins GlnB and GlnK, are not necessary for effectiveness. We introduced a Tyr→Phe variant of GlnB, which cannot be uridylylated, into a glnBglnK background to approximate the expected state in a glnD-sm2 mutant, and this strain was effective. These results suggested that unmodified PII does not inhibit effectiveness. We also generated a glnBglnK-glnD triple mutant and used this and other mutants to dissect the role of these proteins in regulating the free-living NSR and nitrogen metabolism in symbiosis. The glnD-sm2 mutation was dominant to the glnBglnK mutations in symbiosis but recessive in some free-living phenotypes. The data show that the GlnD protein has a role in free-living growth and in symbiotic nitrogen exchange that does not depend on the PII proteins, suggesting that S. meliloti GlnD can communicate with the cell by alternate mechanisms.
为了向植物生长提供氮,并与紫花苜蓿建立有效的共生关系,根瘤菌 Sinorhizobium meliloti Rm1021 需要 GlnD 蛋白的正常运作,该蛋白是一种双功能尿苷酰基转移酶/尿苷酸裂解酶,可衡量细胞的氮状态并启动氮胁迫反应 (NSR)。然而,GlnD 修饰的唯二已知靶标,PII 蛋白 GlnB 和 GlnK,对于有效性并非必需。我们引入了 GlnB 的 Tyr→Phe 变体,该变体不能被尿苷酰化,进入 glnBglnK 背景以近似 glnD-sm2 突变体中的预期状态,该菌株是有效的。这些结果表明,未修饰的 PII 不会抑制有效性。我们还生成了 glnBglnK-glnD 三突变体,并使用该突变体和其他突变体来剖析这些蛋白在调节自由生活 NSR 和共生氮代谢中的作用。在共生中,glnD-sm2 突变是对 glnBglnK 突变的显性,但在一些自由生活表型中是隐性的。数据表明,GlnD 蛋白在自由生活生长和共生氮交换中具有作用,而不依赖于 PII 蛋白,这表明 S. meliloti GlnD 可以通过替代机制与细胞进行通讯。