O'Connell Kevin P, Raffel Sandra J, Saville Barry J, Handelsman Jo
Department of Plant Pathology,University of Wisconsin-Madison1630 Linden Drive, Madison, WI 53706USA.
Department of Bacteriology and University of Wisconsin-Madison1630 Linden Drive, Madison, WI 53706USA.
Microbiology (Reading). 1998 Sep;144 ( Pt 9):2607-2617. doi: 10.1099/00221287-144-9-2607.
Type B strains of Rhizobium tropici induce severe foliar chlorosis when applied at planting to seeds of symbiotic host and non-host dicotyledonous plants. A Tn5-induced mutant, designated CT4812, or R. tropici strain CIAT899 that was unable to induce chlorosis was isolated. Cloning and sequencing of the DNA flanking the transposon in CT4812 revealed that the Tn5 insertion is located in a gene similar to glnD, which encodes uridylyltransferase/uridylyl-removing enzyme in enteric bacteria. Two marker-exchange mutants with insertions in glnD also failed to induce chlorosis in bean (Phaseolus vulgaris) plants. The 5'-most insertion in glnD (in mutant strain ME330) abolished the ability of R. tropici to utilize nitrate as a sole carbon source, whereas a mutation in glnD further downstream (in mutant strain ME245) did not have an obvious effect on nitrate utilization. A gene similar to the Salmonella typhimurium virulence gene mviN overlaps the 3' end of the R. tropici glnD homologue. A mutation in mviN had no effect on the ability of CIAT899 to induce chlorosis in bean plants. Therefore the glnD homologue, but not mviN, appears to be required for induction of chlorosis in plants by R. tropici strain CIAT899. A high nitrogen: carbon ratio in the rhizosphere of bean plants also prevented R. tropici from inducing chlorosis in bean plants. Mutations in either the glnD homologue or mviN had no significant effect on root nodule formation or acetylene reduction activity. A mutation in mviN eliminated motility in R. tropici. The sequence data, the inability of the glnD mutant to utilize nitrate, and the role of the R. tropici glnD gene in chlorosis induction in plants, a process that is nitrogen regulated, suggest that glnD plays a role in nitrogen sensing in R. tropici as its homologues do in other organisms.
热带根瘤菌B型菌株在播种时施用于共生宿主和非宿主双子叶植物种子时，会引发严重的叶片黄化。分离出了一株Tn5诱导的突变体，命名为CT4812，以及一株不能诱导黄化的热带根瘤菌CIAT899菌株。对CT4812中转座子侧翼DNA进行克隆和测序，结果显示Tn5插入位于一个与glnD相似的基因中，该基因在肠道细菌中编码尿苷酰转移酶/尿苷酰去除酶。两个在glnD中插入的标记交换突变体在菜豆（Phaseolus vulgaris）植株中也未能诱导黄化。glnD中最上游的插入（在突变体菌株ME330中）消除了热带根瘤菌利用硝酸盐作为唯一碳源的能力，而glsD下游更远处的一个突变（在突变体菌株ME245中）对硝酸盐利用没有明显影响。一个与鼠伤寒沙门氏菌毒力基因mviN相似的基因与热带根瘤菌glnD同源物的3'端重叠。mviN中的突变对CIAT899在菜豆植株中诱导黄化的能力没有影响。因此，似乎CIAT899菌株在植物中诱导黄化需要glnD同源物，而不是mviN。菜豆植株根际的高氮碳比也能防止热带根瘤菌在菜豆植株中诱导黄化。glnD同源物或mviN中的突变对根瘤形成或乙炔还原活性没有显著影响。mviN中的突变消除了热带根瘤菌的运动性。序列数据、glnD突变体不能利用硝酸盐以及热带根瘤菌glnD基因在植物黄化诱导（一个受氮调节的过程）中的作用表明，glnD在热带根瘤菌的氮感知中发挥作用，就像其在其他生物体中的同源物一样。
