Szeto W W, Nixon B T, Ronson C W, Ausubel F M
J Bacteriol. 1987 Apr;169(4):1423-32. doi: 10.1128/jb.169.4.1423-1432.1987.
We show here that Rhizobium meliloti, the nitrogen-fixing endosymbiont of alfalfa (Medicago sativa), has a regulatory gene that is structurally homologous to previously characterized ntrC genes in enteric bacteria. DNA sequence analysis showed that R. meliloti ntrC is homologous to previously sequenced ntrC genes from Klebsiella pneumoniae and Bradyrhizobium sp. (Parasponia) and that an ntrB-like gene is situated directly upstream from R. meliloti ntrC. Similar to its counterparts in K. pneumoniae and Escherichia coli, R. meliloti ntrC is expressed when the cells are grown in nitrogen-limiting media. In addition, R. meliloti ntrC is required for growth on media containing nitrate as the sole nitrogen source and for the ex planta transcription of several R. meliloti nif genes. On the other hand, root nodules elicited by R. meliloti ntrC mutants fix nitrogen as well as nodules elicited by wild-type R. meliloti. These latter results indicate that R. meliloti has separate regulatory pathways for activating nif gene expression ex planta and during symbiotic nitrogen fixation.
我们在此表明，苜蓿（紫花苜蓿）的固氮内共生菌苜蓿中华根瘤菌具有一个调控基因，该基因在结构上与先前在肠道细菌中鉴定的ntrC基因同源。DNA序列分析表明，苜蓿中华根瘤菌ntrC与肺炎克雷伯菌和慢生根瘤菌属（帕拉豆科根瘤菌）中先前测序的ntrC基因同源，并且一个类似ntrB的基因直接位于苜蓿中华根瘤菌ntrC的上游。与肺炎克雷伯菌和大肠杆菌中的对应基因相似，苜蓿中华根瘤菌ntrC在细胞于限氮培养基中生长时表达。此外，苜蓿中华根瘤菌ntrC是在以硝酸盐作为唯一氮源的培养基上生长以及苜蓿中华根瘤菌几个固氮基因在植物外转录所必需的。另一方面，苜蓿中华根瘤菌ntrC突变体诱导形成的根瘤与野生型苜蓿中华根瘤菌诱导形成的根瘤一样能够固氮。后一结果表明，苜蓿中华根瘤菌在激活植物外固氮基因表达和共生固氮过程中有不同的调控途径。
