Fellay R, Hanin M, Montorzi G, Frey J, Freiberg C, Golinowski W, Staehelin C, Broughton W J, Jabbouri S
LBMPS, Université de Genève, Chambésy/Genève, Switzerland.
Mol Microbiol. 1998 Mar;27(5):1039-50. doi: 10.1046/j.1365-2958.1998.00761.x.
Transcriptional regulators of the lysR family largely control the expression of bacterial symbiotic genes. Rhizobium sp. NGR234 contains at least four members of this family: two resemble nodD, while two others are more closely related to syrM. Part of the extremely broad host range of NGR234 can be attributed to nodD1, although the second gene shares a high degree of DNA sequence homology with nodD2 of R. fredii USDA191. A nodD2 mutant of NGR234 was constructed by insertional mutagenesis. This mutant (NGR omega nodD2) was deficient in nitrogen fixation on Vigna unguiculata and induced pseudonodules on Tephrosia vogelii. Several other host plants were tested, but no correlation could be drawn between the phenotype and nodule morphology. Moreover, nodD2 has a negative effect on the production of Nod factors: mutation of this gene results in a fivefold increase in Nod factor production. Surprisingly, while the structure of Nod factors from free-living cultures of NGR omega nodD2 remained unchanged, those from V. unguiculata nodules induced by the same strain are non-fucosylated and have a lower degree of oligomerization. In other words, developmental regulation of Nod factor production is also abolished in this mutant. Competitive RNA hybridizations, gene fusions and mobility shift assays confirmed that nodD2 downregulates expression of the nodABC operon.
赖斯R家族的转录调节因子在很大程度上控制着细菌共生基因的表达。根瘤菌属NGR234至少含有该家族的四个成员：两个类似于nodD，而另外两个与syrM关系更密切。NGR234极广的宿主范围部分可归因于nodD1，尽管第二个基因与费氏中华根瘤菌USDA191的nodD2具有高度的DNA序列同源性。通过插入诱变构建了NGR234的nodD2突变体。该突变体（NGRωnodD2）在豇豆上固氮能力不足，在非洲山毛豆上诱导形成假根瘤。对其他几种宿主植物进行了测试，但未发现表型与根瘤形态之间存在相关性。此外，nodD2对结瘤因子的产生有负面影响：该基因突变导致结瘤因子产量增加五倍。令人惊讶的是，虽然来自NGRωnodD2自由生活培养物的结瘤因子结构保持不变，但来自同一菌株诱导的豇豆根瘤中的结瘤因子是非岩藻糖基化的，且寡聚化程度较低。换句话说，该突变体中结瘤因子产生的发育调控也被消除。竞争性RNA杂交、基因融合和迁移率变动分析证实，nodD2下调nodABC操纵子的表达。
