Schlaman Helmi R M, Olsthoorn Maurien M A, Harteveld Marga, Dörner Lucie, Djordjevic Michael A, Thomas-Oates Jane E, Spaink Herman P
Institute of Biology Leiden, Leiden University, Clusius Laboratory, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands.
Mol Plant Microbe Interact. 2006 Mar;19(3):215-26. doi: 10.1094/MPMI-19-0215.
A proportion of the Nod factors of some Rhizobium leguminosarum bv. trifolii strains is characterized by the presence of highly unsaturated fatty acyl chains containing trans double bonds in conjugation with the carbonyl group of the glycan oligosaccharide backbone. These fatty acyl chains are C18:3, C20:3, C18:4, or C20:4 and have UV-absorption maxima at 303 and 330 nm. These Nod factors are presumed to be important for host-specific nodulation on clover species. However, in wild-type R. leguminosarum bv. trifolii ANU843, Nod factors with these characteristic acyl chains were not observed using standard growth conditions. They were observed only when nod genes were present in multiple copies or when transcription was artificially increased to higher levels by introduction of extra copies of the transcriptional regulator gene nodD. In a screen for the genetic requirements for production of the Nod factors with these characteristic structures, it was found that the region downstream of nodF and nodE is essential for the presence of highly unsaturated fatty acyl moieties. Mu-lacZ insertion in this region produced a mutant that did not produce detectable levels of the highly unsaturated fatty acyl-bearing Nod factors. The Mu-lacZ insertion was translationally fused to a putative new gene, designated nodR, in the nodE-nodL intergenic region; however, no predicted function for the putative NodR protein has been obtained from database homology searches. In a set of 12 wild-type strains of R. leguminosarum by. trifolii originating from various geographical regions that were analyzed for the presence of a nodR-like gene, it was found that seven strains carry a homologous NodR open reading frame. Taken together, our results suggest a tightly controlled regulation of nod genes, in which we propose that it is the balance of transcriptional levels of nodFE and the nodRL genes that is critical for determining the presence of highly unsaturated fatty acyl moieties in the Nod factors produced by R. leguminosarum bv. trifolii.
一些三叶草根瘤菌（Rhizobium leguminosarum bv. trifolii）菌株的部分结瘤因子（Nod因子）的特征是，其聚糖寡糖主链的羰基与含有反式双键的高度不饱和脂肪酰链相连。这些脂肪酰链为C18:3、C20:3、C18:4或C20:4，在303和330 nm处有最大紫外吸收峰。这些结瘤因子被认为对三叶草属植物的宿主特异性结瘤很重要。然而，在野生型三叶草根瘤菌ANU843中，使用标准生长条件未观察到具有这些特征酰链的结瘤因子。只有当结瘤基因以多拷贝形式存在，或者通过引入转录调节基因nodD的额外拷贝使转录人工增加到更高水平时，才会观察到它们。在筛选产生具有这些特征结构的结瘤因子的遗传需求时，发现nodF和nodE下游区域对于高度不饱和脂肪酰部分的存在至关重要。该区域的Mu - lacZ插入产生了一个突变体，该突变体不能产生可检测水平的带有高度不饱和脂肪酰的结瘤因子。Mu - lacZ插入与nodE - nodL基因间隔区中一个假定的新基因（命名为nodR）翻译融合；然而，通过数据库同源性搜索未获得假定的NodR蛋白的预测功能。在一组来自不同地理区域的12株野生型三叶草根瘤菌中分析是否存在nodR样基因，发现7株菌株携带同源的NodR开放阅读框。综上所述，我们的结果表明结瘤基因受到严格调控，我们提出，nodFE和nodRL基因转录水平的平衡对于决定三叶草根瘤菌产生的结瘤因子中高度不饱和脂肪酰部分的存在至关重要。
