Osterås M, Stanley J, Finan T M
Department of Biology, McMaster University, Hamilton, Ontario, Canada.
J Bacteriol. 1995 Oct;177(19):5485-94. doi: 10.1128/jb.177.19.5485-5494.1995.
Analysis of the DNA regions upstream of the phosphoenolpyruvate carboxykinase gene (pckA) in Rhizobium meliloti and Rhizobium sp. strain NGR234 identified an open reading frame which was highly homologous to the Agrobacterium tumefaciens chromosomal virulence gene product ChvI. A second gene product, 500 bp downstream of the chvI-like gene in R. meliloti, was homologous to the A. tumefaciens ChvG protein. The homology between the R. meliloti and A. tumefaciens genes was confirmed, because the R. meliloti chvI and chvG genes complemented A. tumefaciens chvI and chvG mutants for growth on complex media. We were unable to construct chvI or chvG insertion mutants of R. meliloti, whereas mutants carrying insertions outside of these genes were readily obtained. A 108-bp repeat element characterized by two large palindromes was identified in the chvI and chvG intergenic regions of both Rhizobium species. This element was duplicated in Rhizobium sp. strain NGR234. Another structurally similar element with a size of 109 bp was present in R. meliloti but not in Rhizobium sp. strain NGR234. These elements were named rhizobium-specific intergenic mosaic elements (RIMEs), because their distribution seems to be limited to members of the family Rhizobiaceae. A homology search in GenBank detected six more copies of the first element (RIME1), all in Rhizobium species, and three extra copies of the second element (RIME2), only in R. meliloti. Southern blot analysis with a probe specific to RIME1 showed the presence of several copies of the element in the genome of R. meliloti, Rhizobium sp. strain NGR234, Rhizobium leguminosarum, and Agrobacterium rhizogenes, but none was present in A. tumefaciens and Bradyrhizobium japonicum.
对苜蓿根瘤菌和根瘤菌属NGR234菌株中磷酸烯醇丙酮酸羧激酶基因（pckA）上游的DNA区域进行分析，鉴定出一个开放阅读框，它与根癌土壤杆菌染色体毒性基因产物ChvI高度同源。在苜蓿根瘤菌中，第二个基因产物位于类ChvI基因下游500 bp处，与根癌土壤杆菌的ChvG蛋白同源。苜蓿根瘤菌和根癌土壤杆菌基因之间的同源性得到了证实，因为苜蓿根瘤菌的chvI和chvG基因在复杂培养基上生长时可互补根癌土壤杆菌的chvI和chvG突变体。我们无法构建苜蓿根瘤菌的chvI或chvG插入突变体，而携带这些基因以外插入片段的突变体则很容易获得。在两种根瘤菌的chvI和chvG基因间区域鉴定出一个由两个大回文序列组成的108 bp重复元件。该元件在根瘤菌属NGR234菌株中是重复的。苜蓿根瘤菌中存在另一个大小为109 bp、结构相似的元件，但在根瘤菌属NGR234菌株中不存在。这些元件被命名为根瘤菌特异性基因间镶嵌元件（RIME），因为它们的分布似乎仅限于根瘤菌科成员。在GenBank中进行的同源性搜索又检测到第一个元件（RIME1）的另外六个拷贝，均存在于根瘤菌物种中，以及第二个元件（RIME2）的另外三个拷贝，仅存在于苜蓿根瘤菌中。用RIME1特异性探针进行的Southern杂交分析表明，该元件的几个拷贝存在于苜蓿根瘤菌、根瘤菌属NGR234菌株、豆科根瘤菌和发根土壤杆菌的基因组中，但在根癌土壤杆菌和日本慢生根瘤菌中不存在。
