Finan T M, Kunkel B, De Vos G F, Signer E R
J Bacteriol. 1986 Jul;167(1):66-72. doi: 10.1128/jb.167.1.66-72.1986.
Using physical and genetic data, we have demonstrated that Rhizobium meliloti SU47 has a symbiotic megaplasmid, pRmeSU47b, in addition to the previously described nod-nif megaplasmid pRmeSU47a. This plasmid includes four loci involved in exopolysaccharide (exo) synthesis as well as two loci involved in thiamine biosynthesis. Mutations at the exo loci have previously been shown to result in the formation of nodules which lack infection threads (Inf-) and fail to fix nitrogen (Fix-). Thus, both megaplasmids contain genes involved in the formation of nitrogen-fixing root nodules. Mutations at two other exo loci were not located on either megaplasmid. To mobilize the megaplasmids, the oriT of plasmid RK2 was inserted into them. On alfalfa, Agrobacterium tumefaciens strains containing pRmeSU47a induced marked root hair curling with no infection threads and Fix- nodules, as reported by others. This plant phenotype was not observed to change with A. tumefaciens strains containing both pRmeSU47a and pRmeSU47b megaplasmids, and strains containing pRmeSU47b alone failed to curl root hairs or form nodules.
利用物理和遗传数据，我们已经证明，除了先前描述的结瘤-固氮大质粒pRmeSU47a外，苜蓿中华根瘤菌SU47还具有一个共生大质粒pRmeSU47b。该质粒包含四个参与胞外多糖（exo）合成的基因座以及两个参与硫胺素生物合成的基因座。先前已表明，exo基因座的突变会导致形成缺乏感染丝（Inf-）且无法固氮（Fix-）的根瘤。因此，这两个大质粒都含有参与固氮根瘤形成的基因。另外两个exo基因座的突变并不位于任何一个大质粒上。为了转移这些大质粒，将质粒RK2的oriT插入其中。如其他人所报道的，在苜蓿上，含有pRmeSU47a的根癌土壤杆菌菌株诱导显著的根毛卷曲，但没有感染丝且形成Fix-根瘤。含有pRmeSU47a和pRmeSU47b大质粒的根癌土壤杆菌菌株未观察到这种植物表型发生变化，而仅含有pRmeSU47b的菌株未能使根毛卷曲或形成根瘤。
