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将苜蓿中华根瘤菌（Ｓｉｎｏｒｈｉｚｏｂｉｕｍｍｅｌｉｌｏｔｉ）０４２Ｂ总ＤＮＡ提取，用Ｓａｕ３ＡＩ部分酶切后回收２０～３０ｋｂ长的片段，以ｐＬＡＦＲ３为载体，构建基因文库。用苜蓿中华根瘤菌ｎｏｄＡＢＣ作探针，经Ｓｏｕｔｈ－ｅｒｎ杂交获得了三个阳性克隆Ａ、Ｂ和Ｃ。将这３个阳性克隆所携带的质粒分别命名为ｐＸＣＡ，ｐＸＣＢ，ｐＸ－ＣＣ。在辅助质粒ｐＲＫ２０１３的帮助下，将这３个质粒分别转入ｎｏｄＣ－突变株ＡＫ１６５７后接种苜蓿，携带ｐＸ－ＣＢ质粒的接合子在苜蓿上结瘤。将ｐＸＣＢ用多种酶切后结合Ｓｏｕｔｈｅｒｎ杂交，得到ｐＸＣＢ的物理图谱。将包含ｎｏｄＡＢＣ的约２．８ｋｂ长的片段克隆到ｐＵＣ１８，得到质粒ｐＸＣＢ１。对ｐＸＣＢ１进行亚克隆得到测序克隆。
The total DNA of Sinorhizobium meliloti 042B was extracted and partially digested with Sau3AI to recover a 20-30 kb fragment. The pLAFR3 vector was used to construct a gene library. Using Sinorhizobium meliloti nodABC as probe, three positive clones A, B and C were obtained by Southern-ern hybridization. The plasmids carried by the three positive clones were named as pXCA, pXCB and pX-CC, respectively. With the aid of the helper plasmid pRK2013, the three plasmids were transformed into nodC-mutant AK1657 respectively and inoculated with alfalfa. The conjugate carrying pX-CB plasmid was nodulated on alfalfa. The pXCB was digested with various enzymes and combined with Southern blot to obtain the physical map of pXCB. A fragment of about 2.8 kb containing nodABC was cloned into pUC18 to give plasmid pXCB1. The cloned pXCB1 was subcloned.