Zhang X X, Turner S L, Guo X W, Yang H J, Debellé F, Yang G P, Dénarié J, Young J P, Li F D
Department of Microbiology, Huazhong Agricultural University, Wuhan 430070, China.
Appl Environ Microbiol. 2000 Jul;66(7):2988-95. doi: 10.1128/AEM.66.7.2988-2995.2000.
The nodulation genes of Mesorhizobium sp. (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mutants. The common nod genes, nodD, nodA, and nodBC, were identified by heterologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribed. The 2. 0-kb nodDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes and geographic origins. No polymorphism was found in the size of PCR products, suggesting that the separation of nodA from nodBC is a common feature of A. sinicus rhizobia. Sequence analysis of the PCR-amplified nodA gene indicated that seven strains representing different 16S and 23S ribosomal DNA genotypes had identical nodA sequences. These data indicate that, whereas microsymbionts of A. sinicus exhibit chromosomal diversity, their nodulation genes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria.
中华根瘤菌（紫云英）菌株7653R的结瘤基因通过苜蓿中华根瘤菌结瘤突变体的功能互补进行克隆。通过异源杂交和序列分析鉴定出了共同的结瘤基因nodD、nodA和nodBC。发现nodA基因与nodBC基因相隔约22 kb，并且转录方向相反。通过PCR从24株结瘤紫云英的根瘤菌菌株中扩增出2.0 kb的nodDBC区域，这些菌株代表了不同的染色体基因型和地理来源。在PCR产物大小上未发现多态性，这表明nodA与nodBC的分离是紫云英根瘤菌的一个共同特征。对PCR扩增的nodA基因进行序列分析表明，代表不同16S和23S核糖体DNA基因型的7个菌株具有相同的nodA序列。这些数据表明，虽然紫云英的微共生体表现出染色体多样性，但其结瘤基因是保守的，这支持了结瘤基因在不同受体细菌之间水平转移的假说。
