紫云英根瘤菌159中两个结构和功能不同的nodD基因

Rhizobium astragali 159 Has Two nodD Genes That Differ in Primary Structure and Function

紫云英根瘤菌159含2个拷贝的nodD基因。核苷酸序列测定表明由nodD1和nodD2基因所推定编码的nodD1和nodD2蛋白其氨基酸顺序同源性为69％。功能分析显示，在紫云英根瘤菌中nodD1和nodD2基因均能自主表达。由nodD1推定的nodD1能与紫云英种子浸出液（诱导物）共同激活nod基因的表达，而由nodD2推定的nodD2则无此功能。

Rhizobium astragali strain159 has two homologs of the nodulationregulatory gene, nodD. These homologs,nodD1 and nodD2, are located upstreamof the nodBC gene cluster and the nodAgene respectively. The two predctedNodD proteins, as derived from the nucleotide sequence of the respectivegenes, shared 69% identical amino acidresidues at corresponding positions andhad the structural characteristics knownfor NodD proteins from other rhizobia: ahighly conserved sequence containing thehelix-turn-helix motif for DNA binding.However, compared with NodD1 protein, NodD2 had 67 amino acid deletionsin the N-terminal region (Fig. 3). Measurements of β-galactosidase activitiesrevealed tha nodD1-lacZ and nodD2-lacZ fusion were expressed constitutivelyin R. astragali 159 (Table 1 ). Thepredicted NodD1 could activate the expression of R. meliloti nodC-lacZ fusionin the presence of Astragalus sinicus seedexudate, but NodD2 could not (Table2).