Hernández-Lucas I, Pardo M A, Segovia L, Miranda J, Martínez-Romero E
Departamento de Genética Molecular, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, México.
Appl Environ Microbiol. 1995 Nov;61(11):3992-7. doi: 10.1128/aem.61.11.3992-3997.1995.
Two genes encoding citrate synthase, a key enzyme in the Krebs cycle, have been found in Rhizobium tropici. One of them is in the bacterial chromosome, while the other is in the symbiotic plasmid. We sequenced the chromosomal gene and found that it is very similar to the previously reported plasmidic gene sequence in its structural region but not in its regulatory region. The chromosomal gene is able to complement an Escherichia coli citrate synthase mutant. In R. tropici, a mutant in the chromosomal citrate synthase gene has a diminished citrate synthase activity (in free-living bacteria), a diminished nodulation capacity, and forms nitrogen-fixing nodules. In contrast, the citrate synthase double mutant forms ineffective nodules devoid of bacteroids and forms less nodules than the single chromosomal mutant. It is inferred that both genes are functional and required during the nodulation process in R. tropici.
在热带根瘤菌中发现了两个编码柠檬酸合酶（三羧酸循环中的关键酶）的基因。其中一个位于细菌染色体上，另一个位于共生质粒中。我们对染色体基因进行了测序，发现它在结构区域与先前报道的质粒基因序列非常相似，但在调控区域则不然。该染色体基因能够互补大肠杆菌柠檬酸合酶突变体。在热带根瘤菌中，染色体柠檬酸合酶基因的突变体具有降低的柠檬酸合酶活性（在自由生活的细菌中）、降低的结瘤能力，并形成固氮根瘤。相比之下，柠檬酸合酶双突变体形成的根瘤无效，没有类菌体，且形成的根瘤比单个染色体突变体少。据推测，这两个基因在热带根瘤菌的结瘤过程中都是有功能且必需的。
