Somerville J E, Shatters R G, Kahn M L
Department of Microbiology, Washington State University, Pullman 99164-6340.
J Bacteriol. 1989 Sep;171(9):5079-86. doi: 10.1128/jb.171.9.5079-5086.1989.
The glutamine synthetase (GS)-glutamate synthase pathway is the primary route used by members of the family Rhizobiaceae to assimilate ammonia. Two forms of glutamine synthetase, GSI and GSII, are found in Rhizobium and Bradyrhizobium species. These are encoded by the glnA and glnII genes, respectively. Starting with a Rhizobium meliloti glnA mutant as the parent strain, we isolated mutants unable to grow on minimal medium with ammonia as the sole nitrogen source. For two auxotrophs that lacked any detectable GS activity, R. meliloti DNA of the mutated region was cloned and partially characterized. Lack of cross-hybridization indicated that the cloned regions were not closely linked to each other or to glnA; they therefore contain two independent genes needed for GSII synthesis or activity. One of the cloned regions was identified as glnII. An R. meliloti glnII mutant and an R. meliloti glnA glnII double mutant were constructed. Both formed effective nodules on alfalfa. This is unlike the B. japonicum-soybean symbiosis, in which at least one of these GS enzymes must be present for nitrogen-fixing nodules to develop. However, the R. meliloti double mutant was not a strict glutamine auxotroph, since it could grow on media that contained glutamate and ammonia, an observation that suggests that a third GS may be active in this species.
谷氨酰胺合成酶（GS）-谷氨酸合酶途径是根瘤菌科成员用于同化氨的主要途径。在根瘤菌属和慢生根瘤菌属物种中发现了两种形式的谷氨酰胺合成酶，即GSI和GSII。它们分别由glnA和glnII基因编码。以苜蓿中华根瘤菌glnA突变体作为亲本菌株，我们分离出了在以氨作为唯一氮源的基本培养基上无法生长的突变体。对于两个缺乏任何可检测到的GS活性的营养缺陷型菌株，对突变区域的苜蓿中华根瘤菌DNA进行了克隆并进行了部分特性分析。缺乏交叉杂交表明克隆区域彼此之间或与glnA没有紧密联系；因此，它们包含GSII合成或活性所需的两个独立基因。其中一个克隆区域被鉴定为glnII。构建了苜蓿中华根瘤菌glnII突变体和苜蓿中华根瘤菌glnA glnII双突变体。两者都在苜蓿上形成了有效的根瘤。这与日本慢生根瘤菌-大豆共生关系不同，在后者中，为了形成固氮根瘤，这些GS酶中至少必须有一种存在。然而，苜蓿中华根瘤菌双突变体不是严格的谷氨酰胺营养缺陷型，因为它可以在含有谷氨酸和氨的培养基上生长，这一观察结果表明该物种中可能有第三种GS具有活性。
