SYMBIOTIC NITROGEN-FIXATION BY A NIFA DELETION MUTANT OF RHIZOBIUM-MELILOTI - THE ROLE OF AN UNUSUAL NTRC ALLELE
SYMBIOTIC NITROGEN-FIXATION BY A NIFA DELETION MUTANT OF RHIZOBIUM-MELILOTI - THE ROLE OF AN UNUSUAL NTRC ALLELE
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DOI：
10.1128/jb.175.9.2662-2673.1993
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发表时间：
1993-05-01
影响因子：
3.2
通讯作者：
FINAN, TM
中科院分区：
文献类型：
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作者：
LABES, M;RASTOGI, V;FINAN, TM
在固氮紫花苜蓿共生体meliloti根瘤菌中，三种依赖sigma54 （NTRA）的正作用调节蛋白NIFA、NTRC和DCTD分别是激活参与固氮（pnifHDKE和pfixABCX）、氮吸收（pglnII）和c4 -二羧酸转运（pdctA）启动子所必需的。在这里，我们描述了ntrC的一个等位基因，它导致上述ntrC -、NIFA-和dctd调控的启动子的组成性激活。野生型NTRC的表达和激活是对氮有效性的响应，而在携带ntrC283等位基因的细胞中，ntrC283蛋白表现为组成性活性并组成性表达。结果显示，ntrC283等位基因携带一个单一突变，导致ntrC283的螺旋-转-螺旋基序中的一个Asp被一个Tyr残基取代。在nifA缺失突变体中引入ntrC283等位基因可将n2固定能力恢复到野生型水平的70 - 80%。因此，nifA基因对于共生固氮是不可缺少的。
In the N2-fixing alfalfa symbiont Rhizobium meliloti, the three sigma54 (NTRA)-dependent positively acting regulatory proteins NIFA, NTRC, and DCTD are required for activation of promoters involved in N2 fixation (pnifHDKE and pfixABCX), nitrogen assimilation (pglnII), and C4-dicarboxylate transport (pdctA), respectively. Here, we describe an allele of ntrC which results in the constitutive activation of the above NTRC-, NIFA-, and DCTD-regulated promoters. The expression and activation of wild-type NTRC occur in response to nitrogen availability, whereas in cells carrying the ntrC283 allele, the NTRC283 protein appears constitutively active and is constitutively expressed. The ntrC283 allele was shown to carry a single mutation resulting in the replacement of an Asp by a Tyr residue in the helix-turn-helix motif of ntrC283. Introduction of the ntrC283 allele into a nifA deletion mutant restores the N2-fixation ability to 70 to 80% of the wild-type level. Thus, the nifA gene is dispensable for symbiotic N2 fixation.