Schwab Stefan, Souza Emanuel M, Yates Marshall G, Persuhn Darlene C, Steffens M Berenice R, Chubatsu Leda S, Pedrosa Fábio O, Rigo Liu U
Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Paraná, Curitiba, Brazil.
Can J Microbiol. 2007 Jan;53(1):100-5. doi: 10.1139/w06-113.
Herbaspirillum seropedicae is an endophytic bacterium that fixes nitrogen under microaerophilic conditions. The putative promoter sequences glnAp1 (sigma70-dependent) and glnAp2 (sigma54), and two NtrC-binding sites were identified upstream from the glnA, ntrB and ntrC genes of this microorganism. To study their transcriptional regulation, we used lacZ fusions to the H. seropedicae glnA gene, and the glnA-ntrB and ntrB-ntrC intergenic regions. Expression of glnA was up-regulated under low ammonium, but no transcription activity was detected from the intergenic regions under any condition tested, suggesting that glnA, ntrB and ntrC are co-transcribed from the promoters upstream of glnA. Ammonium regulation was lost in the ntrC mutant strain. A point mutation was introduced in the conserved -25/-24 dinucleotide (GG-->TT) of the putative sigma54-dependent promoter (glnAp2). Contrary to the wild-type promoter, glnA expression with the mutant glnAp2 promoter was repressed in the wild-type strain under low ammonium levels, but this repression was abolished in an ntrC background. Together our results indicate that the H. seropedicae glnAntrBC operon is regulated from two functional promoters upstream from glnA, which are oppositely regulated by the NtrC protein.
血清固氮螺菌是一种在微需氧条件下固氮的内生细菌。在这种微生物的谷氨酰胺合成酶基因（glnA）、ntrB基因和ntrC基因上游，鉴定出了假定的启动子序列glnAp1（依赖于σ70）和glnAp2（依赖于σ54），以及两个NtrC结合位点。为了研究它们的转录调控，我们构建了血清固氮螺菌glnA基因、glnA-ntrB基因间区域和ntrB-ntrC基因间区域与lacZ的融合体。在低铵条件下，glnA的表达上调，但在任何测试条件下，基因间区域均未检测到转录活性，这表明glnA、ntrB和ntrC是从glnA上游的启动子共同转录的。在ntrC突变株中，铵调控丧失。在假定的依赖于σ54的启动子（glnAp2）的保守-25/-24二核苷酸（GG→TT）处引入了一个点突变。与野生型启动子相反，在低铵水平下，带有突变型glnAp2启动子的glnA表达在野生型菌株中受到抑制，但在ntrC背景下这种抑制作用被消除。我们的结果共同表明，血清固氮螺菌glnAntrBC操纵子由glnA上游的两个功能启动子调控，这两个启动子受NtrC蛋白的反向调控。
