Foster-Hartnett D, Cullen P J, Monika E M, Kranz R G
Department of Biology, Washington University, St. Louis, Missouri 63130.
J Bacteriol. 1994 Oct;176(20):6175-87. doi: 10.1128/jb.176.20.6175-6187.1994.
The enteric NtrC (NRI) protein has been the paradigm for a class of bacterial enhancer-binding proteins (EBPs) that activate transcription of RNA polymerase containing the sigma 54 factor. Activators in the NtrC class are characterized by essentially three properties: (i) they bind to sites distant from the promoters that they activate (> 100 bp upstream of the transcriptional start site), (ii) they contain a conserved nucleotide-binding fold and exhibit ATPase activity that is required for activation, and (iii) they activate the sigma 54 RNA polymerase. We have characterized the NtrC protein from a photosynthetic bacterium, Rhodobacter capsulatus, which represents a metabolically versatile group of bacteria found in aquatic environments. We have shown that the R. capsulatus NtrC protein (RcNtrC) binds to two tandem sites that are distant from promoters that it activates, nifA1 and nifA2. These tandem binding sites are shown to be important for RcNtrC-dependent nitrogen regulation in vivo. Moreover, the conserved nucleotide-binding fold of RcNtrC is required to activate nifA1 and nifA2 but is not required for DNA binding of RcNtrC to upstream activation sequences. However, nifA1 and nifA2 genes do not require the sigma 54 for activation and do not contain the highly conserved nucleotides that are present in all sigma 54-type, EBP-activated promoters. Thus, the NtrC from this photosynthetic bacterium represents a novel member of the class of bacterial EBPs. It is probable that this class of EBPs is more versatile in prokaryotes than previously envisioned.
肠道NtrC（NRI）蛋白一直是一类细菌增强子结合蛋白（EBP）的范例，这类蛋白可激活含有σ54因子的RNA聚合酶的转录。NtrC类激活蛋白主要具有三个特性：（i）它们结合于与其激活的启动子相距较远的位点（转录起始位点上游>100 bp）；（ii）它们含有保守的核苷酸结合结构域，并表现出激活所需的ATP酶活性；（iii）它们激活σ54 RNA聚合酶。我们对光合细菌荚膜红细菌的NtrC蛋白进行了表征，该细菌代表了在水生环境中发现的代谢功能多样的一类细菌。我们已经表明，荚膜红细菌NtrC蛋白（RcNtrC）结合于两个串联位点，这些位点与其激活的启动子nifA1和nifA2相距较远。这些串联结合位点在体内对RcNtrC依赖的氮调节很重要。此外，RcNtrC保守的核苷酸结合结构域是激活nifA1和nifA2所必需的，但不是RcNtrC与上游激活序列DNA结合所必需的。然而，nifA1和nifA2基因激活不需要σ54，也不包含所有σ54型EBP激活启动子中存在的高度保守核苷酸。因此，这种光合细菌的NtrC代表了细菌EBP类的一个新成员。这类EBP在原核生物中可能比以前设想的更具多样性。
