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== NolR ==
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The symbiosis between rhizobial microbes and legume plants is fundamental to sustainable agriculture and ecological nitrogen cycling. This partnership requires coordinated expression of multiple genes to establish nitrogen-fixing nodules, with **NolR serving as a global transcriptional regulator** controlling this critical developmental process across diverse *Rhizobium* species.
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<span style="font-size:160%"><b>Structural basis for regulation of rhizobial nodulation and symbiosis gene expression by the regulatory protein NolR </b></span>
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We present the first high-resolution X-ray crystal structures of NolR in both unliganded and DNA-bound forms, revealing its complex interactions with asymmetric operator sequences. Analysis of NolR complexed with two different 22-base pair operator DNA sequences (oligos AT and AA) demonstrates that this **homodimeric transcription factor adopts a winged helix-turn-helix fold** and recognizes DNA through a combination of positively charged surface residues that engage the DNA phosphate backbone and specific base contacts.
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Soon Goo Lee, Hari B.Krishnan and Joseph M.Jez
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The most striking finding is a **conformational switching mechanism involving Gln56**, which alters its position to accommodate variation in target DNA sequences without changing overall binding affinity. This elegant mechanism allows NolR to regulate multiple nodulation and symbiosis genes with different operator sequences through modulation of thermodynamic binding contributions. The conformational flexibility of this key residue represents a novel regulatory strategy in the ArsR/SmtB transcription factor family.
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PNAS, April 29, 2014, Vol. 111, No.17[https://doi.org/10.1073/pnas.1402243111]
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These structural studies provide unprecedented molecular insight into **how NolR functions as a global regulatory hub**, proposing two distinct regulatory models for differential gene expression during nodule formation and symbiotic nitrogen fixation. This work illuminates the structural basis for one of nature's most important agricultural and ecological partnerships.
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==Structure Tour==
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<StructureSection load='4omz' size='350' side='right' caption='Crystal Structure of NolR from Sinorhizobium fredii (PDB entry [[4omz]])' scene=''>
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==Structural and Biological Significance==
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===Global Regulation of Nitrogen Fixation Symbiosis===
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NolR is a transcriptional regulator that fine-tunes the expression of nodulation (nod) and symbiosis genes across diverse Rhizobium species. Despite its critical ecological importance, the molecular basis of NolR's regulatory mechanism remained largely unknown until the comprehensive structural characterization presented in this paper.
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=== Structural Architecture and DNA-Binding Mechanism===
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The crystallographic structures of NolR reveal a homodimeric winged '''helix-turn-helix''' transcription factor, comprising two α-helical regions ('''α1 and α5''') forming the dimerization interface and a triangular configuration of helices ('''α2–α4''') that positions the conserved helix-turn-helix motif ('''α3–α4''') for DNA major groove binding. Notably, a distinctive "wing" composed of antiparallel β-sheets extends into the DNA minor groove. This architectural arrangement enables NolR to recognize '''asymmetric operator sequences'''—a remarkable feature that confers specificity and versatility in binding diverse target genes.
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</StructureSection>

Current revision

Structural basis for regulation of rhizobial nodulation and symbiosis gene expression by the regulatory protein NolR

Soon Goo Lee, Hari B.Krishnan and Joseph M.Jez

PNAS, April 29, 2014, Vol. 111, No.17[1]

Structure Tour

Crystal Structure of NolR from Sinorhizobium fredii (PDB entry 4omz)

Drag the structure with the mouse to rotate
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