Structural and Biological Significance
Global Regulation of Nitrogen Fixation Symbiosis
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.
Structural Architecture and DNA-Binding Mechanism
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.
