The crystal structure of trp aporepressor at 1.8 Å shows how binding tryptophan enhances DNA affinity

@article{Zhang1987TheCS,
  title={The crystal structure of trp aporepressor at 1.8 {\AA} shows how binding tryptophan enhances DNA affinity},
  author={R.-g. Zhang and Andrzej J Joachimiak and Catherine L. Lawson and Richard Walter Schevitz and Zbyszek Otwinowski and Paul B. Sigler},
  journal={Nature},
  year={1987},
  volume={327},
  pages={591-597}
}
Comparison of the crystal structure of inactive unliganded trp aporepressor with that of trp repressor shows that binding tryptophan activates the dimer a thousandfold by moving two symmetrically-disposed flexible bihelical motifs. These flexible 'DNA-reading heads' flank a highly inflexible core domain formed by an unusual arrangement of interlocking α-helices from both subunits. 
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