2RBF:
reference: Structural basis of the transcriptional regulation of the proline utilization regulon by multifunctional PutA., Zhou Y, Larson JD, Bottoms CA, Arturo EC, Henzl MT, Jenkins JL, Nix JC, Becker DF, Tanner JJ, J Mol Biol. 2008 Aug 1;381(1):174-88. Epub 2008 Jun 7. PMID: 18586269

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).