3KGD:
reference: Structure of the RNA 3'-phosphate cyclase-adenylate intermediate illuminates nucleotide specificity and covalent nucleotidyl transfer., Tanaka N, Smith P, Shuman S, Structure. 2010 Mar 14;18(4):449-57. PMID: 20399182

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