4KGK:
reference: Structural Studies of a Bacterial tRNA(HIS) Guanylyltransferase (Thg1)-Like Protein, with Nucleotide in the Activation and Nucleotidyl Transfer Sites., Hyde SJ, Rao BS, Eckenroth BE, Jackman JE, Doublie S, PLoS One. 2013 Jul 3;8(7):e67465. doi: 10.1371/journal.pone.0067465. Print 2013. PMID: 23844012

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