3R1L:
reference: The structural basis of RNA-catalyzed RNA polymerization., Shechner DM, Bartel DP, Nat Struct Mol Biol. 2011 Aug 21;18(9):1036-42. doi: 10.1038/nsmb.2107. PMID: 21857665

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