4JKR:
reference: The Mechanism of E. coli RNA Polymerase Regulation by ppGpp Is Suggested by the Structure of their Complex., Zuo Y, Wang Y, Steitz TA, Mol Cell. 2013 May 9;50(3):430-6. doi: 10.1016/j.molcel.2013.03.020. Epub 2013, Apr 25. PMID: 23623685

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