4R3A:
reference: Full-length structure of a monomeric histidine kinase reveals basis for sensory regulation., Rivera-Cancel G, Ko WH, Tomchick DR, Correa F, Gardner KH, Proc Natl Acad Sci U S A. 2014 Dec 16;111(50):17839-44. doi:, 10.1073/pnas.1413983111. Epub 2014 Dec 2. PMID: 25468971

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