2AHQ:
reference: The C-terminal RpoN domain of sigma54 forms an unpredicted helix-turn-helix motif similar to domains of sigma70., Doucleff M, Malak LT, Pelton JG, Wemmer DE, J Biol Chem. 2005 Dec 16;280(50):41530-6. Epub 2005 Oct 6. PMID: 16210314

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