The Bd1971 cNMP dimer (light/dark blue) situates its output domain (EAL) on the opposite side to that of CRP (DNA-binding subdomain, dimer coloured pink/red). Both Bd1971 and CRP dimerize via a central C-helix:C-helix interface. Close-up of hinge helix to C-helix crossover (circled). Tilt of the hinge helix relative to the C-helix allows Bd1971 to bridge from the cNMP domain body into the C-helix of the opposing monomer; the arrangement in CRP differs by pairing the C-helix with the cNMP body of the same monomer. Despite this difference, the respective C-helix dimers remain in the same orientation, as observed in panel (A). Overlay of the cAMP-binding pocket of Bd1971 and CRP (aligned on cAMP ligand, Bd1971 residue numbering given first), ligand-contacting sidechains shown in stick form, residues from the opposing monomer labeled by a '. 