Different forms coloured thus - cAMP-bound full-length (dark/light blue), cGMP-bound full-length (red), apo Bd1971∆EAL (yellow/orange) and half-occupied cAMP Bd1971∆EAL-mid (dark/light green). Residues from the opposing monomer labeled by a '. (A) Comparative conformational shifts when aligned on C-helix dimer. One monomer from the half-occupied form is empty (mid A) and adopts a similar pose to the apo form; the other monomer has bound cAMP (mid B) and adopts a pose halfway between the apo and bound forms. (B and C) The occupied cNMP monomer of Bd1971∆EAL-mid structure (green) possesses other features conformationally intermediate between the apo (yellow) and bound (blue) states, namely the position of M59 and the juxtaposition of the D63:R110 pair. (D) Comparison of nucleotide conformations between the three differing bound states of the Bd1971 cNMP sensor. The cAMP ligand in the Bd1971∆EAL-mid structure (green), is present in the syn form, with nucleotide base pointing toward the cyclic ribose phosphate. This conformation is like that observed for cGMP (red), but opposite to that of cAMP in the full-length (blue, occupied at both sites) complex. The interaction of R111' with the syn cAMP in Bd1971∆EAL-mid kinks R108 such that it cannot make a contact with the backbone carbonyl of I55, leaving M59 in upward conformation. 