The crystal structure of cyclic AMP–catabolite activator protein (CAP) fromEscherichia colicontaining cobalt(II) chloride and ammonium sulfate is reported at 1.97 Å resolution. Each of the two CAP subunits in the asymmetric unit binds one cobalt(II) ion, in each case coordinated by N-terminal domain residues His19, His21 and Glu96 plus an additional acidic residue contributedviaa crystal contact. The three identified N-terminal domain cobalt-binding residues are part of a region of CAP that is

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... mportant for transcription activation at class II CAP-dependent promoters. Sulfate anions mediate additional crystal lattice contacts and occupy sites corresponding to DNA backbone phosphate positions in CAP–DNA complex structures.