Crystallization of Doc and the Phd–Doc toxin–antitoxin complex

The phd/doc addiction system is responsible for the stable inheritance of lysogenic bacteriophage P1 in its plasmidic form in Escherichia coli and is the archetype of a family of bacterial toxin–antitoxin modules. The His66Tyr mutant of Doc (Doc^H66Y) was crystallized in space group P2₁, with unit-cell parameters a = 53.1, b = 198.0, c = 54.1 Å, β = 93.0°. These crystals diffracted to 2.5 Å resolution and probably contained four dimers of Doc in the asymmetric unit. Doc^H66Y in complex with a 22-amino-acid C-terminal peptide of Phd (Phd^52-73Se) was crystallized in space group C2, with unit-cell parameters a = 111.1, b = 38.6, c = 63.3 Å, β = 99.3°, and diffracted to 1.9 Å resolution. Crystals of the complete wild-type Phd–Doc complex belonged to space group P3₁21 or P3₂21, had an elongated unit cell with dimensions a = b = 48.9, c = 354.9 Å and diffracted to 2.4 Å resolution using synchrotron radiation.