Crystallization and preliminary X-ray diffraction studies of the ζ addiction system encoded by Streptococcus pyogenes plasmid pSM19035

The proteins encoded by the Streptococcus pyogenes broad-host range and low copy-number plasmid pSM19035 form a toxin–antitoxin module that secures stable maintenance by causing the death of plasmid-free segregants. The ζ protein complex was crystallized in four different forms at pH 5.0 and pH 7.0 using the vapour-diffusion method with PEG 3350 and ethylene glycol as precipitants. Three of the crystal forms were obtained in the same droplet under identical conditions at pH 5.0. One form belongs to the enantiomorphic space groups P4₃2₁2 or P4₁2₁2. For the other two, the X-­ray reflection conditions match those of space group P2₁2₁2₁, one representing a superlattice of the other. A crystal form growing at pH 7.0 also belongs to space group P2₁2₁2₁, but there is no indication of a structural relationship to the other orthorhombic forms. Initially, the crystals diffracted to 2.9 Å resolution and diffracted to 1.95 Å after soaking at pH 7.0. A preparation of selenomethionyl ζ protein complex yielded single crystals suitable for X-ray diffraction experiments using synchrotron sources.