Abstract
The paaR2-paaA2-parE2 operon is a three-component toxin-antitoxin module encoded in the genome of the human pathogen Escherichia coli O157. The toxin (ParE2) and antitoxin (PaaA2) interact to form a nontoxic toxin-antitoxin complex. In this paper, the crystallization and preliminary characterization of two variants of the ParE2-PaaA2 toxin-antitoxin complex are described. Selenomethionine-derivative crystals of the full-length ParE2-PaaA2 toxin-antitoxin complex diffracted to 2.8 Å resolution and belonged to space group P41212 (or P43212), with unit-cell parameters a = b = 90.5, c = 412.3 Å. It was previously reported that the full-length ParE2-PaaA2 toxin-antitoxin complex forms a higher-order oligomer. In contrast, ParE2 and PaaA213-63, a truncated form of PaaA2 in which the first 12 N-terminal residues of the antitoxin have been deleted, form a heterodimer as shown by analytical gel filtration, dynamic light scattering and small-angle X-ray scattering. Crystals of the PaaA213-63-ParE2 complex diffracted to 2.7 Å resolution and belonged to space group P6122 (or P6522), with unit-cell parameters a = b = 91.6, c = 185.6 Å.