(A) Shared common core of the BECR fold illustrated with representative structures from Barnase, RelE, ColE5, ColD, and EndoU families. PDB ids are shown in brackets. All structural cartoons are shown in an approximately similar orientation. The α-helices are colored red, β-sheets yellow and loops gray. The predicted and known active site residues are labeled. Representative domain architectures of polymorphic toxins containing (B) Tox-Barnase, (C) Tox-RelE, (D)Colicin E5 (Tox-ColE5), (E) Tox-EndoU, (F) Colicin D (Tox-ColD), and (G) several other novel toxin domains predicted to contain the BECR fold. The labeling scheme for domain architectures and alignments, and the coloring scheme and consensus abbreviations are as in Figure 3.