Mtd protein structure and SMPL diversity generation.

<p>(A) <i>Bordetella</i> phage expresses six distal tail fibers, with the Mtd protein located at the end of each tail fiber. The Mtd structure consists of a homotrimer found on the tail fibers of the BP (PDB: 1YU4) <a href="" target="_blank">[25]</a>. During SMPL formation, the variable region (VR) of the trimer (purple spheres) is diversified, and is responsible for determining binding specificity to target proteins on the surface of <i>Bordetella bronchiseptica </i><a href="" target="_blank">[25]</a>, <a href="" target="_blank">[26]</a>. (B) This schematic gene map of the diversity generating retroelement identifies mutable codons within the <i>tr</i>. The targeted codons include adenines outside the wobble position, and encode the underlined amino acids to generate 9.2×10<sup>12</sup> variants of the Mtd protein. (C) Generation of the BP-SMPLs through phage production by infecting Bvg<sup>+</sup><i>Bordetella bronchiseptica</i>, followed by propagation in either Bvg<sup>+</sup> or Bvg<b><sup>−</sup></b> phase <i>Bordetella</i> strains.</p>