Mtd protein structure and SMPL diversity generation.

(A) Bordetella 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) [25]. 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 Bordetella bronchiseptica [25], [26]. (B) This schematic gene map of the diversity generating retroelement identifies mutable codons within the tr. The targeted codons include adenines outside the wobble position, and encode the underlined amino acids to generate 9.2×10¹² variants of the Mtd protein. (C) Generation of the BP-SMPLs through phage production by infecting Bvg⁺Bordetella bronchiseptica, followed by propagation in either Bvg⁺ or Bvg⁻ phase Bordetella strains.