Unprecedented Protein Divergence within a T3SS Family

Selection pressure drives rapid emergence of antibiotic resistance mechanisms promoting searches for therapeutic targets in bacterial processes needed for virulence, not viability, which include the Type Three Secretion System (T3SS). Distinct T3SS families evolved from the flagellar export apparatus where remaining homology hinders development of anti-T3SS specific therapies. Around 15 proteins that are highly-conserved within, but not between T3SS families yet such divergence is rarely leveraged, to promote understanding, due to unknown evolutionary histories. Here we document unprecedented divergence in two ‘LEE’ T3SS family members. Interchangeability studies uncover unusual LEE biology (eg 2-orf genes) and illustrate each T3SS protein can tolerate dramatic change. Functional defects (12 proteins) and novel phenotypes enabled studies that reveal i) pathotype-specific protein functionality, ii) T3SS crosstalk with other processes, and iii) potential therapeutic targets. The work provides resources and testable predictions for further discoveries and will promote comparable studies between distinct T3SS families. ### Competing Interest Statement The authors have declared no competing interest.