Tail-specific protease is an essential <i>Chlamydia</i> virulence factor that mediates the differentiation of elementary bodies into reticulate bodies

Tail-specific proteases (Tsp) are members of a widely distributed family of serine proteases that commonly target and process periplasmic proteins in Gram-negative bacteria. The obligately intracellular, Gram-negative <i>Chlamydia</i> encode a highly conserved Tsp homolog whose target and function are unclear. We identified a <i>Chlamydia muridarum</i> mutant with a nonsense mutation in <i>tsp</i>. Differentiation of the <i>tsp</i> mutant elementary bodies into vegetative reticulate bodies was delayed at 37°C and completely blocked at 40°C. Tsp localized to <i>C. muridarum</i> cells but was not detected outside the inclusion, suggesting that it targets chlamydial rather than host proteins. The abundance of key chlamydia outer membrane complex and virulence-related proteins differed in wild-type and <i>tsp</i> mutant elementary bodies, consistent with the possibility that Tsp regulates developmental cycle progression. The altered abundances of chlamydial structural and virulence factors could explain why the mutant, but not an isogenic recombinant with wild-type <i>tsp</i>, was highly attenuated in a mouse intravaginal infection model. Thus, chlamydial Tsp is required for timely differentiation of elementary bodies into reticulate bodies <i>in vitro</i> and is an essential virulence factor <i>in vivo</i>.