HtrA conservation between Chlamydia and E. coli shows likely functional redundancy between bacterial species. (A) Schematic of the HtrA protein from C. trachomatis is shown. The predicted N-terminal secretion signal sequence (shaded) and signal protease cleavage site (A25-S26), serine protease domain and two C-terminal PDZ domains are shown. The percentage of conservation of amino acid sequences (identical residues) for each of these regions between Chlamydia species, between all Chlamydia and Parachlamydia, and between E. coli HtrA and C. trachomatis HtrA (E. coli) are indicated below. An alignment of the residues surrounding the three essential residues for serine protease catalytic triad (H146, D179, S247; diamonds) from C. trachomatis L2 and E. coli HtrA is shown. (B) Immunohistochemistry using HtrA polyclonal sera with HEp-2 cells infected with C. pneumoniae AR39 at 72 h PI. C. pneumoniae AR39 infections were centrifuged for 30 mins at 2000 rpm and media changed at 2 h PI to media with 1 mg/ml cyclohexamide. Cells were fixed and stained as described in Methods.