Isolates ofChlamydia trachomatisThat Occupy Nonfusogenic Inclusions Lack IncA, a Protein Localized to the Inclusion Membrane

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Abstract
The chlamydiae are obligate intracellular pathogens that occupy a nonacidified vacuole, termed an inclusion, throughout their developmenal cycle. When an epithelial cell is infected with multipleChlamydia trachomatiselementary bodies, they are internalized by endocytosis into individual phagosomal vacuoles that eventually fuse to form a single inclusion. In the course of large-scale serotyping studies in which fluorescent antibody staining of infected cells was used, a minority of strains that had an alternate inclusion morphology were identified. These variants formed multiple nonfusogenic inclusions in infected cells, with the number of independent inclusions per cell varying directly with the multiplicity of infection. Overall the nonfusogenic phenotype was found in 1.5% (176 of 11,440) of independent isolates. Nonfusing variants were seen inC. trachomatisserovars B, D, D−, E, F, G, H, Ia, J, and K. The nonfusing phenotype persisted through repeated serial passage, and the phenotype was consistent in four mammalian host cell lines. Fluorescence microscopy and immunoblotting with antisera directed at proteins in theC. trachomatisinclusion membrane revealed that one such protein, IncA, was not detected in the inclusion membrane in each tested nonfusogenic strain. The distributions of other chlamydial proteins, including one additional Inc protein, were similar in wild-type and variant strains. TheincAcoding and upstream regions were amplified and sequenced from the prototype serovar D and two nonfusing serovar D(s)strains. Three nucleotide changes were discovered in the D(s)incAgene, leading to two amino acid changes within the predicted D(s)IncA sequence. These studies demonstrate a subgroup of variantC. trachomatisisolates that form nonfusing inclusions; the variant phenotype is associated with the absence of detectable IncA and with an alteredincAsequence that modifies the characteristic hydrophobic domain of the IncA protein.