Background: The Bacteroidetes and Chlorobi species constitute two main groups of the Bacteria that are closely related in phylogenetic trees. The Bacteroidetes species are widely distributed and include many important periodontal pathogens. In contrast, all Chlorobi are anoxygenic obligate photoautotrophs. Very few (or no) biochemical or molecular characteristics are known that are distinctive characteristics of these bacteria, or are commonly shared by them.

Comparative genomic studies have identified many proteins that are found only in various Chlamydiae species and exhibit no significant sequence similarity to any protein in organisms that do not belong to this group. The CT670 protein of Chlamydia trachomatis is one of the proteins whose genes are in one of the type III secretion gene clusters but whose cellular functions are not known. CT670 shares several characteristics with the YscO protein of Yersinia pestis, including the neighboring genes, size, charge, and secondary structure, but the structures and/or functions of these proteins remain to be determined. Although a BLAST search with CT670 did not identify YscO as a related protein, our analysis indicated that these two proteins exhibit significant sequence similarity. In this paper, we report that the CT670 crystal, solved at a resolution of 2 Å, consists of a single coiled coil containing just two long helices. Gel filtration and analytical ultracentrifugation studies showed that in solution CT670 exists in both monomeric and dimeric forms and that the monomer predominates at lower protein concentrations. We examined the interaction of CT670 with many type III secretion system-related proteins (viz., CT091, CT665, CT666, CT667, CT668, CT669, CT671, CT672, and CT673) by performing bacterial two-hybrid assays. In these experiments, CT670 was found to interact only with the CT671 protein (YscP homolog), whose gene is immediately downstream of ct670. A specific interaction between CT670 and CT671 was also observed when affinity chromatography pull-down experiments were performed. These results suggest that CT670 and CT671 are putative homologs of the YcoO and YscP proteins, respectively, and that they likely form a chaperone-effector pair.Chlamydiae are obligate intracellular bacteria that infect a variety of eukaryotes, including humans, animals, insects, and free-living amoebae (8, 31, 51). They are highly pathogenic and cause genital tract, ocular, and respiratory infections in humans (9, 55). A key characteristic of Chlamydiae is their biphasic developmental cycle, in which the bacteria alternate between two morphologies: the elementary body and the reticulate body (1, 31). The Chlamydiae species, like many other Gram-negative pathogenic bacteria, contain a type III secretion (T3S) system, which plays a major role in their pathogenicity (4, 7, 44). This key system aids pathogenicity by exporting bacterial proteins, termed effectors, into the host cell via a syringe-like nanomachine called an injectisome (4, 7). Once secreted into the host cell, these effectors may manipulate host cell functions to the advantage of the pathogen (44, 54).Because of the unique chlamydial developmental cycle currently there are no tractable methods for genetic manipulation of these organisms (25,49). Detailed bioinformatic investigations have identified approximately 200 proteins unique to various taxonomic levels of the Chlamydiae phylum (19, 21). Included in this pool of proteins are proteins whose genes occur in chlamydi...