Chlamydia trachomatis is a Gram-negative obligate intracellular bacterium that is the causative agent of common sexually transmitted diseases and the leading cause of preventable blindness worldwide. It has been observed that YtgA (CT067) is very immunogenic in patients with chlamydial genital infections. Homology analyses suggested that YtgA is a soluble periplasmic protein and a component of an ATP-binding cassette (ABC) transport system for metals such as iron. Since little is known about iron transport in C. trachomatis, biochemical assays were used to determine the potential role of YtgA in iron acquisition. (59)Fe binding and competition studies revealed that YtgA preferentially binds iron over nickel, zinc or manganese. Western blot and densitometry techniques showed that YtgA concentrations specifically increased 3-5-fold in C. trachomatis, when cultured under iron-starvation conditions rather than under general stress conditions, such as exposure to penicillin. Finally, immuno-transmission electron microscopy provided evidence that YtgA is more concentrated in C. trachomatis during iron restriction, supporting a possible role for YtgA as a component of an ABC transporter.