CLONING AND SEQUENCING OF THE GENE FOR A 120-KDA IMMUNODOMINANT PROTEIN OF EHRLICHIA-CHAFFEENSIS

Ehrlichia chaffeensis is the tick-borne, obligately intracellular bact erium that causes human monocytic ehrlichiosis. A 120-kDa protein is o ne of the immunodominant proteins of E. chaffeensis that stimulates pr oduction of specific antibodies in infected humans. A genomic library of E. chaffeensis was constructed in a lambda ZAP II phage vector, and a clone expressing the 120-kDa protein of E. chaffeensis was identifi ed using canine anti-E chaffeensis serum. DNA sequence analysis of the cloned 120-kDa protein gene of E. chaffeensis identified a 1884-bp op en reading frame with an ehrlichial promoter. Five identical 240-bp ta ndem repeat units were identified in the 120-kDa protein gene of E. ch affeensis, comprising 60% of the entire gene. Aside from the first rep eat unit, all the other repeat units are identical. In the first repea t unit there are four nucleotides that are different from the other re peats. Hydropathy analysis of the deduced amino-acid sequence demonstr ated that the repeat domain contains highly hydrophilic segments. The 120-kDa protein should be evaluated for a role in stimulating protecti ve immunity.