Diversity of immunodominant 56-kDa type-specific antigen (TSA) of Rickettsia tsutsugamushi. Sequence and comparative analyses of the genes encoding TSA homologues from four antigenic variants.

There are several antigenic variants in Rickettsia tsutsugamushi, and a type-specific antigen (TSA) of 56-kilodaltons located on the rickettsial surface is responsible for the variation. The primary structures of the protein in two variants, Gilliam and Karp, have been reported independently by us and Stover et al. by cloning and sequencing the corresponding genes (Ohashi, N., Nashimoto, H., Ikeda, H., and Tamura, A. (1990) Gene (Amst.) 91, 119-122; Stover, C. K., Marana, D. P., Carter, J. M., Roe, B. A., Mardis, E., and Oaks, E. V. (1990) Infect. Immun. 58, 2076-2084). In the present study, genes encoding the TSA homologues of the other four variants, Kato, Kawasaki, Kuroki, and Shimokoshi, which are all distinguishable serologically, were cloned and sequenced, and consequently, it became possible to compare the primary structures of the six antigenic variants. The sequence analyses revealed a complete open reading frame encoding 55,308-56,745-dalton proteins with 521-532 amino acids, in which a putative signal peptide consisting of 22 amino acids was recognized at the NH2-terminal end. Transcription of the gene is regulated by several tandem promoters. All TSA molecules have the characteristics of transmembrane proteins with alternating hydrophobic and hydrophilic regions, and contain four variable domains with spans of 16-40 amino acids which are located in the hydrophilic regions in the molecule and show different amino acid sequences among the strains. Phylogenetic classification among the R. tsutsugamushi strains based on TSA homologues supports the antigenic relationships known in the closely and distantly related strains.