Study on correlation between virulence and molecular structure of type-specific antigen of Rickettsia tsutsugamushi

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Various antigenic variants are recognized in Rickettsia tsutsugamushi, and our previous studies indicated the relationship between the antigenic type of this rickettsia and its virulence to mice. Because the antigenic variety of this microorganism is due to the type-specific antigen of 56-KDa protein located on the rickettsial surface, the difference of virulence in each strain seems to depend on the molecular structure of this protein. In this study, the whole molecular structure of 56-KDa proteins originated from high-virulent strains of Gilliam, Karp and Kato, and from low-virulent strains of Kawasaki, Kuroki and Shimokoshi, were determined by chemical analysis of purified 56-KDa proteins, and by cloning and sequencing methods of 56-KDa protein genes, and the structures were compared among the strains. The results obtained are as follows :(1) the proteins consisted of 521-532 amino acids (corresponding to 55308-56745 dalton),(2) a putative signal peptide consisting of 22 amino acids was recognized at the N-terminal end,(3) transcription of the gene is regulated by several tandem promoters,(4) the homologies of these proteins among the six strains varied from 60 to 82% in the amino acid sequences,(5) 4 variable domains with spans of 16-40 amino acids were recognized in the molecules,(6) 56-KDa protein genes from several newly isolated strains amplified by PCR method showed polymorphism in the restriction enzyme fragment analyzes, and serotype strains of Gilliam, Karp and Kuroki strains were further classified into several subtypes.