Search for pathogenic factor in Rickettsia tsutsupamushi.

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There are several antigenic variants in Rickettsia tsutsugamushi. This antigenic variant is due to the antigenicities of 56-kilodalton(K)protein located on the rickettsial surface. We found recently that the virulence of individual strain to mice relates to the antigenic type of rickettsiae. This result suggests that the 56K protein may decide the virulence of this rickettsia. Therefore we analyzed, in this study, the molecular structure of 56K proteins from 3 virulent strains of Gilliam, Karp, and Kato, and 3 avirulent strains of Shimokoshi, Kawasaki, and Kuroki, by biochemical and gene-cloning methods, and compared the differences of the molecular structure among the strains. The results obtained are summarized as follows. (1)Promoter sequences and ribosome-binding Mite were deduced in the 5'-side flanking regions of cloned genes, of which the sequences are similar with the consensus and Shine-Dalgarno sequences in Escherichia coli, respectively. (2)Signal peptide composed of 22-amino acids was observed at the N-terminal side of the deduced amino acid sequences from all the six strains. (3)In the 56K protein molecules, 4 variable domains showing remarkable variations of amino acid sequences among the strains were recognized. (4)Hydrophobic and hydrophilic regions appeared alternatively in the molecules, showing typical properties of membrane protein. All the 4 variable domains located in the hydrophilic regions, suggesting that these domains are exposed on the surface of rickettsiae and antigenic site(epitope)may exist in these areas. (5)A hairpin loop structure was seen at the 3'-side flanking region of the gene about 20 bp downstream from the stop codon except the case of Kawasaki strain. (6)The variations of amino acid sequences among the strains were larger and more complex than we expected. To identify the sequence which relates to the virulence of the rickettsia, more detail studies on this protein are required in future.