Abstract
A major outer membrane protein (the 75-kDa protein) from Porphyromonas gingivalis 381 has recently been purified and characterized. In this study, the 75-kDa protein was further investigated from molecular genetic and immunological aspects. To clone a gene for the protein, its N-terminal amino acid sequence was analyzed and DNA probes based on this sequence were synthesized. Using the probes, a recombinant plasmid clone carrying a single 4.2 kb BamH I fragment was isolated from pUC 19 libraries of P. gingivalis 381. The cloned 4.2 kb BamH I fragment was transferred to the bacteriophage T7 RNA polymerase/promoter vector system which produced a slightly larger 77 kDa protein immunoreactive to the antibody against the 75-kDa protein. Genomic Southern analysis revealed a single copy of the 75-kDa protein gene per genome among all P. gingivalis strains tested, and that no homologous genes were present in the other black-pigmented Bacteroides species. The 75-kDa protein gene, therefore, may be useful as a specific DNA probe to classify or detect this organism.
The 75-kDa protein was shown to be immunologically species-specific when immunoblot analysis was done using whole cell lysates from the same strains and the antibody against the 75-kDa protein described above. None of the species, except for P. gingivalis strains, showed any immunoreactivity to the 75-kDa protein. To detect specific antibodies against the 75-kDa protein, immunoblot analysis with sera from periodontal patients and healthy subjects was done. As a result, 65.6% of adult periodontitis patients and 100% of rapidly progressive periodontitis patients were found to have specific antibodies against the 75-kDa protein, while the sera of gingivitis patients and healthy subjects did not show positive reactions to the protein. These findings suggested that the 75-kDa protein is an immunologically species-specific and immunodominant surface antigen. The etiology of periodontitis, especially rapidly progressive periodontitis, may be related to P. gingivalis.
