Domain‐Specific Rearrangement between the Two Arg‐Gingipain‐Encoding Genes in Porphyromonas gingivalis: Possible Involvement of Nonreciprocal Recombination

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Abstract
Porphyromonas gingivalis has two functional genes (rgpA and rgpB) encoding an arginine‐specific cysteine proteinase (Arg‐gingipain, RGP). Several RGP‐encoding genes have been cloned and sequenced from various P. gingivalis strains, but all of the genes seem to be essentially equivalent to rgpA. In this study, we cloned and sequenced the second rgp gene (rgpB). A comparison of the rgpB gene and the rgp1 gene, one of the rgpA‐equivalent genes, revealed that their gene structures were very similar to each other, except that the rgpB gene did not possess most of the hemagglutinin domain present in the C‐terminal region of the rgpl gene, and provided strong evidence for homologous recombination between the proteinase domain regions of the two rgp genes. The presence of nonreciprocal recombination in P. gingivalis was experimentally proven using suicide/integration plasmid systems. The results provide one of the hypothetical scenarios of the generation of the two rgp genes; that is, they have been generated through the duplication of an ancestor rgp gene, insertion of the hemagglutinin domain region into one copy of the two resulting rgp genes (or deletion of the region from one rgp) and homologous recombination between the proteinase domain regions of the two rgp genes.