<title>ABSTRACT</title>

Subtractive hybridization was employed to isolate specific genes from virulent
<italic>Porphyromonas gingivalis</italic>
strains that are possibly related to abscess formation. The genomic DNA from the virulent strain
<italic>P. gingivalis</italic>
W83 was subtracted with DNA from the avirulent strain ATCC 33277. Three clones unique to strain W83 were isolated and sequenced. The cloned DNA fragments were 885, 369, and 132 bp and had slight homology with only
<italic>Bacillus stearothermophilus</italic>
IS
<italic>5377</italic>
, which is a putative transposase. The regions flanking the cloned DNA fragments were isolated and sequenced, and the gene structure around the clones was revealed. These three clones were located side-by-side in a gene reported as an outer membrane protein. The three clones interrupt the open reading frame of the outer membrane protein gene. This inserted DNA, consisting of three isolated clones, was designated IS
<italic>1598</italic>
, which was 1,396 bp (i.e., a 1,158-bp open reading frame) in length and was flanked by 16-bp terminal inverted repeats and a 9-bp duplicated target sequence. IS
<italic>1598</italic>
was detected in
<italic>P. gingivalis</italic>
W83, W50, and FDC 381 by Southern hybridization. All three
<italic>P. gingivalis</italic>
strains have been shown to possess abscess-forming ability in animal models. However, IS
<italic>1598</italic>
was not detected in avirulent strains of
<italic>P. gingivalis</italic>
, including ATCC 33277. The IS
<italic>1598</italic>
may interrupt the synthesis of the outer membrane protein, resulting in changes in the structure of the bacterial outer membrane. The IS
<italic>1598</italic>
isolated in this study is a novel insertion element which might be a specific marker for virulent
<italic>P. gingivalis</italic>
strains.