Cloning and characterization of a gene encoding an immunosuppressive factor from Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a pathogen of localized juvenile periodontitis and adult periodontitis. Immunomodulating activity is generally thought to be important in colonization by such pathogenic bacteria. Among the proteins possessing these activities, a 14 kDa immunosuppressive factor of A. actinomycetemcomitans has been reported by Kurita-Ochiai and Ochiai (Infect Immun 64: 50-54, 1996). To evaluate this factor, we cloned and characterized the gene encoding it. The immunosuppressive factor was screened from a genomic library of A. actinomycetemcomitans using an oligonucleotide probe based on the amino acid sequence of the factor. The clone obtained, pHI13, contained a 1.5 kbp fragment. The immunosuppressive factor located in its center. Southern blot analysis showed that this factor is common among A. actinomycetemcomitans strains. The open reading frame consisted of 324 bp coding for 107 amino acid residues. The relative molecular mass of the deduced amino acid sequence was calculated to be 11,595. BLAST analysis indicated that the amino acid sequence is highly homologous with those of thioredoxins from Haemophilus influenzae (76.6%), Neisseria meningitidis (67.3%), and Pseudomonas aeruginosa (59.3%). These results suggest that the 14 kDa immunosuppressive factor characterized in this study is a thioredoxin.