Characterization of two genes encoding ferritin-like protein in Actinobacillus actinomycetemcomitans

Two genes encoding ferritin-like protein, designated afnA and afnB, were id entified in the upstream region of actX on the Actinobacillus actinomycetem comitans chromosomal DNA. The actX has been reported to be a regulatory gen e homologous to the Escherichia coli fnr, which controls the growth and vir ulence of A. actinomycetemcomitans under anaerobic conditions. The afnB loc ated 340 bp-upstream from the actX, and the afnA located just 15 bp-upstrea m from afnB. The afnA and afnB encoded 161 and 165 amino acid residues, res pectively, which were similar to ferritin-like proteins of other microorgan isms. Western immunoblotting using rabbit antiserum against E. coli ferriti n showed these two proteins, which are reactive with the serum with 19-kDa molecular masses, are produced from A. actinomycetemcomitans. The N-termina l amino acid sequences of the two proteins were consequent with those deduc ed from afnA and afnB. Northern hybridization revealed that the afnA and af nB constituted a bicistronic operon and the accumulation of afnA and afnB m RNA was upregulated under aerobic conditions. These findings suggested that the operon was regulated by the presence of oxygen. The two ferritin-like proteins may have important roles in the adaptation of A. actinomycetemcomi tans to oxidative environmental changes.