intermedia ATCC 25611. In E. coli, the transcribed leader region of tnaA contains a 72-basepair (bp) region, tnaC, which encodes a 24-residue leader peptide that is necessary for tnaA operon expression. No such sequence corresponding to the leader peptide region was identified in P. intermedia ATCC 25611. The genes upstream (nhaD) and downstream (orfY) of tnaA in P. intermedia 25611 were homologues of the genes for Na+/H+ antiporter and inner membrane

protein, respectively. There was no significant level of identity between these sequences and any of the flanking genes of P. gingivalis W83, E. coli K-12, or F. nucleatum ATCC 25586 (Fig. 1a). The transcriptional regulation of the tnaA region in P. intermedia ATCC 25611 was characterized by RT-PCR. Transcripts corresponding to the regions spanning the borders buy AUY-922 of nhaD/tnaA and tnaA/orfY were undetectable, which indicated that tnaA of P. intermedia is not cotranscribed with any flanking genes (Fig. 1b). Thus, gene organization within the tnaA region of P. intermedia ATCC 25611 was more like that of P. gingivalis W83

than F. nucleatum ATCC 25586 and E. coli K-12. Given the high degree of amino acid similarity between TnaA of P. intermedia and P. gingivalis, these results suggested that the genetic origin of the tnaA region in these two bacteria may be similar. As to why tnaB was not identified at the tnaA locus in P. intermedia, it is possible that it may be located EPZ-6438 chemical structure at another locus, or may be unnecessary in these species of bacteria. Recombinant P. intermedia ATCC 25611 TnaA was expressed as a glutathione S-transferase fusion protein and then purified by cleavage of the protein bound to glutathione-sepharose 4B. Recombinant TnaA was sufficiently pure for

enzymatic characterization based on SDS-PAGE analysis. The molecular mass of the denatured polypeptide was in good agreement with the predicted molecular mass of the protein (51 kDa) (Fig. 2). To evaluate the quaternary structure of TnaA, the IMP dehydrogenase protein was examined by gel-filtration chromatography. The enzyme eluted at approximately 107.8 kDa, as estimated using a standard curve generated using commercially available protein molecular weight standards (data not shown), which corresponded to dimers of P. intermedia TnaA. This was different from the quaternary structure of P. gingivalis TnaA, which is 70% identical to P. intermedia TnaA at the amino acid level, but is stable as a tetramer (Yoshida et al., 2009). By contrast, incubation of the tetrameric form of E. coli TnaA in potassium phosphate buffer at 5 °C led to the conversion of approximately 24% of the protein to a dimeric form (Erez et al., 1998). The kinetic activity of recombinant TnaA from P. intermedia ATCC 25611 was evaluated by spectroscopy, and the results are summarized in Table 2. The Km of P. intermedia TnaA (0.23 ± 0.01 mM) was similar to that of other bacteria, including E. coli (0.32 mM), Bacillus alvei (0.27 mM), P. gingivalis (0.20 mM), and F. nucleatum (0.