The Porphyromonas gingivalis Hybrid Cluster Protein Hcp Is Required for Growth with Nitrite and Survival with Host Cells

Although the periodontal pathogen Porphyromonas gingivalis must withstand high levels of nitrosative stress while in oral cavity the mechanisms of nitrosative stress defense are not well understood in this organism. Previously we have shown that the transcriptional regulator HcpR plays a significant role in the defense and here we further defined its regulon. Our study shows that hcp (PG0893), a putative nitric oxide (NO) reductase, is the only gene significantly upregulated in response to nitrite (NO 2 ) and this regulation is dependent on HcpR. Isogenic mutant deficient in hcp is not able to grow with 200 μM of nitrite thus demonstrating that the sensitivity of the HcpR mutant is mediated through Hcp. We further define the molecular mechanisms of HcpR interaction with the hcp promoter through mutational analysis of the inverted repeat present within the promoter. Although other putative nitrosative stress protection mechanisms present on the nrfAH operon, are also encoded on the P. gingivalis genome we show that their gene products play no role in growth of the bacterium with nitrite. As growth of the hcp deficient strain was also significantly diminished in the presence of nitric oxide producing compound, S-nitrosoglutathione (GSNO), Hcp appears to be the primary means by which P. gingivalis responds to NO 2 - based stress. Finally, we show that Hcp is required for survival with host cells but loss of Hcp has no effect on association and entry of P. gingivalis into human oral keratinocytes.