Reassessment Of DNA Binding Protein Dps in Porphyromonas gingivalis

The periodontal pathogen Porphyromonas gingivalis can grow in the presence of low levels of oxygen and exhibit resistance to peroxide stress. In the microenvironment of the periodontal pocket, concentration of reactive oxygen species such as hydrogen peroxide is 4 to 5 µM and this increases under inflammatory conditions. As an iron and porphyrin auxotroph, P. gingivalis acquires and accumulates extraneous haem for growth. However, in the presence of hydrogen peroxide, ferrous iron as well as free haem contributes to the generation of hydroxyl radicals by the Fenton reaction. The ability to control intracellular haem and detoxify ferrous iron is critical to understand the aerotolerance of P. gingivalis during the infective process. Objectives: To characterize the iron and haem binding property of Dps (DNA protective protein from starved cells) protein and determine its role in haem detoxification and oxidative protection. Methods: The recombinant P. gingivalis Dps protein was expressed and purified from E. coli hosts. Iron and haem binding properties of recombinant Dps were investigated by a series of biochemical and biophysical assays. The DNA protection assays were performed to understand the role of Dps in haem detoxification and oxidative protection. Results: Isothermal titration calorimetry and fluorescence quenching studies revealed that Dps bind haem with high affinity and is independent to the iron binding site. Controversially, P. gingivalis Dps cannot bind DNA as binding to iron or haem could not induce DNA aggregation. However, consumption of ferrous iron and binding of haem by Dps can shield DNA from damage by oxidative radicals. Conclusions: This is the first report of a bacterial Dps protein possessing haem binding property. The haem binding site of Dps is independent to the iron binding site and the protein plays a critical role in DNA protection against oxidative damage via iron/haem toxicity in P. gingivalis.