| Porphyromonas gingivalis is a gram negative, anaerobic bacterium implicated as a major etiological agent in chronic adult periodontitis. In the inflammatory environment of the periodontal pocket, the organism must overcome oxidative stress as a result of exposure to air in the oral cavity as well as to reactive oxygen species (ROS) generated by host immune cells. A comprehensive mechanism for oxidative stress resistance in P. gingivalis is yet to be fully elucidated. In this study we examined the modulation of gene expression under conditions of oxidative stress by whole-genome DNA microarray analysis. Our findings indicated that numerous genes are involved in oxidative stress defense including genes whose functions have not been previously characterized. Additionally, the patterns of gene expression were observed to be different based on the duration and level of oxidant exposure suggesting that P. gingivalis has the ability to quickly and specifically adapt to changing environmental conditions typical of chronic periodontitis. Under conditions of prolonged oxidative stress we observed the increased expression of five genes, grpE, dnajJ pg1777, pg1778 and pg1779, which appear to be part of a unique transcriptional unit in P. gingivalis. We generated the mutants FLL273 and FLL293 by inactivation of the hypothetical genes pg1777 and pg1779, respectively. Both mutants showed increased sensitivity to and decreased survival after treatment with hydrogen peroxide. Additionally, we demonstrated that the genes, though appearing to be co-transcribed can also be independently expressed suggesting differential regulation of this putative operon. The pg1777 gene which is predicted to be involved in iron-sulfur cluster assembly was further characterized by over-expression of its protein product. Our studies indicated that this small protein may be involved in iron binding either as part of the repair of oxidatively damaged iron-sulfur cluster proteins or protection from iron-mediated DNA damage. Characterization of this unique operon which was observed to be most highly up-regulated during prolonged oxidative stress suggests that P. gingivalis survival is critically dependent on protein repair or other cellular processes. This ability to adapt is important for its survival and may give further insights into what makes P. gingivalis a successful pathogen. |
