Role of superoxide dismutase activity in the physiology of Porphyromonas gingivalis

Porphyromonas gingivalis is a gram-negative, obligate anaerobe strongly associated with chronic adult periodontitis. A previous study has demonstrated that this organism requires superoxide dismutase (SOD) for its modest aerotolerance. In this study, we have constructed a mutant deficient in SOD activity by insertional inactivation as well as a sod::lacZ reporter translational fusion construct to study the regulation of expression of this gene. We have confirmed that SOD is essential for tolerance to atmospheric oxygen but does not appear to be protective against hydrogen peroxide or exogenously generated reactive oxygen species. Furthermore, the sod mutant appeared to be no more sensitive to killing by neutrophils than the parental strain 381. SOD appears to be protective against oxygen-dependent DNA damage as measured by increased mutation to rifampin resistance by the sod mutant. Use of the sod::lacZ construct confirmed that SOD expression is maximal at mid-log phase and is influenced by oxygen, temperature, and pH. However, expression does not appear to be significantly affected by iron depletion, osmolarity, or nutrient depletion. The transcription start site of the sod gene was determined to be 315 bp upstream of the sod start codon and to be within an upstream open reading frame. Our studies demonstrate the essential role that SOD plays in aerotolerance of this organism as well as the selective induction of this enzyme by environmental stimuli.