A<i>Salmonella enterica</i>Serovar Typhimurium<i>hemA</i>Mutant Is Highly Susceptible to Oxidative DNA Damage

<jats:title>ABSTRACT</jats:title><jats:p>The first committed step in the biosynthesis of heme, an important cofactor of two catalases and a number of cytochromes, is catalyzed by the<jats:italic>hemA</jats:italic>gene product.<jats:italic>Salmonella enterica</jats:italic>serovar Typhimurium<jats:italic>hemA26</jats:italic>::Tn<jats:italic>10d</jats:italic>(<jats:italic>hemA26</jats:italic>) was identified in a genetic screen of insertion mutants that were sensitive to hydrogen peroxide. Here we show that the<jats:italic>hemA26</jats:italic>mutant respires at half the rate of wild-type cells and is highly susceptible to the effects of oxygen species. Exposure of the<jats:italic>hemA26</jats:italic>strain to hydrogen peroxide results in extensive DNA damage and cell death. The chelation of intracellular free iron fully abrogates the sensitivity of this mutant, indicating that the DNA damage results from the iron-catalyzed formation of hydroxyl radicals. The inactivation of heme synthesis does not change the amount of intracellular iron, but by diminishing the rate of respiration, it apparently increases the amount of reducing equivalents available to drive the Fenton reaction. We also report that hydrogen peroxide has opposite effects on the expression of<jats:italic>hemA</jats:italic>and<jats:italic>hemH</jats:italic>, the first and last genes of heme biosynthesis pathway, respectively.<jats:italic>hemA</jats:italic>mRNA levels decrease, while the transcription of<jats:italic>hemH</jats:italic>is induced by hydrogen peroxide, in an<jats:italic>oxyR</jats:italic>-dependent manner. The<jats:italic>oxyR</jats:italic>-dependent induction is suppressed under conditions that accelerate the Fenton reaction by a mechanism that is not yet understood.</jats:p>