Oxidative stress promotes degradation of the Irr protein to regulate haem biosynthesis in Bradyrhizobium japonicum

The haem proteins catalase and peroxidase are stress response proteins that detoxify reactive oxygen species. In the bacterium Bradyrhizobium japonicum, expression of the gene encoding the haem biosynthesis enzyme delta-aminolevulinic acid dehydratase (ALAD) is normally repressed by the Irr protein in iron-limited cells. Irr degrades in the presence of iron, which requires haem binding to the protein. Here, we found that ALAD levels were elevated in iron-limited cells of a catalase-deficient mutant, which corresponded with aberrantly low levels of Irr. Irr was undetectable in wild-type cells within 90 min after exposure to exogenous H2O2, but not in a haem-deficient mutant strain. In addition, Irr did not degrade in response to iron in the absence of O2. The findings indicate that reactive oxygen species promote Irr turnover mediated by haem, and are involved in iron-dependent degradation. We demonstrated Irr oxidation in vitro, which required haem, O2 and a reductant. A truncated Irr mutant unable to bind ferrous haem does not degrade in vivo, and was not oxidized in vitro. We suggest that Irr oxidation is a signal for its degradation, and that cells sense and respond to oxidative stress through Irr to regulate haem biosynthesis.