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

Molecular Microbiology (2006) 60(1), 209–218

Speaker：葉駿達                         Commentator：吳俊忠老師

Date：2006/10/25 14:10~15:00                Place：Room 601

Abstract：

  Reactive oxygen species (ROS), such as superoxide and hydroxyl radicals, may generate from aerobic respiration or the environment and cause damage to cells. Bacteria use multiple strategies against oxidative stress, including the detoxification of ROS by catalase and peroxidase, the haem proteins (1). In Bradyrhizobium japonicum, a rhizobia that exist in symbiosis with leguminous plants, the gene expression of the haem synthesis enzyme δ-aminolevulinic acid dehydrotase (ALAD) is repressed by the iron response regulator (Irr) protein in iron-limited cells. It was also found that Irr degraded rapidly when the cell was exposed to iron, which allowed the derepression of haem synthesis (2). The authors wanted to know the interaction between ROS and Irr, and the effect of ROS on haem synthesis. In this study, they found that the ALAD levels increased in iron-limited cells of a catalase-deficient mutant, and this corresponded with low levels of Irr. In addition, Irrwas undetectable in the wild-type cells, but not in the haem-deficient mutant, after exposure to H₂O_2, suggesting that haem was associated with the decreased level of Irr. On the other hand, Irr was degraded in response to iron in the presence, but not in the absence, of O₂ indicating that both iron and O₂ were required for the degradation of Irr. These suggest that ROS might promote Irrturnover mediated by haem. They also found that the degradation of Irr was via oxidation that required haem, O₂ and a reductant. These results demonstrate that Irr regulates haem biosynthesis in response to oxidative stress in this organism.

References：

(1) Jianhua Yang, Heather R. Panek and Mark R. O’Brian (2006) Oxidative stress promotes degradation of the Irr protein to regulate haem biosynthesis in Bradyrhizobium japonicum. Molecular Microbiology 60(1), 209–218

(2) Hamza, I., Chauhan, S., Hassett, R., and O’Brian, M.R. (1998) The bacterial irr protein is required for coordination of heme biosynthesis with iron availability. J Biol Chem 273: 21669–21674.