Fur-independent regulation of iron metabolism by Irr in Bradyrhizobium japonicum

Bradyrhizobium japonicum expresses both Fur and Irr, proteins that mediate iron-dependent regulation of gene expression. Control of irr mRNA accumulation by iron was aberrant in a fur mutant strain, and Fur repressed an irr:: lacZ promoter fusion in the presence of iron. Furthermore, metal-dependent binding of Fur to an irr gene promoter was demonstrated in a region with no significant similarity to the Fur-binding consensus DNA element. These data suggest that the modest control of irr transcription by iron is mediated by Fur. However, Irr protein levels were regulated normally by iron in the fur strain, indicating that Fur is not required for posttranscriptional control of the irr gene. Accordingly, regulation of hemB, a haem biosynthesis gene regulated by Irr, was controlled normally by iron in a fur strain. In addition, the hemA gene was shown to be controlled by Fur, but not by Irr. It was concluded that Fur cannot be the only protein by which B. japonicum cells sense and respond to iron, and that Irr may be involved in Fur-independent signal transduction. Furthermore, iron-dependent regulation of haem biosynthesis involves both Irr and Fur.