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

Abstract

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, metaldependent 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 Furindependent signal transduction. Furthermore, iron-dependent regulation of haem biosynthesis involves both Irr and Fur.

A Novel DNA-binding Site for the Ferric Uptake Regulator (Fur) Protein from Bradyrhizobium japonicum

“…shows that the iron-dependent control of irr mRNA expression is aberrant in a fur mutant and that E. coli extracts that overexpress B. japonicum Fur (BjFur) bind to DNA corresponding to the irr gene upstream region (22). However, a Fur boxlike element was not found in this region.…”

“…irr promoter fragments were generated by PCR amplification of irr sequence in pSKSBIrr using primers 5Ј-TTTGAATTCGTGAC-CAAAATCGGCTAAG-3Ј and 5Ј-TTTCTGCAGGGACGTCGTCGTCGT-GAT-3Ј, producing a 272-bp DNA fragment that terminated 80 bp downstream of the irr transcription start site determined previously (22). Similar PCR reactions were performed on pSKSBIrr derivatives with four base pair alterations that independently affected formation of the high mobility complex or low mobility complex but did not mutate the putative Ϫ35 box.…”

“…In addition to Fur, the Irr protein in B. japonicum is involved in iron metabolism, where it mediates iron-dependent regulation of heme biosynthesis (7). The irr gene is controlled by iron at both transcriptional (22) and post-translational levels (8,23). Evidence for Fur-mediated transcriptional control is based on the observations that iron-dependent accumulation of irr mRNA is aberrant in a fur mutant, and extracts from E. coli cells harboring the B. japonicum fur gene bind to DNA upstream of the irr gene (22).…”

“…It has also been characterized in Rhizobium leguminosarum (21), and homologs are found in the genomes of other taxonomically related organisms within the ␣-proteobacterial group as well. Fur is involved in controlling iron metabolism in B. japonicum (20,22) but appears to play a lesser role in R. leguminosarum (21). In addition to Fur, the Irr protein in B. japonicum is involved in iron metabolism, where it mediates iron-dependent regulation of heme biosynthesis (7).…”

“…The irr gene is controlled by iron at both transcriptional (22) and post-translational levels (8,23). Evidence for Fur-mediated transcriptional control is based on the observations that iron-dependent accumulation of irr mRNA is aberrant in a fur mutant, and extracts from E. coli cells harboring the B. japonicum fur gene bind to DNA upstream of the irr gene (22). However, the irr promoter region does not contain DNA sequence similar to the Fur box consensus, indicating an important difference between the B. japonicum Fur protein and the other proteins on which the Fur box consensus sequence is based.…”