Iron-dependent cytochrome c₁ expression is mediated by the status of heme in Bradyrhizobium japonicum

The heme prosthetic group of heme proteins contains iron, which can be a limiting nutrient. Here, we show that cytochrome c₁ protein from Bradyrhizobium japonicum was strongly affected by the iron status, with low expression in cells grown under iron limitation. This control was not affected in mutants encoding the iron regulator Irr or Fur. Furthermore, cytochrome c₁ mRNA was not influenced by the iron status, suggesting control at a posttranscriptional step. Cytochrome c₁ protein levels were very low in mutants defective in the genes encoding δ-aminolevulinic acid (ALA) synthase and ferrochelatase, enzymes that catalyze the first and final steps of the heme biosynthetic pathway, respectively. Iron-dependent cytochrome c ₁ expression was restored in the ALA synthase mutant by supplementation of the medium with the heme precursor ALA. Supplementation with heme resulted in high levels of cytochrome c₁ protein in the wild type and in both mutants, but expression was no longer iron dependent. Cytochrome c₁ is synthesized as a protein precursor fused with cytochrome b. A plasmid-borne construct encoding only cytochrome c₁ was expressed in an iron- and heme-dependent manner similar to that of the wild-type gene, indicating that control by those effectors is not linked to posttranslational processing of the fusion protein. Mutation of the cytochrome c₁ cysteines involved in covalent binding to heme nearly abolished immunodetectable protein. Thus, defects in heme synthesis or heme binding abrogate cytochrome c₁ accumulation, apparently due to protein degradation. We suggest that iron-dependent cytochrome c₁ expression is mediated by heme availability for heme protein formation.