Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules

In Bradyrhizobium japonicum/soybean symbiosis, the leghemoglobin (legume hemoglobin) apoprotein is a plant product, but the origin of the heme prosthetic group is not known. B. japonicum strain LO505 is a transposon Tn5-induced cytochrome-deficient mutant; it excreted the oxidized heme precursor coproporphyrin III into the growth medium. Mutant strain LO505 was specifically deficient in protoporphyrinogen oxidase (protoporphyrinogen-IX:oxygen oxidoreductase, EC 1.3.3.4) activity, and thus it could not catalyze the penultimate step in heme biosynthesis. Soybean root nodules formed from this mutant did not contain leghemoglobin, but the apoprotein was synthesized nevertheless. Data show that bacterial heme synthesis is required for leghemoglobin expression, but the heme moiety is not essential for apoleghemoglobin synthesis by the plant. Soybean leghemoglobin, therefore, is a product of both the plant and bacterial symbionts.