A SOYBEAN COPROPORPHYRINOGEN OXIDASE GENE IS HIGHLY EXPRESSED IN ROOT-NODULES

In plants the enzyme coproporphyrinogen oxidase catalyzes the oxidativ e decarboxylation of coproporphyrinogen III to protoporphyrinogen IX i n the heme and chlorophyll biosynthesis pathway(s). We have isolated a soybean coproporphyrinogen oxidase cDNA from a cDNA library and deter mined the primary structure of the corresponding gene. The coproporphy rinogen oxidase gene encodes a polypeptide with a predicted molecular mass of 43 kDa. The derived amino acid sequence shows 50% similarity t o the corresponding yeast amino acid sequence. The main difference is an extension of 67 amino acids at the N-terminus of the soybean polype ptide which may function as a transit peptide. A full-length coproporp hyrinogen oxidase cDNA clone complements a yeast mutant deleted of the coproporphyrinogen oxidase gene, thus demonstrating the function of t he soybean protein, The soybean coproporphyrinogen oxidase gene is hig hly expressed in nodules at the stage where several late nodulins incl uding leghemoglobin appear. The coproporphyrinogen oxidase mRNA is als o detectable in leaves but at a lower level than in nodules while no m RNA is detectable in roots. The high level of coproporphyrinogen oxida se mRNA in soybean nodules implies that the plant increases heme produ ction in the nodules to meet the demand for additional heme required f or hemoprotein formation.