Expression of a Soybean Gene Encoding the Tetrapyrrole-Synthesis Enzyme Glutamyl-tRNA Reductase in Symbiotic Root Nodules1

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Abstract
Heme and chlorophyll accumulate to high levels in legume root nodules and in photosynthetic tissues, respectively, and they are both derived from the universal tetrapyrrole precursor δ-aminolevulinic acid (ALA). The first committed step in ALA and tetrapyrrole synthesis is catalyzed by glutamyl-tRNA reductase (GTR) in plants. A soybean (Glycine max) root-nodule cDNA encoding GTR was isolated by complementation of anEscherichia coli GTR-defective mutant for restoration of ALA prototrophy. Gtr mRNA was very low in uninfected roots but accumulated to high levels in root nodules. The induction ofGtr mRNA in developing nodules was subsequent to that of the gene Enod2 (early nodule) and coincided with leghemoglobin mRNA accumulation. Genomic analysis revealed two Gtr genes, Gtr1 and a 3′ portion of Gtr2, which were isolated from the soybean genome. RNase-protection analysis using probes specific toGtr1 and Gtr2 showed that both genes were expressed, but Gtr1 mRNA accumulated to significantly higher levels. In addition, the qualitative patterns of expression ofGtr1 and Gtr2 were similar to each other and to total Gtr mRNA in leaves and nodules of mature plants and etiolated plantlets. The data indicate thatGtr1 is universal for tetrapyrrole synthesis and that aGtr gene specific for a tissue or tetrapyrrole is unlikely. We suggest that ALA synthesis in specialized root nodules involves an altered spatial expression of genes that are otherwise induced strongly only in photosynthetic tissues of uninfected plants.