Expression of the Soybean (Glycine max) Glutamate 1-Semialdehyde Aminotransferase Gene in Symbiotic Root Nodules

Extracts of soybean (Glycine max) root nodules and greening etiolated leaves catalyzed radiolabeled delta-aminolevulinic acid (ALA) formation from 3,4-[3H]glutamate but not from 1-[14C]glutamate. Nevertheless, those tissue extracts expressed the activity of glutamate 1-semialdehyde (GSA) aminotransferase, the C5 pathway enzyme that catalyzes ALA synthesis from GSA for tetrapyrrole formation. A soybean nodule cDNA clone that conferred ALA prototrophy, GSA aminotransferase activity, and glutamate-dependent ALA formation activity on an Escherichia coli GSA aminotransferase mutant was isolated. The deduced product of the nodule cDNA shared 79% identity with the GSA aminotransferase expressed in barley leaves, providing, along with the complementation data, strong evidence that the cDNA encodes GSA aminotransferase. GSA aminotransferase mRNA and enzyme activity were expressed in nodules but not in uninfected roots, indicating that the Gsa gene is induced in the symbiotic tissue. The Gsa gene was strongly expressed in leaves of etiolated plantlets independently of light treatment and, to a much lesser extent, in leaves of mature plants. We conclude that GSA aminotransferase, and possibly the C5 pathway, is expressed in a nonphotosynthetic plant organ for nodule heme synthesis and that Gsa is a regulated gene in soybean.