gsa1 is a universal tetrapyrrole synthesis gene in soybean and is regulated by a GAGA element

Abstract

Expression of plant tetrapyrroles is high in photosynthetic tissues and in legume root nodules in the form of chlorophyll and heme, respectively. The universal tetrapyrrole precursor δ-aminolevulinic acid (ALA) is synthesized from glutamate 1-semialdehyde (GSA) by USA aminotransferase in plants, which is encoded by gsa. Immunoblot analysis showed that GSA aminotransferase was expressed in soybean leaves and nodules, but not in roots, and that protein correlated with enzyme activity. These observations indicate that USA aminotransferase expression is controlled in tetrapyrrole formation and argue against significant activity of an enzyme other than the well described aminotransferase for GSA-dependent ALA formation. gsa mRNA and protein were induced in soybean nodules, and their activation was temporally intermediate between those of the respective early and late genes enod2 and lb. A GSA aminotransferase gene, designated gsa1, was isolated and appears to be one of two gsa genes in the soybean genome. gsa1 mRNA accumulated to high levels in leaves and nodules, but not in uninfected roots as discerned with a gsa1- specific probe. Message levels were higher in leaves from etiolated plantlets than in mature plants, and expression in the former was slightly elevated by light. The expression pattern of gsa1 mRNA was qualitatively similar to that of total gsa. The data strongly suggest that gsa1 is a universal tetrapyrrole synthesis gene and that a gsa gene specific for a tissue, tetrapyrrole, or light condition is unlikely. The gsa1 promoter contained a genetic element found in numerous Drosophila melanogaster genes; the so-called GAGA element displayed single-stranded character in vitro and formed a complex with nuclear factors from nodules and leaves but not from roots. From these observations we infer that the GAGA element is involved in the transcriptional control of gsa1.