GENOMIC STRUCTURE, EXPRESSION AND EVOLUTION OF THE ALFALFA ASPARTATE-AMINOTRANSFERASE GENES

Genomic clones encoding two isozymes of aspartate aminotransferase (AA T) were isolated from an alfalfa genomic library and their DNA sequenc es were determined. The AAT1 gene contains 12 exons that encode a cyto solic protein expressed at similar levels in roots, stems and nodules. In nodules, the amount of AAT1 mRNA was similar at all stages of deve lopment, and was slightly reduced in nodules incapable of fixing nitro gen. The AAT1 mRNA is polyadenylated at multiple sites differing by mo re than 250 bp. The AAT2 gene contains 11 exons, with 5 introns locate d in positions identical to those found in animal AAT genes, and encod es a plastid-localized isozyme. The AAT2 mRNA is polyadenylated at a v ery limited range of sites. The transit peptide of AAT2 is encoded by the first two and part of the third exon. AAT2 mRNA is much more abund ant in nodules than in other organs, and increases dramatically during the course of nodule development. Unlike AAT1, expression of AAT2 is significantly reduced in nodules incapable of fixing nitrogen. Phyloge netic analysis of deduced AAT proteins revealed 4 separate but related groups of AAT proteins; the animal cytosolic AATs, the plant cytosoli c AATs, the plant plastid AATs, and the mitochondrial AATs.