Aspartate Aminotransferase in Alfalfa Root Nodules : III. Genotypic and Tissue Expression of Aspartate Aminotransferase in Alfalfa and Other Species

Genomic structure, expression and evolution of the alfalfa aspartate aminotransferase genes

Genomic clones encoding two isozymes of aspartate aminotransferase (AAT) were isolated from an alfalfa genomic library and their DNA sequences were determined. The AAT1 gene contains 12 exons that encode a cytosolic protein expressed at similar levels in roots, stems and nodules. In nodules, the amount of AAT1 mRNA was similar at all stages of development, and was slightly reduced in nodules incapable of fixing nitrogen. The AAT1 mRNA is polyadenylated at multiple sites differing by more than 250 bp. The AAT2 gene contains 11 exons, with 5 introns located in positions identical to those found in animal AAT genes, and encodes a plastid-localized isozyme. The AAT2 mRNA is polyadenylated at a very 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 abundant 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. Phylogenetic analysis of deduced AAT proteins revealed 4 separate but related groups of AAT proteins; the animal cytosolic AATs, the plant cytosolic AATs, the plant plastid AATs, and the mitochondrial AATs.

Identification and expression of a cDNA clone encoding aspartate aminotransferase in carrot

A full-length cDNA clone encoding aspartate aminotransferase (AAT) has been identified from a carrot root cDNA library. Degenerate oligo primers were synthesized from the known amino acid sequence of AAT form I from carrot (Daucus carota L. cv Danvers). These primers were utilized in a polymerase chain reaction to amplify a portion of a carrot AAT gene from first strand cDNA synthesized from poly(A)(+) RNA isolated from 5-d-old cell suspension cultures. The resulting 750-bp fragment was cloned, mapped, and sequenced. The cloned fragment, mpAAT1, was used as a probe to identify a full-length cDNA clone in a library constructed from poly(A)(+) RNA isolated from carrot roots. A 1.52-kb full-length clone, AAT7, was isolated and sequenced. AAT7 has 54% nucleotide identity with both the mouse cytoplasmic and mitochondrial AAT genes. The deduced amino acid sequence has 52 and 53% identity with the deduced amino acid sequences of mouse cytoplasmic and mitochondrial AAT genes, respectively. Further analysis of the sequence data suggests that AAT7 encodes a cytoplasmic form of carrot AAT; the evidence includes the (a) absence of a transit or signal sequence, (b) lack of "m-residues," or invariant mitochondrial residues, in the carrot AAT sequence, and (c) high degree of sequence similarity with the amino acid sequence previously obtained for form I of carrot, a cytoplasmic isoenzyme. High- and low-stringency hybridizations to Southern blots of carrot nuclear DNA with AAT7 show that AAT is part of a small multigene family. Northern blot analysis of AAT7 suggests that AAT is expressed throughout cell culture up to 7 d and is highly expressed in roots but not in leaves.

Molecular and whole-plant responses to selection for enzyme activity in alfalfa root nodules: evidence for molecular compensation of aspartate aminotransferase expression

The enzyme aspartate aminotransferase (AAT) plays a key role in the assimilation of fixed-N in alfalfa (Medicago sativa L.) root nodules. AAT activity in alfalfa nodules is due to the activity of two dimeric isozymes, AAT-1 and AAT-2, that are products of two distinct genes. Three forms of AAT-2 (AAT-2a, -2b, and-2c) have been identified. It was hypothesized that two alleles occur at the AAT-2 locus, giving rise to the three AAT-2 enzymes. In a prior study bidirectional selection for root nodule AAT and asparagine synthetase (AS) activities on a nodule fresh weight basis in two diverse alfalfa germ plasms resulted in high nodule enzyme activity subpopulations with about 20% more nodule AAT activity than low enzyme activity subpopulations. The objectives of the study presented here were to determine the inheritance of nodule AAT-2 production and to evaluate the effect of bidirectional selection for AAT and AS on AAT-2 allelic frequencies, the relative contributions of AAT-1 and AAT-2 to total nodule activity, nodule enzyme concentration, and correlated traits. Two alleles at the AAT-2 locus were verified by evaluating segregation of isozyme phenotypes among F1 and S1 progeny of crosses or selfs. Characterization of subpopulations for responses associated with selection was conducted using immunoprecipitation of in vitro nodule AAT activity, quantification of AAT enzyme protein by ELISA, and AAT activity staining of native isozymes on PAGE. Results indicate that selection for total AAT activity specifically altered the expression of the nodule AAT-2 isozyme. AAT-2 activity was significantly greater in high compared to low activity subpopulations, and high AAT subpopulations from both germ plasms had about 18% more AAT-2 enzyme (on a nodule fresh weight basis). No significant or consistent changes in AAT-2 genotypic frequencies in subpopulations were caused by selection for AAT activity. Since changes in AAT activity were not associated with changes in AAT-2 genotype, selection must have affected a change(s) at another locus (or loci), which indirectly effects the expression of nodule AAT.

Aspartate aminotransferase in effective and ineffective alfalfa nodules : cloning of a cDNA and determination of enzyme activity, protein, and mRNA levels

Aspartate aminotransferase (AAT) is a key plant enzyme affecting nitrogen and carbon metabolism, particularly in legume root nodules and leaves of C(4) species. To ascertain the molecular genetic characteristics and biochemical regulation of AAT, we have isolated a cDNA encoding the nodule-enhanced AAT (AAT-2) of alfalfa (Medicago sativa L.) by screening a root nodule cDNA expression library with antibodies. Complementation of an Escherichia coli AAT mutant with the alfalfa nodule AAT-2 cDNA verified the identity of the clone. The deduced amino acid sequence of alfalfa AAT-2 is 53 and 47% identical to animal mitochondrial and cytosolic AATs, respectively. The deduced molecular mass of AAT-2 is 50,959 daltons, whereas the mass of purified AAT-2 is about 40 kilodaltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the protein's N-terminal domain (amino acids 1-59) contains many of the characteristics of plastid-targeting peptides. We postulate that AAT-2 is localized to the plastid. Southern blot analysis suggests that AAT-2 is encoded by a small, multigene family. The expression of AAT-2 mRNA in nodules is severalfold greater than that in either leaves or roots. Northern and western blots showed that expression of AAT activity during effective nodule development is accompanied by a sevenfold increase in AAT-2 mRNA and a comparable increase in enzyme protein. By contrast, plant-controlled ineffective nodules express AAT-2 mRNA at much lower levels and have little to no AAT-2 enzyme protein. Expression of root nodule AAT-2 appears to be regulated by at least two events: the first is independent of nitrogenase activity; the second is associated with nodule effectiveness.