The Aspartate Aminotransferase-P2 Gene from Lupinus angustifolius

AAT (EC 2.6.1 .l) catalyzes the transamination reaction: aspartate + 2-oxoglutarate $ oxaloacetate + glutamate and plays a key role in carbon and nitrogen metabolism in plants. The enzyme has been shown to be involved in the shuttling of reducing equivalents from the cytoplasm to chlo- roplasts, mitochondria, glyoxysomes, and peroxisomes via the malate-aspartate shuttle (Wightman and Forest, 1978). Up to five separate isoforms of AAT have been reported in plants, differing both kinetically and in levels and place of expression (Wightman and Forest, 1978). In the nitrogen- fixing root nodules of lupin (Lupinus angustifolius), two iso- forms of AAT, AAT-Pl and AAT-P2, have been described and were found to differ in their kinetic properties and expression characteristics throughout nodule development (Reynolds and Famden, 1979). AAT-P2 from L. angustifolius has been shown to be expressed in an inducible manner concomitantly with the onset of biological nitrogen fixation (Reynolds and Famden, 1979) and to be localized in the nodule proplastid (Boland et al., 1982), whereas the AAT-P1 isoform is localized in the cytoplasm and appears to be constitutively expressed. Similar results have been obtained for the AAT isoforms present in alfalfa nodules (Farnham et al., 1990; Robinson et ai., 1994). The cDNAs encoding the