Proline prototrophy was restored to an Escherichia coli proBA proline auxotroph by ornithine and a mothbean (Vigna aconitifolia) cDNA expression library. This novel strategy, "trans-complementation," allowed isolation of a cDNA encoding ornithine delta-aminotransferase (delta-OAT). This enzyme transaminates ornithine to glutamic-gamma-semialdehyde (GSA), thereby bypassing the block in GSA synthesis from glutamate in the proBA mutant. The identity of the mothbean enzyme was confirmed by its high sequence homology to mammalian and yeast delta-OATs as well as to a family of bacterial and fungal omega-aminotransferases and an absence of significant homology to various alpha-aminotransferases. The V. aconitifolia OAT cDNA encodes a polypeptide of 48.1 kDa. The native enzyme expressed in E. coli appears to be a monomer with Km of 2 mM for ornithine and 0.75 mM for alpha-ketoglutarate. Levels of mRNA in V. aconitifolia for delta 1-pyrroline-5-carboxylate synthetase (P5CS) and delta-OAT, the two key enzymes for proline synthesis, were monitored under different physiological conditions. Salt stress and nitrogen starvation induced P5CS mRNA levels and depressed OAT mRNA levels. Conversely, OAT mRNA level was elevated in plants supplied with excess nitrogen while the P5CS mRNA level was reduced. These data suggest that the glutamate pathway is the primary route for proline synthesis in plants during conditions of osmotic stress and nitrogen limitation whereas the ornithine pathway assumes prominence under high nitrogen input.