A NUCLEAR GENE ENCODING MITOCHONDRIAL PROLINE DEHYDROGENASE, AN ENZYME INVOLVED IN PROLINE METABOLISM, IS UP-REGULATED BY PROLINE BUT DOWN-REGULATED BY DEHYDRATION IN ARABIDOPSIS

Proline is one of the most common compatible osmolytes in water-stress ed plants. The accumulation of proline in dehydrated plants is caused both by the activation of proline biosynthesis and by the inactivation of proline degradation; a decrease in the level of accumulated prolin e in rehydrated plants is caused both by the inhibition of proline bio synthesis and by the activation of proline degradation. The proline bi osynthetic pathway has been well characterized, but the degradation of proline is poorly understood. Sequence analysis of an Arabidopsis cDN A clone, ERD5 (for early responsive to dehydration stress), isolated f rom plants dehydrated for 1 hr. revealed that it encodes a protein wit h identity to products of the yeast PUT1 (for proline utilization) gen e (23.6% over 364 amino acids) and the Drosophila sluggish-A gene (34. 5% over 255 amino acids). Their gene products are precursors of prolin e oxidases (dehydrogenase) (EC 1.5.99.8), which are the first enzymes involved in the conversion of proline to glutamic acid. Proline oxidas e is localized in mitochondria. RNA gel blot analysis demonstrated tha t transcripts of the ERD5 gene were undetectable when plants had been dehydrated for 10 hr, but large amounts of the transcript accumulated when plants subsequently were rehydrated. Elevated levels of the trans cript were also found in plants that had been incubated in a medium th at contained proline. Immunologically, we showed that the product of E RD5 is localized in the mitochondrial fraction and accumulates in resp onse to proline in cultured cells. Fusion genes for ERD5 and PUT1 comp lemented a put1 mutant of yeast, allowing put1 to grow with proline as the source of nitrogen. These results suggest that ERD5 encodes a pre cursor of proline dehydrogenase (oxidase), which is regulated at the l evel of mRNA accumulation in both dehydrated and rehydrated plants.