OSMOREGULATION OF A PYRROLINE-5-CARBOXYLATE REDUCTASE GENE IN ARABIDOPSIS-THALIANA

In Arabidopsis thaliana (L.) Heynh. proline can account for up to 20% of the free amino acid pool after salt stress. Proline accumulation oc curs in plants mainly by de novo synthesis from glutamate. The last st ep of the proline biosynthetic pathway is catalyzed by pyrroline-5-car boxylate (P5C) reductase. A gene (AT-P5C1) encoding this enzyme in A. thaliana has been cloned and sequenced. Expression of AT-P5C1 in Esche richia coli resulted in the complementation of a proC mutant to protot rophy. A comparison of the AT-P5C1 primary and secondary structures wi th those of six P5C reductases of other organisms is presented. With t he exception of several functionally important amino acid residues, li ttle conservation in the primary structure is seen; much greater simil arity exists in the putative secondary structure. The AT-P5C1 protein is probably cytosolic. Under normal growth conditions, the P5C reducta se mRNA level was significantly higher in roots and ripening seeds tha n in green tissue. A salt treatment of A. thaliana plants resulted in a 5-fold induction of the AT-P5C1 transcript, suggesting osmoregulatio n of the AT-P5C1 promoter region. Moreover, a time-course experiment i ndicated that this induction precedes proline accumulation.