MOLECULAR-CLONING OF MANNOSE-6-PHOSPHATE REDUCTASE AND ITS DEVELOPMENTAL EXPRESSION IN CELERY

Compared with other primary photosynthetic products (e.g. sucrose and starch), little is known about sugar alcohol metabolism, its regulatio n, and the manner in which it is integrated with other pathways. Manno se-6-phosphate reductase (M6PR) is a key enzyme that is involved in ma nnitol biosynthesis in celery (Apium graveolens L.). The M6PR gene was cloned from a leaf cDNA library, and clonal authenticity was establis hed by assays of M6PR activity, western blots, and comparisons of the deduced amino acid sequence with a celery M6PR tryptic digestion produ ct. Recombinant M6PR, purified from Escherichia coil, had specific act ivity, molecular mass, and kinetic characteristics indistinguishable f rom those of authentic celery M6PR. Sequence analyses showed M6PR to b e a member of the aldo-keto reductase superfamily, which includes both animal and plant enzymes. The greatest sequence similarity was with a ldose-6-phosphate reductase (EC 1.1.1.200), a key enzyme in sorbitol s ynthesis in Rosaceae. Developmental studies showed M6PR to be limited to green tissues and to be under tight transcriptional regulation duri ng leaf initiation, expansion, and maturation. These data confirmed a close relationship between the development of photosynthetic capacity, mannitol synthesis, and M6PR activity.