Identification, cloning, and properties of cytosolic D-ribulose-5-phosphate 3-epimerase from higher plants

Plant cells contain a complete oxidative pentose phosphate pathway in the chloroplasts, but an incomplete pathway was proposed to be present in the cytosol, with cytosolic (cyt) isoforms of ribulose-5-phosphate 3-epimerase (RPEase) and other non-oxidative branch enzymes being undetectable. Here we present for the first time the identification, cloning, and properties of a cyt-RPEase in rice (Oryza sativa) and presence of its homologues in other plant species. Recombinant cyt-RPEase is a homodimer of 24.3-kDa subunits such as in the case of the animal and yeast enzymes, whereas the chloroplast (chl) RPEase is a hexamer. Cytosolic and chloroplastic RPEases cannot be separated by anion exchange chromatography. Since plant cyt-RPEase is more closely related in its primary structure to homologous enzymes in animal and yeast cells than to the chloroplast RPEase, the plant nuclear genes coding for cytosolic and chloroplast RPEases were most likely derived from eubacteria and cyanobacteria, respectively. Accumulation of cyt-RPEase-mRNA and protein is high in root cells, lacking chl-RPEase, and lower in green tissue. These and other observations support the view that green and non-green plant cells possess a complete oxidative pentose phosphate pathway in the cytosol.