Isoprenoid Biosynthesis via a Mevalonate-Independent Pathway in Plants: Cloning and Heterologous Expression of 1-Deoxy-d-xylulose-5-phosphate Reductoisomerase from Peppermint

Two distinct pathways are utilized by plants for the biosynthesis of isopentenyl diphosphate, the universal precursor of isoprenoids. The classical acetate/mevalonate pathway operates in the cytosol, whereas plastidial isoprenoids originate via a novel mevalonate-independent route that involves a transketolase-catalyzed condensation of pyruvate andd-glyceraldehyde-3-phosphate to yield 1-deoxy-d-xylulose-5-phosphate as the first intermediate. Based onin vivofeeding experiments, rearrangement and reduction of deoxyxylulose phosphate have been proposed to give rise to 2-C-methyl-d-erythritol-4-phosphate as the second intermediate of this pyruvate/glyceraldehyde-3-phosphate pathway (1–3). The cloning of anEscherichia coligene encoding an enzyme capable of converting 1-deoxy-d-xylulose-5-phosphate to 2-C-erythritol-4-phosphate was recently reported (4). A cloning strategy was developed for isolating the gene encoding a plant homolog of this enzyme from peppermint (Mentha×piperita), and the identity of the resulting cDNA was confirmed by heterologous expression inE. coli.Unlike the microbial reductoisomerase, the plant ortholog encodes a preprotein bearing an N-terminal plastidial transit peptide that directs the enzyme to plastids where the mevalonate-independent pathway operates in plants. The peppermint gene comprises an open reading frame of 1425 nucleotides which, when the plastidial targeting sequence is excluded, encodes a deduced enzyme of approximately 400 amino acid residues with a mature size of about 43.5 kDa.