A FAMILY OF TRANSKETOLASES THAT DIRECTS ISOPRENOID BIOSYNTHESIS VIA AMEVALONATE-INDEPENDENT PATHWAY

Isopentenyl diphosphate, the common precursor of all isoprenoids, has been widely assumed to be synthesized by the acetate/mevalonate pathwa y in all organisms, However, based on in vivo feeding experiments, iso pentenyl diphosphate formation in several eubacteria, a green alga, an d plant chloroplasts has been demonstrated very recently to originate via a mevalonate-independent route from pyruvate and glyceraldehyde 3- phosphate as precursors, Here we describe the cloning from peppermint (Mentha x piperita) and heterologous expression in Escherichia coli of 1-deoxy-D-xylulose-5-phosphate synthase, the enzyme that catalyzes th e first reaction of this pyruvate/glyceraldehyde 3-phosphate pathway. This synthase gene contains an ORF of 2,172 base pairs, When the propo sed plastid targeting sequence is excluded. the deduced amino acid seq uence indicates the peppermint synthase to be about 650 residues in le ngth, corresponding to a native size of roughly 71 kDa, The enzyme app ears to represent a novel class of highly conserved transketolases and likely plays a hey role in the biosynthesis of plastid-derived isopre noids essential for growth, development, and defense in plants.