In the bacterium Escherichia coli, the mevalonic-acid (MVA)-independent 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway is characterized by two branches leading separately to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). The signature of this branching is the retention of deuterium in DMAPP and the deuterium loss in IPP after incorporation of 1-[4-2H]deoxy-d-xylulose ([4-2H]DX). Feeding tobacco BY-2 cell-suspension cultures with [4-2H]DX resulted in deuterium retention in the isoprene units derived from DMAPP, as well as from IPP in the plastidial isoprenoids, phytoene and plastoquinone, synthesized via the MEP pathway. This labelling pattern represents direct evidence for the presence of the DMAPP branch of the MEP pathway in a higher plant, and shows that IPP can be synthesized from DMAPP in plant plastids, most probably via a plastidial IPP isomerase.
Abbreviations used: DMAPP, dimethylallyl diphosphate; DX, 1-deoxy-d-xylulose; DXP, DX 5-phosphate; FAB, fast atom bombardment; IPP, isopentenyl diphosphate; IDI, IPP isomerase; ME, 2-C-methyl-d-erythritol; MEP, ME 4-phosphate; MVA, mevalonic acid; TBY-2, Tobacco cv. Bright Yellow-2; THF, tetrahydrofuran.