In plant chloroplasts a mevalonate-independent pathway for isopentenyl diphosphate formation is present. This pathway was discovered first by Rohmer when he was studying the biosynthesis of hopanoids (pentacyclic steroid-like molecules) in bacteria, but is used generally by algae and higher plants for plastid isoprenoid production. The pathway has not been fully defined yet but uses 1-deoxy-D-xylulose-5-phosphate rather than mevalo-nate as a precursor of the isoprenoid unit. Because isopentenyl diphosphate can be transported from the plastid, it may contribute to sterol biosynthesis in the cytosolic compartment. However, the portion of sterol precursor carbon originating from this route is not yet known.
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