DISTRIBUTION OF THE MEVALONATE AND GLYCERALDEHYDE PHOSPHATE PYRUVATE PATHWAYS FOR ISOPRENOID BIOSYNTHESIS IN UNICELLULAR ALGAE AND THE CYANOBACTERIUM SYNECHOCYSTIS PCC-6714/

Isopentenyl diphosphate, the universal isoprenoid precursor, can be pr oduced by two different biosynthetic routes: either via the acetate/me valonate (MVA) pathway, or via the more recently identified MVA-indepe ndent glyceraldehyde phosphate/ pyruvate pathway. These two pathways a re easily differentiated by incorporation of [l-C-13]glucose and analy sis of the resulting labelling patterns found in the isoprenoids. This method was successfully applied to several unicellular algae raised u nder heterotrophic growth conditions and allowed for the identificatio n of the pathways that were utilized for isoprenoid biosynthesis. All isoprenoids examined (sterols, phytol, carotenoids) of the green algae Chlorella fusca and Chlamydomonas reinhardtii were synthesized via th e GAP/pyruvate pathway, as in another previously investigated green al ga, Scenedesmus obliquus, which was also shown in this study to synthe size ubiquinone by the same MVA-independent route. In the red alga Cya nidium caldarium and in the Chrysophyte Ochromonas danica a clear dich otomy was observed: as in higher plants, sterols were formed via the M VA route, whereas chloroplast isoprenoids (phytol in Cy. caldarium and O. danica and beta-carotene in O. danica) were synthesized via the GA P/pyruvate route. In contrast, the Euglenophyte Euglena gracilis synth esized ergosterol, as well as phytol, via the acetate/MVA route. Simil ar feeding experiments were performed with the cyanobacterium Synechoc ystis PCC 6714 using [1-C-13]- and [6-C-13]-glucose. The two isoprenoi ds examined, phytol and beta-carotene, were shown to have the typical labelling pattern derived from the GAP/pyruvate route.