In plants (left), carotenoid biosynthesis takes place in plastids, fed with isopentenyl diphosphate (IPP) generated via the methylerythritol 4-phosphate (MEP) pathway, which can be inhibited by fosmidomycin (FSM). IPP and its isomer dimethylallyl diphosphate (DMAPP) are used by the geranylgeranyl diphosphate (GGPPs) synthase to form GGPP. In the next step, phytoene synthase (PSY) condenses two GGPP molecules into 15-cis-phytoene. A series of desaturation and isomerization reactions catalyzed by phytoene desaturase (PDS), -carotene isomerase (Z-ISO), -carotene desaturase (ZDS), and carotenoid isomerase (CRTISO) transform the colorless 15-cis-phytoene into the red all-trans-lycopene (referred to as lycopene). Lycopene -cyclase (LCYb) converts lycopene into -carotene. Its isomer, -carotene, is formed by lycopene--cyclase (LCYe) and lycopene -cyclase (LCYb). Carotenoid biosynthesis of N. crassa (inset) takes place in the cytosol, based on IPP and DMAPP, which arise through the cytosolic mevalonate (MVA) pathway, which involves hydroxymethylglutaryl-coenzyme A (CoA) reductase (HMGR), which can be inhibited by mevinolin (MEV). In this study, we used a truncated version of HMGR (tHMGR) from Arabidopsis. The fungal biosynthesis is catalyzed by the GGPP synthase (AL-3), the bifunctional enzyme AL-2 that mediates phytoene synthesis, as well as lycopene cyclization and the desaturase AL-1, which introduces five double bonds extending the conjugated double system from 3 in phytoene to 13 in torulene and 3-4 didehydrolycopene. Mono-cyclization of lycopene and 3-4 didehydrolycopene, which is catalyzed by AL-2, leads to gamma-carotene and torulene respectively. The conjugated double bonds generated by AL-1-catalyzed desaturation are highlighted in yellow. Fungal and plant enzymes are depicted in red and green, respectively. GA-3-P, glyceraldehyde 3-phosphate; HMG-CoA, 3-hydroxy-3-methylglutaryl CoA.
