Fig. 1From: The SREBP (Sterol Regulatory Element-Binding Protein) pathway: a regulatory bridge between carotenogenesis and sterol biosynthesis in the carotenogenic yeast Xanthophyllomyces dendrorhousBiosynthesis of astaxanthin and ergosterol in X. dendrorhous. The production of carotenoids and sterols requires isopentenyl pyrophosphate (IPP) from the mevalonate pathway [9]. Carotenoid biosynthesis begins with the production of geranylgeranyl pyrophosphate (GGPP) from farnesyl pyrophosphate (FPP) and IPP by GGPP synthase encoded by crtE [66, 68]. Then, the bi-functional enzyme phytoene-β-carotene synthase encoded by crtYB condenses two GGPP molecules producing phytoene [69]. Subsequently, phytoene undergoes four desaturation reactions carried out by the phytoene desaturase enzyme encoded by crtI, producing lycopene [70]. The cyclization of both ends of lycopene by the lycopene cyclase activity of the phytoene-β-carotene synthase produces β-carotene [69], which is then transformed into astaxanthin via intermediate xanthophylls. To date, X. dendrorhous is the only known organism that produces astaxanthin from β-carotene through a cytochrome P450 system [71], which is composed of the cytochrome P450 enzyme astaxanthin synthase (CrtS, encoded by crtS) [72, 73] and a cytochrome P450 reductase (CPR) (named CrtR in X. dendrorhous, encoded by crtR) [64]. CrtS catalyzes the hydroxylation and ketolation of carbons at positions 3 and 4, respectively, of each end ring of β-carotene to finally produce astaxanthin [72], while CrtR assists CrtS in these reactions [64]. In ergosterol biosynthesis, two cytochrome P450 enzymes are involved, which are encoded by CYP51 (lanosterol 14α-demethylase) [74] and CYP61 (C-22 sterol desaturase) [49]. Arrows: catalytic steps with the corresponding enzyme encoding gene. Genes in red: Direct Sre1 targets identified by ChIP-exo [17]. CrtR was included as a redox partner of the cytochrome P450 monooxygenases Cyp51, Cyp61 and CrtS, and squalene epoxidase (ERG1). 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), mevalonate (MVA), dimethylallyl-pyrophosphate (DMAPP). Other genes: ERG10 (acetyl-CoA C-acetyltransferase), HMGS (HMG-CoA synthase), HMGR (HMG-CoA reductase), MVK (mevalonate kinase), PMK (phosphomevalonate kinase), MVD (mevalonate diphosphate decarboxylase), idi (IPP isomerase), FPS (FPP synthase). ERG represent to enzyme-encoding genes involved in ergosterol biosynthesis identified in Saccharomyces cerevisiae. Figure adapted from [17]Back to article page