Phytoene-synthase

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

POTRYKUS I (1997) Transgenic rice Oryza sativa) endosperm expressing daffodil Narcissuspseudonarcissus) phytoene synthase accumulates phytoene, a key intermediate of provitamin A biosynthesis , Plant J, 11, 1071-78. 

FRASER P D, KIANO I W, TRUESDALE M R, SCHUCH W and BRAMLEY P M (1999) PhytOene synthase-2 enzymes activity in tomato does not contribute to carotenoid synthesis in ripening fruit . Plant Mol Biol, 40, 687-98. 

FRASER P D, ROMER S, SHIPTON C A, MILLS P B, KIANO I W, MISAWA N, DRAKE R G, SCHUCH W and BRAMLEY p M (2002) Evaluation of transgenic tomato plants expressing an additional phytoene synthase in a fruit-specific manner , Proc Natl Acad Sci, 99, 1092-7. 

FRAY R, WALLACE A, FRASER P D, VALERO D, HEDDEN P, BRAMLEY P M and GRIERSON D (1995) Constitutive expression of a fruit phytoene synthase gene in transgenic tomatoes causes dwarfism by redirecting metabolites from the gibberellin pathway . Plant J, 8, 696-701. 

Expression of a tomato cDNA coding for phytoene synthase in Escherichia coli, ph)doene formation in vivo and in vitro, and functional analysis of the varions trimcated gene prodncts , J Biochem (Tokyo), 116, 980-85. 

SCHLEDZ M, AL-BABILI S, VON LINTIG J, HAUBRUCK H, RABBANI S, KLEINIG H and BEYER P (1996) Phytoene synthase fcom Narcissus pseudonarcissus functional expression, galactolipid requirement, topological distribution in chromoplasts and induction during flowering . Plant J, 10, 781-92. 

Shewmaker, C.K. et al.. Seed-specific overexpression of phytoene synthase increase in carotenoids and other metabolic effects. Plant J., 20, 401, 1999. 

Over-expression of bacterial phytoene synthase led to only modest increases in pigment accumulation (except in the case of chloroplast-contaiifing tissues). Attention turned to CrtI, one gene that might control flux through the entire four desaturation steps from phytoene to lycopene (discussed in Section 5.3.2.4). Only a modest increase in carotenoid content in tomatoes and a variety of changes in carotenoid composition including more P-carotene, accompanied by an overall decrease in total carotenoid content (no lycopene increase), resulted when CrtI was over-expressed under control of CaMV 35S. Apparently, the bacterial desaturase... 

Plant phytoene synthase (Psy) has been used in a variety of transgenics. As noted above, P yl over-expression under a strong constitutive promoter caused a decrease in carotenoid accumulation, probably due to transcription silencing. Similarly, over-expression of the gene sequence backward (antisense) also silenced activity. In another approach to over-expression of tomato 1 in fruits, a synthetic alternative in which the third position of each codon was changed in order to avoid transcriptional silencing was successful in conditioning an increase in carotenoid accumulation. 

Scolnik, P.A. and Bartley, G.E., Nucleotide sequence of an Arabidopsis cDNA for phytoene synthase. Plant Physiol. 104, 1471, 1993. 

Fray, R.G. and Grierson, D., Identification and genetic analysis of normal and mutant phytoene synthase genes of tomato by sequencing, complementation and co-suppression, Plant Mol. Biol. 22, 589, 1993. 

Fraser, P.D., Schuch, W., and Bramley, P.M., Phytoene synthase from tomato (Lyco-persicon esculentum) chloroplasts partial purification and biochemical properties, Planta 211, 361, 2000. 

Wong, l.C. et al., QTL and candidate genes phytoene synthase and zetacarotene desaturase associated with the accumulation of carotenoids in maize, Theor. Appl. Genetics 108, 349, 2004. 

Palaisa, K. A. et al., Contrasting effects of selection on seqnence diversity and linkage disequilibrium at two phytoene synthase loci, Plant Cell 15, 1795, 2003. 

Figure 73. The carotenoid biosynthetic pathway. Enzymes are named according to the designation of their genes Ccs, capsanthin-capsorubin synthase CrtL-b, lycopene-b-cyclase CrtL-e, lycopene-e-cyclase CrtR-b, b-ring hydroxylase, CrtR-e, e-ring hydroxylase DMADP, dimethylallyl diphosphate GGDP, geranylgeranyl diphosphate Ggps, geranylgeranyl-diphosphate synthase IDP, isopentenyl diphosphate Ipi, IDP isomerase Pds, phytoene desaturase Psy, phytoene synthase Vde, violaxanthin de-epoxidase Zds, z-carotene desaturase Zep, zeaxanthin epoxidase. (From van den Berg and others 2000.)...

Fig. 8.2 Carotene and xanthophyll biosynthetic pathways in Capsicum. Isopentenyl pyrophosphate (IPP) phytoene synthase (PSY) lycopene fi-cyclase (LCYB) lycopene e-cyclase (LCYE) P-carotene hydroxylase (CrtZ-2) zeaxanthin epoxidase (Ze) and capsanthin-capsorubin synthase (CCS)...

Hurtado-Hernandez and Smith [33] demonstrated that there are three independent loci Cl, C2, and Y determining fruit color and eight phenotypes, using an F2 population of a hybridization between white and red peppers. In this hybridization, red color is dominant, and the F2 progeny displayed fruit colors ranging from red, orange, peach, and white. Lefebvre et al. [34] demonstrated that Ccs is a candidate gene for the Y locus, while Huh et al. [35] proposed that phytoene synthase, Psy, may be encoded at the C2 locus. The biochemical function at the Cl locus, however, has not been determined. 

Huh JH, Kan BC, Nahm SH, Kim S, Ha KS, Lee MH, Kim BD (2001) A candidate gene approach identified phytoene synthase as the locus for mature fruit color in red pepper Capsicum spp.). Theor Appl Genet 102 524-530... 

Phytoene synthase [EC 2.5.1.32] (also known as gera-nylgeranyl-diphosphate geranylgeranyltransferase and prephytoene-diphosphate synthase) catalyzes the reaction of two geranylgeranyl diphosphate to produce pyrophosphate (or, diphosphate) and prephytoene diphosphate. Isopentenyl pyrophosphate isomerase [EC 5.3.3.2] catalyzes the interconversion of isopentenyl diphosphate and dimethylallyl diphosphate. See also Geranylgeranyl Diphosphate Geranylgeranyltransferase... 

---