Lycopene cyclases

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). 

CUNNINGHAM F X Jr, POGSON B, SUN Z, MCDONALD K A, DELLAPENNA D and GANTT E (1996) Functional analysis of the (3 and e lycopene cyclase enzymes of Arabidopsis reveals a mechanism for control of cyclic carotenoid formation . Plant Cell, 8, 1613-26. 

QTLs were also detected in tomato introgression lines (domestic tomatoes with single, defined chromosome regions selected from the progeny of a cross with a wild, non-pigmented variety) that correspond to candidate loci for which genes and mutants have been previously characterized. Detected QTLs included the r locus Psy), the Del locus (LcyE), and the B locus. Similar to the tissue-specific expression of maize Yl, the B locus codes a fruit-specific lycopene cyclase associated with higher levels of P-carotene. ... 

The stability of phytoene desaturase and lycopene cyclase transcripts also influenced accumulation of carotenoids. Efforts in directed evolution of carotenogenic enzymes have also continued. Alternate approaches using systematic and combinatorial gene knockout targets have allowed for enhancement of carotenoid production in the absence of a priori assumptions of regulatory mechanisms. 

Cunningham, F.X. Jr. et ah. Cloning and functional expression in Escherichia coli of a cyanobacterial gene for lycopene cyclase, the enzyme that catalyzes the biosynthesis of beta-carotene, FEBS Lett. 328, 130, 1993. 

Generation of mutants is also a starting point in optimization experiments, and now is the time for metabolic engineering of the astaxanthin biosynthetic pathway. Researchers should be able to manage carbon fluxes within the cells and resolve competitions between enzymes such as phytoene desaturase and lycopene cyclase. 

Neoxanthin, a precursor of the plant hormone abscisic acid, is an allenic xantho-phyll recognized as the last product of carotenoid synthesis in green plants. A cDNA for neoxanthin synthase (NSY) was isolated from tomato cv. Philippino using a molecular approach based on the mechanistic and structural similarities of NSY to two other closely related carotenogenic enzymes, lycopene cyclase (LCY) and capsanthin-capsorubin synthase (CCS) (Bouvier et al., 2000). 

Fig. 4 Reconstruction of the carotenoids biosynthetic pathway in S. cerevisiae. Heterologous activities t-HMGl 3 hydroxy-3-methylglutaryl-coenzymeA, ctrE geranylgeranyldiphosphate synthase (from Xanthophyllomyces dendrorhous), Ctrl phytoene desaturase, ctrYB bifunctional enzyme phytoene synthase and lycopene cyclase. Endogenous yeast activities ISIS I geranylgeranyldiphosphate synthase. Metabolites IPP isopentenyl diphosphate, GPP geranyl diphosphate, FPP famesyl-diphosphate, GGPP geranylgeranyl diphosphate...

Phytoene desaturase and lycopene cyclase New carotenoid pathway (substrate and reaction specificity) Production of torulene in E.coli DNA family shuffling+ screening E. coli [209]... 

Fig. 11.3 Simplified diagram of isoprenoid metabolic pathway relative to the conversions among carotenoids that have been genetically manipulated. Enzymes catalysing some key reactions are indicated by the following abbreviations IPI, isopentenyl pyrophosphate isomerase PSY, phytoene synthase PDS, phytoene desaturase (crti, gene from Erwinia uredovora) ZDS, -carotene desaturase LCYB, lycopene / -cyclase CRTR-b, /3-carotene 3,3 -hydroxylase CCS, capsanthin-capsombin synthase CRTO, /3-carotene ketolase.

Bishop N I, Urbig T and Senger H (1995) Complete separation of the beta, epsilon- and beta, beta-carotenoid biosynthetic pathways by a unique mutation of the lycopene cyclase in the green alga, Scenedesmus obliquus. FEBS Letters 367 158-162... 

CunninghamFX Jr, SunZ, ChamovitzD, Hirschberg J and Gantt E. (1994) Molecular structure and enzymatic function of lycopene cyclase from the cyanobacterium Synechococcus sp strain PCC7942. Plant Cell 6 1107-1121. 

It is unknown whether y-carotene synthase is the same with lycopene cyclase or not. 

CUNNINGHAM JR., F. X., POGSON, B., SUN, Z., MCDONALD, K. A., DELLAPENNA, D., GANTT, E., Functional analysis of the P and E lycopene cyclase enzjmies of Arahidopsis reveals a mechanism for control of cyclic carotenoid formation. Plant Cell 1996,8,1613-1626. 

Expression of an herbicide insensitive target has also been reported to provide resistance to diclofop and sethoxydim (ACCase inhibitors), various dinitroani-lines, and inhibitors of phytoene desaturase, lycopene cyclase and hydroxyphenyl-pyruvate dioxygenase. None of these traits are currently incorporated into commercial products. 

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