Carotenoid biosynthesis and encoding genes

Although carotenogenesis in plants takes place in plastids, all of the carotenoid biosynthesis genes are nuclear encoded and their polypeptide products are imported into the plastids. Therefore, they contain a N-terminal transit peptide sequence. For example, the size of the transit peptide of PSY from ripe tomato fruit is approximately 9 kDa, corresponding to about 80 amino acid residues (Misawa et al, 1994). 

Since carotenoids are isoprenoids, they share a common early pathway with other biologically important isoprenoids such as sterols, gibberellins, phytol and the terpenoid quinones (Fig. 13.3). In all cases, these compounds are derived from the C5 isoprenoid, isopentenyl diphosphate (IPP). Until a few years ago it was believed that a single pathway from the Cg precursor mevalonic acid (MVA) formed IPP, which itself was synthesised from hydroxymethylglutaryl coenzyme A (HMG CoA) by the action of HMG 

IPP = isopentenyl diphosphate GPP = geranyl diphosphate FPP = farnesyl diphosphate  

GA-3-P = glyceraldehyde 3-phosphate TPP = thiamine pyrophosphate DOXP = 1-deoxy-D-syiuiose CTP = oytosine triphosphate ATP = adenosine triphosphate  

CDP-ME-2-P = 4-diphosphocytidyi-2-methyi-D-erythritoi 2-phosphate CDP-MEP = 4-diphosphocytidyi 2-C-methy-D-erythritoi 4-phosphate MEP = 2-C-methyi-D-erythritoi 4-phosphate MECPP 2-C-methyi-D-erythritoi 2,4-eyoiodiphosphate  

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