Carotenogenesis gene

Some eubacteria and archaea do not possess carotenogenesis gene clusters. The genes for early steps can be found from homologous searches. Some genes and enzymes are found by molecular biological techniques (Table 106.3), but some genes for the final steps of complex carotenoids remain unknown. 

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). 

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). 

A further consideration in the choice of gene relates to those that are members of a gene family. Only one member of such a family may be involved in carotenogenesis in a particular tissue. A good example of this is Psy-1 and-2 of tomato. PSY-1 is responsible for phytoene synthesis in ripening fruit (Fraser et al, 1999) whereas PSY-2 is not functional in chromoplasts, even if the protein is produced. 

Careful empirical selection of the expression platform for carotenogenesis has included selection of the best strains for stability and degree of accumulation and the selection of compatible drug-resistance combinations and low copy number polycistronic plasmids to enhance product accumulation by decrease of metabolic burden." 5 Matthews and Wurtzel discussed culture and induction conditions - that have been explored in most studies. Most efforts to engineer carotenoid biosynthesis in E. coli focused on the genes and enzymes of the pathway and had a modest effect on improved accumulation. For example, substitution and over-expression of a GGPPS that uses IPP directly (discussed in... 

Masamoto, K. et al.. Identification of a gene required for cis-to-trans carotene isomerization in carotenogenesis of the cyanobacterium Synechocystis sp. PCC 6803, Plant Cell Physiol. 42, 1398, 2001. 

Table 106.3 Functionally confirmed carotenogenesis enzymes and genes from some organisms ...

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