Origin and evolution of yeast carotenoid pathways.

Carotenoid pathways exist in nature in all domains. Comparison of the genes involved and their distribution allowed the elucidation of the origin and evolution of carotenoid biosynthesis from an early common ancestor of prokaryotes to Bacteria and Archaea. From the latter domain, carotenogenic genes are inherited by fungi as the only phylum of Eukarya. Carotenoid biosynthesis in the algal-plant lineage emerged independently by endosymbiotic gene transfer from an engulfed carotenogenic cyanobacterium. The early set of carotenogenic genes included crtB of phytoene synthase, the desaturase gene crtI, and the lycopene cyclase gene crtYcd for the synthesis of β-carotene. This carotenoid is further metabolised either to zeaxanthin and retinal due to the presence of crtZ and ccd or elongated to a C carotenoids by the crtEb gene product. The diversified pathways, especially in bacteria and fungi, result from gene modifications altering the substrate and product specificities of the corresponding enzymes or from the acquisition of novel genes. This was highlighted in more detail for the carotenoid pathways in the red yeasts of Basidiomycota leading to torularhodin, 2'-plectaniaxanthin, and astaxanthin.