Carotenoid biosynthesis pathway in higher plants and the corresponding steps in bacteria. MEP, methylerythritol 4-phosphate; GGPP, geranylgeranylpyrophosphate; PSY, phytoene synthase; PDS, phytoene desaturase; ZDS, ζ-carotene desaturase; CRTISO carotenoid isomerase; LYCB, lycopene β-cyclase; LYCE, lycopene ε-cyclase; HYDB (also known as BCH), β-carotene hydroxylase; CYP97A and CYP97C, heme-containing cytochrome P450 carotene β- and ε-ring hydroxylases; CRTB, bacterial phytoene synthase; CRTI, bacterial phytoene desaturase/isomerase; CRTY, bacterial lycopene β-cyclase; CRTZ, bacterial β-carotene hydroxylase. Provitamin A carotenoids are boxed.