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Genes, enzymes, and chemical reactions for carotene biosynthesis in Phycomyces. Two molecules of geranylgeranyl pyrophosphate are converted to one molecule of β-carotene by phytoene synthase (diamonds), four copies of phytoene dehydrogenase (squares), and two copies of lycopene cyclase (circles). The scheme on the left represents the genes that code for these enzymes. The chemical changes are marked with arrowheads.
