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From 2011.igem.org

Modeling

The pathway for producing beta-carotene. Yeast naturally produce GGPP but no other intermediates in beta-carotene pathway, crtYB and crtI are needed to to complete the pathway for beta-carotene production. Notice crtYB codes for two proteins needed in the pathway. CrtE further improves beta-carotene production as it aides in the production of the starting product, GGPP.