Nonheme diiron monooxygenase involved in the biosynthesis of xanthophylls. Specific for beta-ring hydroxylations of beta-carotene. Uses ferredoxin as an electron donor. all-trans-beta-carotene + 4 H(+) + 2 O2 + 4 reduced [2Fe-2S]-[ferredoxin] = all-trans-zeaxanthin + 2 H2O + 4 oxidized [2Fe-2S]-[ferredoxin] Homodimer. Expressed in flower buds and lips. Detected in roots and leaves. Expressed during floral development. The histidine box domains may contain the active site and/or be involved in iron binding. High expression of beta-hydroxylase (BHY) and zeaxanthin epoxidase (ZEP) results in the accumulation of violaxanthin, 9-cis-violaxanthin and neoxanthin in the yellow cultivar Gower Ramsey, while the down-regulation of BHY and ZEP results in the accumulation of beta-carotene and orange coloration in floral tissues of the cultivar Sunkist. Belongs to the sterol desaturase family.