Isomerase involved in the biosynthesis of carotenoids. Catalyzes the cis- to trans-conversion of the 15-cis-bond in 9,15,9'-tri-cis-zeta-carotene. 9,9',15-tri-cis-zeta-carotene = 9,9'-di-cis-zeta-carotene Expressed in leaves and at lower levels in roots. Up-regulated by light. Lacks carotenoids in the dark and exhibits delayed greening when exposed to light. Major isoform in both etiolated and green leaves.