FUNCTIONAL-ANALYSIS OF THE BETA-LYCOPENE AND EPSILON-LYCOPENE CYCLASEENZYMES OF ARABIDOPSIS REVEALS A MECHANISM FOR CONTROL OF CYCLIC CAROTENOID FORMATION

Carotenoids with cyclic end groups are essential components of the pho tosynthetic membranes in all plants, algae, and cyanobacteria. These l ipid-soluble compounds protect against photooxidation, harvest light f or photosynthesis, and dissipate excess light energy absorbed by the a ntenna pigments. The cyclization of lycopene (psi,psi-carotene) is a k ey branch point in the pathway of carotenoid biosynthesis. Two types o f cyclic end groups are found in higher plant carotenoids: the beta an d epsilon rings. Carotenoids with two beta rings are ubiquitous, and t hose with one beta and one epsilon ring are common; however, carotenoi ds with two epsilon rings are rare. We have identified and sequenced c DNAs that encode the enzymes catalyzing the formation of these two rin gs in Arabidopsis. These beta and epsilon cyclases are encoded by rela ted, single-copy genes, and both enzymes use the linear, symmetrical l ycopene as a substrate. However, the epsilon cyclase adds only one rin g, forming the monocyclic delta-carotene (epsilon,psi-carotene), where as the beta cyclase introduces a ring at both ends of lycopene to form the bicyclic beta-carotene (epsilon,beta-carotene). When combined, th e beta and epsilon cyclases convert lycopene to alpha-carotene (beta,e psilon-carotene), a carotenoid with one beta and one epsilon ring. The inability of the epsilon cyclase to catalyze the introduction of a se cond epsilon ring reveals the mechanism by which production and propor tions of beta,beta- and beta,epsilon-carotenoids may be controlled and adjusted in plants and algae, while avoiding the formation of the ina ppropriate epsilon,epsilon-carotenoids.