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Functional Analysis of the β and ε Lycopene Cyclase Enzymes of Arabidopsis Reveals a Mechanism for Control of Cyclic Carotenoid Formation
Francis X. Cunningham, Jr., Barry Pogson, Zairen Sun, Kelly A. McDonald, Dean DellaPenna and Elisabeth Gantt
The Plant Cell
Vol. 8, No. 9 (Sep., 1996), pp. 1613-1626
Published by: American Society of Plant Biologists (ASPB)
Stable URL: http://www.jstor.org/stable/3870254
Page Count: 14
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Carotenoids with cyclic end groups are essential components of the photosynthetic membranes in all plants, algae, and cyanobacteria. These lipid-soluble compounds protect against photooxidation, harvest light for photosynthesis, and dissipate excess light energy absorbed by the antenna pigments. The cyclization of lycopene (Ψ,Ψ-carotene) is a key branch point in the pathway of carotenoid biosynthesis. Two types of cyclic end groups are found in higher plant carotenoids: the β and ε rings. Carotenoids with two β rings are ubiquitous, and those with one β and one ε ring are common; however, carotenoids with two ε rings are rare. We have identified and sequenced cDNAs that encode the enzymes catalyzing the formation of these two rings in Arabidopsis. These β and ε cyclases are encoded by related, single-copy genes, and both enzymes use the linear, symmetrical lycopene as a substrate. However, the ε cyclase adds only one ring, forming the monocyclic δ-carotene (ε,Ψ-carotene), whereas the β cyclase introduces a ring at both ends of lycopene to form the bicyclic β-carotene (β,β-carotene). When combined, the β and ε cyclases convert lycopene to α-carotene (β,ε-carotene), a carotenoid with one β and one ε ring. The inability of the ε cyclase to catalyze the introduction of a second ε ring reveals the mechanism by which production and proportions of β,β- and β,ε-carotenoids may be controlled and adjusted in plants and algae, while avoiding the formation of the inappropriate ε,ε-carotenoids.
The Plant Cell © 1996 American Society of Plant Biologists (ASPB)