Abstract
Enzymes that are able to oxidatively cleave carotenoids at specific positions have been identified in animals and plants. The first such enzyme to be identified was a nine-cis-epoxy carotenoid dioxygenase from maize, which catalyzes the rate-limiting step of abscisic acid biosynthesis. Similar enzymes are necessary for the synthesis of vitamin A in animals and other carotenoid-derived molecules in plants. In the model plant, Arabidopsis, there are nine hypothetical proteins that share some degree of sequence similarity to the nine-cis-epoxy carotenoid dioxygenases. Five of these proteins appear to be involved in abscisic acid biosynthesis. The remaining four proteins are expected to catalyze other carotenoid cleavage reactions and have been named carotenoid cleavage dioxygenases (CCDs). The hypothetical proteins, AtCCD7 and AtCCD8, are the most disparate members of this protein family in Arabidopsis. The max3 and max4 mutants in Arabidopsis result from lesions in AtCCD7 and AtCCD8. Both mutants display a dramatic increase in lateral branching and are believed to be impaired in the synthesis of an unidentified compound that inhibits axillary meristem development. To determine the biochemical function of AtCCD7, the protein was expressed in carotenoid-accumulating strains of Escherichia coli. The activity of AtCCD7 was also tested in vitro with several of the most common plant carotenoids. It was shown that the recombinant AtCCD7 protein catalyzes a specific 9-10 cleavage of beta-carotene to produce the 10 black triangle down-apo-beta-carotenal (C27) and beta-ionone (C13). When AtCCD7 and AtCCD8 were co-expressed in a beta-carotene-producing strain of E. coli, the 13-apo-beta-carotenone (C18) was produced. The C18 product appears to result from a secondary cleavage of the AtCCD7-derived C27 product. The sequential cleavages of beta-carotene by AtCCD7 and AtCCD8 are likely the initial steps in the synthesis of a carotenoid-derived signaling molecule that is necessary for the regulation lateral branching.
Highlights
The Biochemical Characterization of Two Carotenoid Cleavage Enzymes from Arabidopsis Indicates That a Carotenoid-derived Compound Inhibits Lateral Branching*□S
Two major compounds were detected in the pACBETA/AtCCD7 strain that were not detected in the pACBETA strain with an empty vector
Because of an inability to repress the outgrowth of axillary buds, the max mutants display an increase in lateral branching and have a bushy appearance
Summary
Vol 279, No 45, Issue of November 5, pp. 46940 –46945, 2004 Printed in U.S.A. The Biochemical Characterization of Two Carotenoid Cleavage Enzymes from Arabidopsis Indicates That a Carotenoid-derived Compound Inhibits Lateral Branching*□S. Abscisic acid is necessary for seed development and adaptation to various environmental stresses The synthesis of these apocarotenoids and others is catalyzed by a class of oxygenases that cleave specific double bonds resulting in two products with carbonyls at the site of cleavage. Four members of this protein family in Arabidopsis do not cluster with previously characterized NCEDs and are considered unlikely to be abscisic acid biosynthetic enzymes These proteins may, catalyze other carotenoid cleavage reactions and are more appropriately referred to as carotenoid cleavage dioxygenases (CCDs). The max mutant in Arabidopsis and the rms mutant in pea result from a lesion in AtCCD8 and a pea ortholog (16) Both mutants display an increase in lateral branching and are believed to be impaired in the synthesis of an unknown compound that inhibits axillary meristem development or bud outgrowth.
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