Cloning and functional analysis of the beta-carotene hydroxylase of Arabidopsis thaliana.

An Arabidopsis thaliana cDNA encoding the enzyme beta-carotene hydroxylase was identified by functional complementation in Escherichia coli. The product of this cDNA adds hydroxyl groups to both beta rings of the symmetrical beta-carotene (beta,beta-carotene) to form zeaxanthin (beta,beta-carotene-3,3'-diol) and converts the monocyclic beta-zeacarotene (7',8'-dihydro-beta,psi-carotene) to hydroxy-beta-zeacarotene (7',8'-dihydro-beta,psi-carotene-3-ol). The epsilon rings of delta-carotene (epsilon,psi-carotene) and alpha-zeacarotene (7',8'-dihydro-epsilon,psi-carotene) are poor substrates for the enzyme. The predicted amino acid sequence of the A. thaliana enzyme resembles the four known bacterial beta-carotene hydroxylase enzymes (31-37% identity) but is much longer, with an N-terminal extension of more than 130 amino acids. Truncation of the cDNA to produce a polypeptide lacking the first 69 amino acids does not impair enzyme activity in E. coli. Truncation to yield a polypeptide of a length comparable with the bacterial enzymes (lacking 129 N-terminal amino acids) resulted in the accumulation of the monohydroxy intermediate beta-cryptoxanthin (beta,beta-carotene-3-ol), predominantly, when beta-carotene was provided as the substrate. It is suggested that amino acid residues 70-129 of the A. thaliana enzyme may play a role in formation of a functional homodimer.

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