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J. Biol. Chem., Vol. 279, Issue 40, 41866-41872, October 1, 2004
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From the Plant Science Center, RIKEN (The Institute of Physical and Chemical Research), Suehiro 1-7-22, Tsurumi, Yokohama 230-0045, Japan
Cytokinins (CKs), a group of phytohormones, are adenine derivatives that carry either an isoprene-derived or an aromatic side chain at the N6 terminus. trans-Zeatin (tZ), an isoprenoid CK, is assumed to play a central physiological role because of its general occurrence and high activity in bioassays. Although hydroxylation of isopentenyladenine-type CKs is a key step of tZ biosynthesis, the catalyzing enzyme has not been characterized yet. Here we demonstrate that CYP735A1 and CYP735A2 are cytochrome P450 monooxygenases (P450s) that catalyze the biosynthesis of tZ. We identified the genes from Arabidopsis using an adenosine phosphate-isopentenyltransferase (AtIPT4)/P450 co-expression system in yeast. Co-expression of AtIPT4 and CYP735A enabled yeast to excrete tZ and the nucleosides to the culture medium. In vitro, both CYP735As preferentially utilized isopentenyladenine nucleotides rather than the nucleoside and free base forms and produced tZ nucleotides but not the cis-isomer. The expression of CYP735A1 and CYP735A2 was differentially regulated in terms of organ specificity and response to CK. Root-specific induction of CYP735A2 expression by CK suggests that the trans-hydroxylation is involved in the regulation of CK metabolism and signaling in roots.
Received for publication, June 7, 2004 , and in revised form, July 26, 2004.
* This work was partly supported by Grants-in-aid for Scientific Research 12142202 (to H. S.) and 15770035 (to K. T.) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
To whom correspondence should be addressed. Tel.: 81-45-503-9576; Fax: 81-45-503-9609; E-mail: sakaki{at}postman.riken.go.jp.
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