Cis-Zeatin

The only report of an effort to develop a functional assay for a cytokinin receptor is the work in barley by Kulaeva et al. [54-56]. They isolated CBPs, added them to isolated nuclei and looked for a stimulation in RNA synthesis. They isolated three different CBPs of 28-30, 40 and 67 kD. The 28-30 and 67 kD proteins were purified on BA-Sepharose and zeatin-Sepharose, respectively. Both proteins were able to stimulate total RNA synthesis by 2-3 fold only in presence of cytokinins (100 nM). The 67 kD protein activated polymerase I and II dependent transcription specifically with trans and not cis-zeatin. However, the binding affinities and specificities for other cytokinins were not indicated, therefore, this report needs to be viewed with some caution until it can be carefully reproduced with added controls. 

A tremendous upsurge in synthetic activity in this field is evident. A stereoselective synthesis of cis-zeatin (22), whose geometrical isomer was first isolated from Zea mays and shown to be a stimulant of cell division in plant tissue cultures, has been reported (Scheme 2). The Diels-Alder reaction of 1-chloro-l-nitro-socyclohexane (19) with isoprene gave the dihydro-l,2-oxazine hydrochloride (20) in moderate yield. Liberation of the base of (20) followed by zinc-acetic acid reduction gave the required amino-alcohol (21), which upon treatment with 6-chloropurine provided a separable mixture of cis-zeatin (22) and trans-zeatin. Standard tobacco callus bioassay for cytokinin activity showed that the natural trans-zeatin is at least 50 times more active than the synthetic cis-isomer (22). 

Fig. 29.5 The two cytokiiim biosynthesis and activation pathways. DMAPP dimethylallyl diphosphate, iP A/ -(A -isopenteaiyl)adenme, iPRMP iP riboside S -mraiophosphate, tZ trans-zeatin, tZRTP tZ riboside 5 -triphosphate, tZRDP tZ riboside 5 -diphosphate, tZRMP tZ riboside 5 -monophosphate, cZ cis-zeatin, cZRMP cZ riboside 5 -monophosphate, cZR cZ riboside. Enzymes iPT isopentenyltransfoase, CYP735A cytochrome P450 CYP735A, tRNA-iPT transfer RNA isopentenyltransferase, LOG lonely guy, cytokinin nucleoside 5 -monophosphate phosphor-ibohydrolase (After Hirose et al. [26])...

Cw-P-apo-10 -carotenal, 3588 9-Cw-P-carotene, 3588 Cw-3-butyl-4-vinylcyclopentene, 2895 Cis conformation, 1756 Cis-isomers, 4066, 4069 c -Nepetalactone, 3029 C20 isoprenoid precursor, 1730 Cisplatin, 2215 Cis-zeatin, 958, 963... 

Martin, R.C. et al. (2001) A maize cytokinin gene encoding an 0-glucosyltransferase specific to cis-zeatin. Proc. Natl Acad. Sci. USA 98,5922-5926... 

Another derivative is the zeatin. Zeatin belongs to the plant-growth hormones, cytokinins. Cytokinins are adenine derivatives with isoprenoid side chain [108, 109]. The rate-limiting step of tra/is-zeatin biosynthesis is catalyzed by adenylate-isopentenyltransferase (cytokinin synthase) [110]. The plant enzyme (EC 2.5.1.27) utilize ADP, which is different from the microbial one (EC 2.5.1.112) which recognizes ATP/or ADP [111, 112]. Afterward, the adenylate-isopentenyl converts to the active tra/is-zeatin by a single hydroxylation reaction (cytokinin tra/is-hydroxylase). On the opposite site, cis-zeatin biosynthesis starts with a different transferase, tRNA-dimethylallyl-transferase (EC 2.5.1.75) [113-116], which mediates the transfer of an isopentenyl group of DMAPP to adenine of tRNAs. 

ZEATIN CIS-TRANS ISOMERASE ZEATIN REDUCTASE ZEOLITE ZERO ORDER... 

There are good reasons presently for believing also that free cytokinins have important biological activity independently of any direct association with tRNA (1) there are results showing direct effects of exogenous cytokinin independently of any apparent incorporation into tRNA (2) ethanolic extracts of corn kernels contain the trans-isomer of zeatin, while the tRNA hydrolysates of corn kernels contain the cis-isomer, suggesting that zeatin is not a precursor in the synthesis of tRNA in that material (3) dihydrozeatin... 

Anti-zeatinriboside Anti-cis- Anti-dihydro- Anti-isopentenyladenosine zeatin zeatin riboside riboside... 

The existence of the enzyme catalysing the conversion between cis- and frany-isomers of zeatin is the prerequisite for possible involvement of tRNA as a source of free cytokinins (c/. part 2.1.3. in this chapter). Indeed, the c/y-rra y-isomerase was isolated and partially purified from the endosperm of immature Phaseolus vulgaris seeds. The reaction may proceed in the presence of FAD or FMN cofactors and light in both directions, but the conversion of the cis- to the frany-isomer is preferred. The enzyme seems to be a glycoprotein and is specific for both free bases (Z, cis-Z) and their ribosides [83,84],... 

McCloskey, J.A., B. Basile, K. Kimura, and T. Hashizume Presence and levels of cis- and trans-meth-ylthiozeatin riboside and cis- and trans-zeatin ribodise in tobacco plant determined by mass spectrometry Proc. Jap. Acad. Ser. B 57 (1981) 276-281. 

The low affinity of the AtIPTs for AMP implies that most of the natural isopentenyl riboside 5 -monophosphate (iPRMP) is formed by dephosphorylation of iPRDP and iPRTP, phosphorylation of isopentenyl riboside (iPR) by adenosine kinase, and conjugation of a phosphoribosyl moiety to iP by adenine PT [266]. Arabidopsis CYP735A1 and CYP735A2, cytocrome P450 monooxygenases, encode CK hydroxylases that catalyze tZ biosynthesis via the iPRMP-dependent pathway. The reduction of the double bond in the tZ side chain, catalyzed by a zeatin reductase, forms 6-(4-hydroxy-3-methylbutylamino)purine, whose trivial name is DZ. cZ and tZ can be enzymatically interconverted by zeatin cis-trans isomerase. 

Figure 6.63 Isoprenoid cytokinin biosynthesis pathway in Arabidopsis. IPT, phosphate-isopentenyl transferase AK, adenosine kinase 1, phosphatase 2,5 -ribonucleotide phosphohydrolase 3, adenosine nucleosidase 4, purine nucleoside phosphorylase 5, zeatin cis-trans isomerase and 6, zeatin reductase.

N ci -y-Methyl-y-hydroxyniethylallyladeno-sine 6-(4-hydroxy-3-methyl-but-cis-2-enyl)-amino-purine, an adenine derivative, and the cis isomer of Zeatin (see). It is a rare nucleic add component in certain tRNA, and it is biosynthesized by modification of an adenosine residue in the nucleic add. The free compound shows cytokinin activity. 

Zeatin 6-(4-hydroxy-3-methyl-but-(ranr-2-enyl)-aminopurine, a naturally occurring Cytokinin (see for formula). Z. occurs free in many plants, especially in immature maize kernels, and is identical with the previously described maize factor (MF). Its derivatives, Dihydrozeatin (see), Z. riboside and Z. ribotide are also cytokinins. The cis compound (see M-cu-y-Methyl-y-hydroxymethylallyladenosine) is a rare nucleic component in certain species of RNA. 

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