This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: cis-cytokinins biosynthesis, tRNA-dependent cytokinin biosynthesis
|Superclasses:||Biosynthesis → Hormones Biosynthesis → Plant Hormones Biosynthesis → Cytokinins Biosynthesis|
Some taxa known to possess this pathway include : Arabidopsis thaliana col
Expected Taxonomic Range: Embryophyta
Cytokinins have profound physiological roles in plants, e.g. promotion of cell division, release of lateral buds from apical dominance, and delay of senescence. The biological activity of cytokinins have long been studied. However, the uncertainty persists as to how different classes of cytokinins play different roles in plant development. The main form of cytokinin in plant cytoplasmic prenylated-tRNAs is cis-zeatin [Taller]. Although cis-zeatins can sometimes be found in plants in the tRNA-free form [Nicander95], and even be the predominant form of cytokinins in the species Cicer arietinum [Emery98], the physiological relevance of cis-hydroxylated cytokinins remains unclear. Indeed, in plants, it has been shown that cis-cytokinins have only low activity. The most biologically-relevant cytokinins have a trans-hydroxylated N6 side-chain.
With the uncovered existence of a zeatin cis-trans isomerase [Bassil93] came the postulate that cis-type cytokinins could contribute to the synthesis of trans-type cytokinins. This possibility is however thought to be improbable [Kasahara04]. In Arabidopsis thaliana the trans-type cytokinins are synthesized in plastids from DMAPP precursors originating from the plastidic methylerythritol phosphate pathway I. The cis-type cytokinins, on the other hand, are synthesized through prenylation of tRNA molecules in the cytosol using DMAPP precursors originating from the cytosolic mevalonate pathway.
About This Pathway
So far only one enzyme has been characterized for the tRNA-dependent cytokinin biosynthesis in Arabidopsis. The tRNA isopentenyltransferase activity of the enzyme encoded by IPT2 was demonstrated by heterologous expression of the gene in yeast [Golovko02]. To date no enzyme able to catalyze the stereo-specific hydroxylation of the isoprene side chain has been found and it is not known whether this hydroxylation step occurs after pentenylation of the tRNA adenine using DMAPP as a substrate or whether an hydroxylated analogue of DMAPP could give cis-zeatin-tRNA directly [Golovko02].
Unification Links: AraCyc:PWY-2781
Emery98: Emery RJ, Leport L, Barton JE, Turner NC, Atkins CA (1998). "cis-isomers of cytokinins predominate in chickpea seeds throughout their development." Plant Physiol 117(4);1515-23. PMID: 9701607
Kasahara04: Kasahara H, Takei K, Ueda N, Hishiyama S, Yamaya T, Kamiya Y, Yamaguchi S, Sakakibara H (2004). "Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis." J Biol Chem 279(14);14049-54. PMID: 14726522
Caillet88: Caillet J, Droogmans L (1988). "Molecular cloning of the Escherichia coli miaA gene involved in the formation of delta 2-isopentenyl adenosine in tRNA." J Bacteriol 170(9);4147-52. PMID: 3045085
Leung97: Leung HC, Chen Y, Winkler ME (1997). "Regulation of substrate recognition by the MiaA tRNA prenyltransferase modification enzyme of Escherichia coli K-12." J Biol Chem 272(20);13073-83. PMID: 9148919
Moore00: Moore JA, Mathis JR, Poulter CD (2000). "Escherichia coli dimethylallyl diphosphate:tRNA dimethylallyltransferase: pre-steady-state kinetic studies." Biochim Biophys Acta 1479(1-2);166-74. PMID: 11004538
Rosenbaum72: Rosenbaum N, Gefter ML (1972). "Delta 2 -isopentenylpyrophosphate: transfer ribonucleic acid 2 -isopentenyltransferase from Escherichia coli. Purification and properties of the enzyme." J Biol Chem 247(18);5675-80. PMID: 4341485
Soderberg01a: Soderberg T, Poulter CD (2001). "Escherichia coli dimethylallyl diphosphate:tRNA dimethylallyltransferase: site-directed mutagenesis of highly conserved residues." Biochemistry 40(6);1734-40. PMID: 11327834
Takei01: Takei K, Sakakibara H, Sugiyama T (2001). "Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana." J Biol Chem 276(28);26405-10. PMID: 11313355
Takei04a: Takei K, Yamaya T, Sakakibara H (2004). "Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-Zeatin." J Biol Chem 279(40);41866-72. PMID: 15280363
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