Isopentenyl transferase

The enzyme A -isopentenyl pyrophosphate tRNA-A -isopentenyl-transferase from Escherichia coli was shown to have a molecular weight of about 55 000. 

The enzyme A -isopentenyl pyrophosphate tRNA A -isopentenyl transferase has been further characterized. It has a molecular weight of about 55 000, needs dimethylallyl pyrophosphate (3), and is highly stereospecific in its action. The tertiary structure of the tRNA is necessary before a reaction will occur, and the enzyme then modifies the adenosine unit adjacent to the 3 -end of the anticodon. 

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.

Experiments demonstrating incorporation of radioactive precursors, commonly adenine, into cytokinins in plant tissues have been used to examine the de novo pathway of cytokinin biosynthesis [15,16,22 and references therein]. Some of these in vivo labelling experiments have highlighted the importance of cytokinin nucleotides as the primary products of biosynthesis [19]. Further confirmation for this proposal was provided by the in vitro work which demonstrated that a cell free preparation from a slime mold catalyzed the formation of iPA 5 -monophosphate from 5 -AMP and IPP [26]. Neither adenine nor adenosine acted as a substrate for the enzyme, A--IPP AMP-A--isopentenyl transferase, a prenyl transferase commonly termed cytokinin synthase. The enzyme has subsequently been purified from a number of higher plant sources [7, 11]. In addition it has been established that cytokinin (and auxin) production in crown gall tissues is under the control of T-DNA, and genes have been identified which directly code for the production of these phytohormones [17]. 

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