Four ribonucleosides responsible for cytokinin activity in wheat germ tRNA have been isolated and identified as 6-(3-methyl-2-butenylamino)-9-β-D-ribofuranosylpurine, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-β-D-ribofuranosyl-purine, 6-(3-methyl-2-butenylamino)-2-methylthio-9-β-D-ribofuranosylpurine, and 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9-β-D-ribofuranosylpurine. tRNA was extracted from defatted wheat germ and was hydrolyzed enzymatically. The ribonucleosides were extracted with ethyl acetate, and fractionated on Sephadex LH-20 columns and on paper. Cytokinin activity in the eluates was followed by the tobacco callus bioassay. The structures of these compounds were assigned on the basis of their chromatographic properties and ultraviolet and mass spectra, which were identical with those of the corresponding synthetic compounds.
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