Subcellular Localization of Cytokinins in Transfer Ribonucleic Acid

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Abstract
Evidence on the localization of cytokinins in chloroplast tRNA was obtained by comparison of Euglena gracilis var. bacillaris light-grown and dark-grown wild type cultures and chloroplast-bleached mutant strains. The several cytokinins characteristic of tRNA were separated by Sephadex LH-20 column chromatography of the hydrolysates and were quantitatively determined by tobacco bioassays of the eluates. The results indicate that 6-(3-methyl-2-butenylamino)-9-β-d-ribofuranosylpurine (i6 A) is formed in both the cytoplasmic and chloroplast tRNA, whereas 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9-β-d- ribofurano-sylpurine (c-io6A) is produced mainly in the cytoplasmic tRNA and 6-(4-hydroxy-3-methyl-2-butenylamino)-2-methylthio-9-β-d- ribofurano-sylpurine (ms2io6A) is localized exclusively in chloroplast tRNA. The restriction of the methiolation reaction to the chloroplast is supported by results of radioisotope experiments showing that 35S-labeled MgSO4 is incorporated into ms2io6A in the wild type cultures, but not in the chloroplast-bleached mutant strains.