Transfer RNA from yeast, liver, and Escherichia coli has cytokinin activity in the tobacco callus bioassay, whereas ribosomal RNA from yeast is inactive. In contrast to fractions of yeast transfer RNA rich in serine acceptor and cytokinin activity, preparations (70 to 90 percent pure) of arginine transfer RNA(2), glycine transfer RNA, phenylalanine transfer RNA, and valine transfer RNA(1) and of highly purified alanine transfer RNA from yeast were inactive at concentrations of 20 to 2500 micrograms per liter. One molecule of 6-(gamma,gamma-dimethylallylamino) purine per 20 molecules of yeast tRNA would account for the observed cytokinin activity. The number of major molecular species contributing to cytokinin activity of transfer RNA, therefore, must be small.
Until recently, the presence in transfer ribonucleic acid (tRNA) ofthe hydroxylated cytokinin ribosylzeatin [N6-(4-hydroxy-3-methylbut-2-enyl)adenosine] was thought to be unique to higher plants. This extension of work from several laboratories indicates the presence of 2-methylthioribosylzeatin in the tRNA of the plant-associated bacteria Rhizobium leguminosarum, Agrobacterium tumefaciens, and Corynebacterium fascians, but not in that of Erwinia amylovora. This cytokinin has the cis configuration, as is normally found in the tRNA's of plants. The tRNA thionucleotide patterns in these bacteria are different from those of Escherichia coli, Bacillus subtilis, and Salmonella typhimurium, which contain the unhydroxylated analogs of ribosylzeatin or 2-methylthioribosylzeatin.
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