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Frequency of N⁶-Benzyladenine Incorporation into Major RNA Fractions of Tobacco Cell Suspensions Grown at Stimulatory or Cytostatic Cytokinin Concentration ¹

Laboratoire de Biochimie Fonctionnelle des Plantes, Faculté des Sciences de Luminy, Case 901, 13288 Marseille Cedex 9, France

The frequency of incorporation of the cytokinin N⁶-[p-³H]benzyladenine into major RNA species of tobacco (Nicotiana tabacum cv W 38) cells steadily increased as a function of its concentration in the culture medium, up to a 10 micromolar cytostatic overdose. During a 55-hour incubation of cells with 0.4 micromolar benzyladenine (BA), which is the optimal concentration for cell division, the incorporation frequency increased to one BA per 1.5 to 2.0 x 10⁴ conventional bases in total RNA. Frequencies of BA incorporation into 18S and 25S rRNA and into RNA precursors were very similar, 2- to 3-fold higher than the frequency of BA incorporation into the 4S + 5S RNA fraction. In cells incubated with 10 micromolar BA, the rate of RNA synthesis between 24 and 55 hours was lower than at optimal growth conditions; 18S and 25S rRNA synthesis was depressed more than the synthesis of 4S + 5S RNA. At 55 hours, BA was incorporated into total RNA at the steady state frequency of one per 1,300 conventional bases. All major RNA species were BA-labeled to approximately the same level, except that the labeling of the RNA precursors was 2-fold higher than the labeling of mature RNA species. These results may reflect an alteration in the processing of the RNA precursors at supra-optimal cytokinin concentration.

¹ Supported by the `Centre National de la Recherche Scientifique,' (ER 104) and the `Délégation Générale à la Recherche Scientifique et Technique' (Contract No. 70-5-079).