Preferential incorporation of an exogenous cytokinin, N6-benzyladenine, into 18S and 25S ribosomal RNA of tobacco cells in suspension culture.

Cytokinin-requiring tobacco cells were incubated for 10 h in the presence of a labeled cytokinin. N6-benzyl-[2-3H]Ade, and of [8-14C]Ado. After alkaline hydrolysis of total RNA and fractionation of the resulting nucleotides, 80 per cent of the 3H radioactivity of RNA were recovered as the N6-benzyl-Ado nucleotide, covalently inserted into polynucleotidic chains. The N6-benzyl-Ado nucleotide was not significantly labled by 14C: at most one part of this nucleotide per 10 000 may result from a transfer of the benzyl moiety to adenyl residues in preformed RNA. Thus, the covalent insertion of N6-benzyl-Ade into RNA involves the intact N6-substituted base. Total RNA was fractionated either by sucrose density gradient centrifugation or by polyacrylamide gel electrophoresis. All identified RNA species were shown to contain N6-benzyl-Ade. The insertion frequency, measured as the molecular proportion of N6-benzyl-Ade to the total base content, was 3 to 4 times larger in 25S and 18S rRNA than in 5S and 4S RNA. The amount of N6-benzyl-Ade inserted into cytoplasmic ribosomal RNA accounted for about 90 per cent of the amount incorporated into total RNA. Electrophoresis of denatured RNA in the presence of formamide provided additional evidence that N6-benzyl-Ade was indeed incorporated into RNA molecules.