Contribution a l'etude d'acides ribonucleiques a marquage rapide de la racine de lentille

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.

Ribosomal cistrons in higher plant cells. I. A definitive scheme for the maturation pathway of the primary transcriptional product of ribosomal cistrons in Acer pseudoplatanus L. cells

1. Four precursor ribosomal RNAs have been previously characterized in cultured sycamore cells (Acer pseudoplatanus L.). The present paper reports a study of the two smallest ones. 2. (Me-3H)-labelling of the precursor and mature rRNAs has been submitted to kinetic analysis. From pulse-chase experiments it is concluded that the 27S and 20S precursor rRNAs behave like intermediate molecules in the maturation of the 42S and 36S precursor rRNAs into the 26S and 17S components. They are direct precursors to the cytoplasmic and mature rRNAs. 3. Base-composition analyses have been performed on carefully purified molecules. They show that the 27S and 20S precursor rRNAs arise from a single cleavage of the 36S precursor rRNA. In addition, they strongly suggest that the 27S and 20S precursor rRNAs can be taken as specific precursors to the 26S and 17S rRNAs, respectively. These results are reinforced by DNA - RNA hybridization experiments, the measured values being in good agreement with the theoretical ones expected from a specific relationship between the four smallest molecules. Base-compositions of the conserved and non-conserved nucleotide sequences of the primary transcriptional product have been estimated or calculated. 4. On the basis of the present results and those previously reported, a scheme for the biosynthesis of ribosomal RNA in sycamore cells is proposed.

Hormonal control of transcription in higher plants

1. Nucleolar RNA polymerase Ib obtained from auxin-treated lentil roots exhibits a higher transcriptional activity than the enzyme obtained from control roots. This difference is due to a change in the enzyme properties after auxin treatment. It is suggested that the hormonal effect is mediated by a factor that changes the molecular properties of nucleolar RNA polymerase. 2. Four fractions, alpha, beta, gamma and delta, that stimulate the activity of RNA polymerase Ib, have been extracted from lentil roots. Two of them, gamma and delta have been studied. Factor delta can stimulate nucleolar polymerase Ib and the nucleoplasmic enzyme II equally well, while factor gamma is specific for polymerase Ib. 3. The curve of UMP incorporation in vitro, with and without factors gamma or delta suggests that they are initiation factors. This conclusion is reinforced by the analysis of simultaneous incorporation of [gamma-32P]ATP and [3H]UMP in the RNAs synthesized in vitro. 4. Although the level of factor delta is independent of auxin treatment, that of factor gamma is doubled in auxin-treated roots. These results suggest that factor gamma is an auxin-induced protein that modulates the specific activity of the nucleolar RNA polymerase. 5. A general model of the mode of action of auxins at the molecular level is proposed. It integrates into a unified scheme the above results as well as those obtained by other workers.