Sequence determination of the 3' terminal T1 oligonucleotide of 18S ribosomal RNA

We have reexamined the primary structure of the 3' terminal oligonucleotide of 18S RNA from chicken fibroblasts and have shown, contrary to previously published results that this extremity G-A-U-C-A-U-U-AOH is identical to that of the rabbit, drosophila and bombyx. Furthermore the electrophoretic mobility and composition of the 3' terminal oligonucleotides of 18S RNA from rat and human cells are similar to that of other RNAs and show that the identity of structure for this region of 18S RNA extends to include all tested species between yeast and man. This finding reveals a marked degree of evolutionary constraint on the structure of this region.

Nucleotide sequence study of mouse 5.8S ribosomal RNA

The primary structure of 5.8S mouse ribosomal RNA has been studied and compared to the structures previously established for other animal species. The results obtained show that mouse 5.8S ribosomal RNA yields pancreatic oligonucleotides with the same nucleotide sequence as the homologous oligonucleotides from rat cells. Furthermore T1 oligonucleotides of 5.8S ribosomal RNA from rat, mouse and human cells behave identically on fingerprinting fractionation and have the same composition as judged by pancreatic digestion. These results strongly suggest that the primary structures of 5.8S ribosomal RNA from rat, mouse and human cells are identical. This identity of structure is also found when the presence of several modified bases (psi and methylated bases) is considered. The findings emphasize the remarkable evolutionary stability of ribosomal gene structure. Comparison of the terminal regional of 5.8S RNA with those of 18S RNA reveals differences which imply a more complex mechanism underlying the maturation of 45S precursor RNA than the finding of identical structure would have suggested.

Methylation process of mammalian ribosomal precursor RNA

The analysis of RNA from L5178 Y cells, by electrophoreis in 1.7% polyacrylamide gel, after short time labelling with [methyl-3H] methionine, shows that the three molecular species 47S, 46S and 45S RNA from the ribosomal RNA "45S fraction" are methylated. Comparative studies of the kinetics obtained after labelling for short periods with [methyl-3H] methionine and [3H] uridine, and brief labelling experiments with [methyl-3H] methionine in the presence of actinomycin D, show that the methylation of ribosomal precursor RNA occurs after the transcription process has been completed, and that methylation and transcription are not coordinated. These results lead us to propose the following scheme for ribosomal RNA biosynthesis : 1) transcription of ribosomal genes, 2) methylation of 47S RNA (and possibly 46S RNA), 3) maturation of these molecules which are converted to 45S RNA.

Fingerprinting studies of the maturation of ribosomal RNA in mammalian cells

32P-labelled ribosomal RNA of L 5178 Y cells (a mouse cell line) was digested with T1 ribonuclease and fingerprinted by electrophoresis at pH 3.5 on cellulose acetate and homochromatography on DEAE-cellulose thin-layer plate. From this, it can be concluded that 18-S and 28-S RNA have different and characteristic fingerprints and that the number, the size and the frequency of the large T1 oligonucleotides demonstrate that the guanylic residues are randomly interspaced along the molecule. Using a double-labelling technique with 32P-labelled 45-S RNA and 14C-labelled 18-S RNA or 28-S RNA, long T1 oligonucleotides of the 45-S RNA can be divided into three classes: (a) those which are lost during the transition, (b) those which are present in the 18-S RNA and (c) those which are present in the 28-S RNA. These results provide direct evidence for the existence of one common precursor for the two mature ribosomal RNAs. The comparison of the fingerprints of T1-ribonuclease-digested 47-S, 45-S and 41-S RNA shows that the 47-S and 41-S RNA have a characteristic ribosomal pattern. Finally the size, the number and the mobility of the oligonucleotides present in the different RNA precursors but absent from the mature RNA demonstrate that the non-conversed RNA pieces do not have a monotonous and repetitive sequence.

Sequence determination of 5'-terminal and 3'-terminal T1 oligonucleotides of 18-S ribosomal RNA of a mouse cell line (L 5178 Y)

The 5' and 3'-terminal oligonucleotides of 18-S ribosomal RNA of L 5178 Y (a mouse cell line) obtained after total T1 ribonuclease hydrolysis were isolated by a diagonal procedure. They were localized on the fingerprint of T1-ribonuclease-hydrolysed 18-S RNA. These two oligonucleotides were partially hydrolysed by snake venom and spleen phsophodiesterases and resulting products were fractionated bidimensionally. Their base compositions were determined by total hydrolysis with piperidine or snake venom phosphodiesterase. From these results the following sequences were deduced: pU-A-C-C-U-G for the 5'-terminal oligonucleotide and G-A-U-C-A-U-U-Aoh for the 3'-terminal oligonucleotide. Quantitative studies indicated that these sequences represent at least 70% for the 5' oligonucleotide and 85% for the 3' oligonucleotide of the terminal sequences of the 18-S RNA.

Evidence for the existence of several molecular species in the "45S fraction" of mammalian ribosomal precursor RNA

Analysis of RNA (after 5 or 60 min of labeling with [(3)H]uridine) from L5178Y cells by electrophoresis in 1.7% polyacrylamide gels demonstrates that the ribosomal RNA "45S fraction" is not homogeneous but consists of at least three molecular species, conventionally designated 47S RNA, 46S RNA, and 45S RNA. Alternatives to the classical scheme for the biosynthesis of ribosomal RNA in mammalian cells are discussed.