Nucleotide sequences within the ribosomal ribonucleic acids of HeLa cells, Xenopus laevis and chick embryo fibroblasts

Pseudouridine in the large-subunit (23 S-like) ribosomal RNA. The site of peptidyl transfer in the ribosome?

On evolutionary grounds, it has been advocated for more than 40 years that RNA generally, and more recently rRNA in particular, may participate, catalytically, in protein biosynthesis. A specific molecular mechanism has never been proposed. We suggest here that the N-1 position(s) in one or more of the approximately 4 pseudouridine (omega) residues in E. coli 23 S rRNA catalyzes transfer of the aminoacyl moiety from teh 3'-terminus of peptidyl tRNA in the P site to aminoacyl tRNA in the A site of the ribosome. Evidence that supports the proposal in the case of E. coli ribosomes, and relevant information pertaining to eukaryotic ribosomes, is summarized. Essential features of the evidence are that (i) the N-1 position in 1-acetylthymine (a direct analogue of 1-acetylpseudouridine) has an especially high potential for acyl-group transfer, comparable to that found for N-acetylimidazole (Spector, L.B. and Keller, E.B. (1958) J. Biol. Chem. 232, 185-192), (ii) most of the omega residues in prokaryotic 23 S rRNA are confined to the peptidyl transferase center in E. coli ribosomes, and (iii) Um-Gm-omega, the most densely modified sequence in eukaryotic 26 S rRNA, is universally conserved at a fixed site in the putative peptidyl transferase center of all eukaryotic ribosomes.

Locations of methyl groups in 28 S rRNA of Xenopus laevis and man. Clustering in the conserved core of molecule

28 S ribosomal RNA from several vertebrate species contains some 68 to 70 methyl groups. Evidence described in this paper enables some 58 methyl groups to be located in the primary structure of 28 S ribosomal RNA from Xenopus laevis. Most of the locations are unambiguous but a few are currently tentative. In human 28 S ribosomal RNA the great majority of the same sites are methylated as in Xenopus, but there are a few differences between the respective methyl group distributions. The main features of the methyl group distribution are as follows. (1) All of the identified methyl groups are in conserved core regions of 28 S ribosomal RNA. (2) Methylation is much more heavily concentrated in the 3' region of the molecule than in the 5' region (in contrast to 18 S ribosomal RNA, in which there is a major cluster of 2'-O-methyl groups in the 5' region). (3) In addition to the heavily methylated 3' region, clusters of methyl groups occur elsewhere in 28 S ribosomal RNA in the vicinity of domain boundaries. For domains 3 to 6, the two ends of each domain are united in a helix and are linked to adjacent domains either directly or by short single-stranded regions. It therefore follows that the methyl groups near the boundaries of these domains come together into the same general region of the three-dimensional structure. Within this large-scale pattern of distribution, methyl groups occur in a variety of local environments, examples of which are discussed. The triply methylated sequence Am-Gm-Cm-A occurs in a short single-stranded region which links domain 3 to domain 4. Near the 3' end of domain 5 there is a cluster of 11 methyl groups including a 2'-O-methyl pseudouridine in a tract of 160 nucleotides whose sequence is totally conserved between Xenopus and man. These methyl groups are variously distributed between single-stranded regions and short or imperfect but conserved helices. A further cluster of methyl groups including the highly conserved Um-Gm-psi sequence occurs in a region of domain 6 which is implicated in peptidyl transfer. Possible relationships between methylation and other events in ribosome maturation are discussed.

Pseudouridine distribution in mammalian 18 S ribosomal RNA. A major cluster in the central region of the molecule

Human and rodent 18 S rRNA contain about 38 pseudouridine residues. By correlating RNA oligonucleotide data with complete sequence data derived from ribosomal DNA, 30 pseudouridine residues can be located in the RNA sequence, either exactly or to within two or three residues. Pseudouridine and 2'-O-methyl groups are interspersed throughout mammalian 18 S rRNA, but not in closely parallel fashion. Whereas the largest cluster of 2'-O-methyl groups is in the 5' one-third of the molecule, the greatest concentration of pseudouridine is in the central one-third of the molecule.

Identification of the locations of the methyl groups in 18 S ribosomal RNA from Xenopus laevis and man

The 18 S ribosomal RNA from a variety of vertebrate species contains some 40 to 47 methyl groups. The majority of these are 2'-O-ribose substituents; the remaining few are on bases. Several lines of evidence have permitted the identification of the precise locations of the methyl groups in the primary structure of 18 S ribosomal RNA of Xenopus laevis and man. Digestion of RNA with T1 ribonuclease, followed by analysis of the methylated oligonucleotides yielded data on sequences immediately surrounding the methyl groups. Preparative hybridization of X. laevis 18 S ribosomal RNA restriction fragments of ribosomal DNA, followed by fingerprinting analysis on RNA recovered from the hybrids, allowed each methylated oligonucleotide to be mapped to a specific region within 18 S ribosomal RNA. The data on RNA oligonucleotides were correlated with Xenopus ribosomal DNA sequence data in the regions defined by the mapping experiments to identify the precise locations of most of the methyl groups in the X. laevis 18 S RNA sequence. The remaining uncertainties in Xenopus were solved with the aid of data from ribonuclease A fingerprints and, in a few instances, relevant oligonucleotide or sequence data from other laboratories. The locations of most of the methyl groups in human 18 S ribosomal RNA were deduced from the high degree of correspondence between methylated oligonucleotides from human and X. laevis 18 S RNA, together with knowledge of the human 18 S ribosomal DNA sequence. The remaining methylation sites in human 18 S RNA were located with assistance from relevant published comparative data. In the aligned sequences, human and other mammalian 18 S RNA are methylated at all the same positions as in X. laevis, and there are seven additional 2'-O-methylation sites in mammalian 18 S RNA. Further features of the methyl group distribution are briefly reviewed.

Human 18 S ribosomal RNA sequence inferred from DNA sequence. Variations in 18 S sequences and secondary modification patterns between vertebrates

We have determined the DNA sequences encoding 18 S ribosomal RNA in man and in the frog, Xenopus borealis. We have also corrected the Xenopus laevis 18 S sequence: an A residue follows G-684 in the sequence. These and other available data provide a number of representative examples of variation in primary structure and secondary modification of 18 S ribosomal RNA between different groups of vertebrates. First, Xenopus laevis and Xenopus borealis 18 S ribosomal genes differ from each other by only two base substitutions, and we have found no evidence of intraspecies heterogeneity within the 18 S ribosomal DNA of Xenopus (in contrast to the Xenopus transcribed spacers). Second, the human 18 S sequence differs from that of Xenopus by approx. 6.5%. About 4% of the differences are single base changes; the remainder comprise insertions in the human sequence and other changes affecting several nucleotides. Most of these more extensive changes are clustered in a relatively short region between nucleotides 190 and 280 in the human sequence. Third, the human 18 S sequence differs from non-primate mammalian sequences by only about 1%. Fourth, nearly all of the 47 methyl groups in mammalian 18 S ribosomal RNA can be located in the sequence. The methyl group distribution corresponds closely to that in Xenopus, but there are several extra methyl groups in mammalian 18 S ribosomal RNA. Finally, minor revisions are made to the estimated numbers of pseudouridines in human and Xenopus 18 S ribosomal RNA.

Nucleotide sequence of Xenopus laevis 18S ribosomal RNA inferred from gene sequence

18S ribosomal RNA in Xenopus laevis is 1,825 nucleotides long, as inferred from sequence analysis of an 18S gene. All the 40 rRNA methyl groups can be located in the sequence. Comparison with the yeast (Saccharomyces cerevisiae) 18S sequence reveals extensive regions of high homology interspersed with tracts having little or no homology. Regions of high homology contain almost all the RNa methyl groups. Major regions of low homology area considerably richer in C + G in Xenopus than in yeast.

Characterization of cloned rat ribosomal DNA fragments

Two Charon 4A lambda bacteriophage clones were characterized which contain all and part o the 18S ribosomal DNA of the rat. One clone contained two Eco RI fragments which include the whole 18S ribosomal RNA region and part of 28S ribosomal RNA region. The other clone contained an Eco RI fragment which covers part of 18S ribosomal RNA region. There were differences between the two clones in the non-transcribed spacer regions suggesting that there is heterogeneity in the non-transcribed spacer regions of rat ribosomal genes. The restriction maps of the two clones were compared to the restriction map of the cloned mouse ribosomal DNA. Eco RI, Hind III, Pst I, and Bam HI sites in 18S ribosomal RNA regions were in the same places in mouse and rat DNA but the restriction sites in the 5'-spacer regions were different.

Nucleotide sequence through the 18S-28S intergene region of a vertebrate ribosomal transcription unit

We have determined the nucleotide sequence of part of a cloned ribosomal transcription unit from Xenopus laevis extending from the 3' region of the 18S gene through the 18S-28S intergene region into the start of the 28S gene. The 18S 3' region possess two tracts of high homology with the corresponding segments of other eukaryotic 18S genes (yeast and Bombyx mori) separated by a tract of low homology which in X. laevis is rich in G plus C. The first internal transcribed spacer, between the 18S and 5.8S genes, is 557 nucleotides long, very rich in G plus C (84%) and shows no sequence homology with the corresponding yeast sequence. The 5.8S rRNA sequence is revised slightly in the light of the DNA sequence. The second internal transcribed spacer, between the 5.8S and 28S genes, is 262 nucleotides long and is even richer in G plus C (88%) than the first internal spacer. 28S rRNA starts with the sequence pUCAG. This is encoded at the first of three closely linked TCAG sites in rDNA.

Methylation map of Xenopus laevis ribosomal RNA

One of the most enigmatic features of eukaryotic ribosomal RNA is the presence of many methylated nucleotides. The numbers of RNA methyl groups range from approximately 70 per ribosome in yeast to over 100 in vertebrates. Here it is shown that the methylated nucleotides in Xenopus laevis rRNA are broadly but non-uniformly distributed. In 18S rRNA 2'-O-methylations are partly concentrated in the 5' region and base methylations near the 3' end. In 28S rRNA methyl groups are infrequent in the 5' region, moderately frequent in the central region and abundant in an 1,100-nucleotide tract near the 3' end.

The 3'-terminal primary structure of five eukaryotic 18S rRNAs determined by the direct chemical method of sequencing. The highly conserved sequences include an invariant region complementary to eukaryotic 5S rRNA

The 3'-terminal sequences of 18S rRNA from chicken reticulocyte, mouse sarcoma, rat liver, rabbit reticulocyte and barley embryo were determined by the direct chemical sequencing method. The regions sequenced show complete homology for the first 73 nucleotides. A sequence 5'-proximal to the m6(2)Am6(2)A residues that is complementary to eukaryotic 5S RNAs is totally conserved. This supports the hypothesis that base-paired interaction between 5S and 18S rRNA, which are present in the large and small ribosomal subunits respectively, may be involved in the reversible association of ribosomal subunits during protein synthesis.

Hybridization of denatured RNA and small DNA fragments transferred to nitrocellulose

A simple and rapid method for transferring RNA from agarose gels to nitrocellulose paper for blot hybridization has been developed. Poly(A)+ and ribosomal RNAs transfer efficiently to nitrocellulose paper in high salt (3 M NaCl/0.3 M trisodium citrate) after denaturation with glyoxal and 50% (vol/vol) dimethyl sulfoxide. RNA also binds to nitrocellulose after treatment with methylmercuric hydroxide. The method is sensitive: about 50 pg of specific mRNA per band is readily detectable after hybridization with high specific activity probes (10(8) cpm/microgram). The RNA is stably bound to the nitrocellulose paper by this procedure, allowing removal of the hybridized probes and rehybridization of the RNA blots without loss of sensitivity. The use of nitrocellulose paper for the analysis of RNA by blot hybridization has several advantages over the use of activated paper (diazobenzyloxymethyl-paper). The method is simple, inexpensive, reproducible, and sensitive. In addition, denaturation of DNA with glyoxal and dimethyl sulfoxide promotes transfer and retention of small DNAs (100 nucleotides and larger) to nitrocellulose paper. A related method is also described for dotting RNA and DNA directly onto nitrocellulose paper treated with a high concentration of salt; under these conditions denatured DNA of less than 200 nucleotides is retained and hybridizes efficiently.

Comparative studies of the primary structures of ribosomal RNAs of several eukaryotic cell lines by the fingerprinting method

Comparisons of the primary structures of 18S and 28S ribosomal RNAs of man, rat, mouse and chicken were made by two-dimensional fractionation including electrophoresis at pH 3.5 and homochromatography. All large T1 oligonucleotides were recovered from the different fingerprints and their radioactivity was measured. They were then hydrolysed with pancreatic RNase and the pancreatic products were digested with alkali to determine their base composition and detect modified residues. Finally, residues bearing a modification on the ribose were analysed by hydrolyses with snake venom and spleen phosphodiesterases. For the 18A RNAs 23, 27, 26, 24 oligonucleotides, whose lengths range from 22 to 10 residues, were analyzed respectively for man, rat, mouse and chicken. Among these, 14 are identical in the four species, two at least are common to man, rat, mouse but differ by the presence of A-Cps in chicken spot 4' instead of A-Up in spot 4 and A2-Gp in chicken spot 14 instead of A2-Gp in spot 13. For the 28S RNAs of man, rat, mouse and chicken, 20, 19, 21 and 22 oligonucleotides ranging in length from 27 to 12 residues were analyzed. 11 of them are common to the four species; 4 of them are found in man, rat, mouse and one of these (spot 1) has a corresponding spot in chicken from which it differs only by the existence of A3-Up instead of A2-Up. Another mammalian oligonucleotide (spot 6) differs from its homologous chicken spot (spot 6') bytwo point mutations. The same modified residues as found by Khan and Maden in man, chicken, and xenopus, have been found in rat and mouse. Moreover when these modified residues are common to several species they are found within an identical nucleotide sequence, as can be seen in the case of spots 1, 3, 9, 11 of 18S RNAs and 4, 7, 13 for 28S RNAs. The number of differences observed between the ribosomal RNAs of the four species were compared to the number of differences observed in the same species for several proteins, globins alpha and beta, insulin, cytochrome C and lysozyme.

Analysis of large specific T1 oligonucleotides of 17S and 25S ribosomal RNAs from Saccharomyces cerevisiae

The primary structure of 17S and 25S ribosomal RNAs from Saccharomyces cerevisiae has been analysed by two-dimensional fractionation of T1 oligonucleotides. This method consists of an electrophoresis at pH 3.5 followed by a homochromatography on DEAE-cellulose plates. After the second dimension, the large T1 oligonucleotides were hydrolyzed by pancreatic RNAse, followed by alkaline hydrolysis of the pancreatic products. By fractionating a mixture of tritiated HeLa cell ribosomal RNAs and 32 P yeast cell ribosomal RNAs, two autoradiographs were obtained; one corresponding to the 32P labelled material and the other to the tritiated labelled material. By superposition of the two autoradiographs, the mobility of the various T1 oligonucleotides can be accurately compared and it is shown that yeast 17S rRNA and human 18S rRNA have in common 5 large oligonucleotides and that yeast 25S rRNA and human 28S rRNA have 4 identical oligonucleotides.

Studies on the methylation of cytoplasmic ribosomal RNA from cultured higher plant cells

The methylation of cytoplasmic ribosomal RNA of cultured sycamore cells (Acer pseudoplatanus L.) was investigated. Labelled 17-S and 26-S rRNA were prepared from cells that had been incubated with either [32P]phosphate, [Me-3H]methionine or [Me-14C]methionine. Ion-exchange resin chromatography of 0.3 M KOH or 1 M HCl hydrolysates and two-dimensional chromatographic analyses of phosphodiesterase plus phosphatase digests of 17-S and 26-S rRNA were performed. 17-S and 26-S rRNA contain 49 and 91 methyl groups per molecule, respectively. These values were verified in sevemral ways. The high degree of methylation of sycamore rRNA, particularly for the 26-S rRNA, contrasts with the situation in all other investigated organisms. Several methylated bases were identified. 7-Methylguanine and 5-methylcytosine both occur in 17-S and 26-S rRNA. N6-Methyladenine and N6,N6-dimethyladenine are restricted to the 17-S rRNA while 3-methyluracil and 1-methyladenine occur in the 26-S rRNA. One hypermodified uridine was also tentatively identified in the small rRNA. In 17-S rRNA, there is one copy of 7-methylguanine, N6-methyladenine and hypermodified uridine and two copies of N6,N6-dimethyladenine. 3-Methyluracil, 1-methyladenine and 5-methylcytosine occur twice, twice and three times, respectively, in 26-S rRNA. 7-Methylguanine and 5-methylcytosine are only in submolar amounts in the 26-S and 17-S rRNA, respectively. There are 40 +/- 2 and 83 +/- 3 2'-O-methylriboses per 17-S and 26-S rRNA molecule, respectively. In addition to the four 2'-O-methylnucleosides, one 2'-O-methylpseudouridine is present in the 17-S rRNA. Several lines of evidence argues for a non-random distribution of the methylriboses. In particular, one and seven Nm-Nm-Np structures occur in the 17-S and 26-S rRNA, respectively. The data are discussed comparatively with the methylation pattern of Escherichia coli, yeast and HeLa cell rRNA.

Partial base-methylation and other structural differences in the 17 S ribosomal RNA of sycamore cells during growth in cell culture

Sycamore (Acer pseudoplatanus L.) cytoplasmic rRNA was investigated in rapidly dividing cells, cells starting mitosis after the lag phase of growth (4 days) induced by deconditioning of the culture medium and also in growth-arrested cells from 10 day-old cultures deprived of exogenous auxin (i.e. exponential, early exponential and 2,4-dichlorophenoxyacetic acid (2,4-D)-deprived cultures). rRNA was extracted and purified from mixed 14C-labelled exponential cultures and 3H-labelled early exponential cultures. A 14C-labelled exponential culture and a 3H-labelled 2,4-D-deprived culture were analyzed in the same way. The 17 S rRNA molecules from both early exponential and 2,4-D-deprived cultures displayed a lower electrophoretic mobility on polyacrylamide gels than those from exponential cultures. Alkaline and acid hydrolysates of purified 17 S rRNA labelled on the phosphate groups or the methyl groups were analyzed on ion-exchange resins. There was no change in the extent of ribose methylation of the molecule from the three different cultures. However, the base methylation of the 17 S rRNA was decreased in early exponential cultures and in 2,4-D-deprived cultures. Part of the molecules synthesized in early exponential cultures specifically lacked 7-methylguanine, N6-methyladenine and N6,N6-dimethyladenine. The possible significance of these changes in the 17 S rRNA were discussed.

The pseudouridine contents of the ribosomal ribonucleic acids of three vertebrate species. Numerical correspondence between pseudouridine residues and 2'-O-methyl groups is not always conserved

The pseudouridine contents of the rRNA species of HeLa cells, mouse L-cells and Xenopus laevis cultured kidney cells were examined. Pseudouridine, like 2'-O-methylation, was found to occur relatively frequently in each of the high-molecular-weight rRNA species. However, the numerical data do not support the idea that there is a general one-to-one relationship between pseudoridine residues and 2'-O-methyl groups in vertebrate rRNA.

Studies on the conformation of the 3' terminus of 18-S rRNA

We have studied the conformation of the 3' end of 18-S RNA from human, hamster and Xenopus laevis cells. The 3'-terminal oligonucleotide in a T1 ribonuclease digest of 18-S RNA from HeLa cells was identified, using a standard fingerprinting method. The sequence (G)-A-U-C-A-U-U-A, established by Eladari and Galibert for HeLa 18-S rRNA, was confirmed. An identical 3' terminus is present in hamster fibroblasts and Xenopus laevis cells. The ease of identification of this oligonucleotide has enabled us to quantify its molar yield relative to several other oligonucleotides, and hence to analyse the 3' terminus by several conformation probes. Its sensitivity to S1 nuclease, limited T1 ribonuclease digestion, bisulphite modification and carbodiimide modification was consistent with the terminal oligonucleotide being in a highly exposed conformation. The m6/2A-m6/2A-C-containing sequence of 18-S rRNA also appears to be in an exposed location on the basis of three of these probes.

Conservation of the primary structure at the 3' end of 18S rRNA from eucaryotic cells

DNA sequencing methods have been used to determine a sequence of about 20 nucleotides at the 3' termini of various 18S (small ribosomal subunit) RNA molecules. Polyadenylated rRNA was first synthesized using the enzyme ATP:polynucleotidyl transferase from mainze. Then in the presence of an oligonucleotide primer uniquely complementary to the end of each adenylated rRNA, a cDNA copy was produced using AMV reverse transcriptase. In every case, the cDNA transcript was of finite size, which we ascribe to the appearance of an oligonucleotide containing m62A near the 3' end of the 18S rRNAs. Sequences at the 3' termini of 18S rRNA molecules from the four eucaryotic species examined here (mouse, silk worm, wheat embryo and slime mold) are highly conserved. They also exhibit strong homology to the 3' end of E. coli 16S rRNA. Two important differences, however, are apparent. First, the 16S sequence CCUCC, implicated in mRNA binding by E. coli ribosomes, is absent from each eucaryotic rRNA sequence. Second, a purine-rich region which exhibits extensive complementarity to the 5' noncoding regions of many eucaryotic mRNAs appears consistently.

Conformation of methylated sequences in HeLa cell 18-S ribosomal RNA: nuclease S1 as a probe

18-S rRNA from HeLa cells was digested with nuclease S1. Under the conditions employed 15% of the total nucleotides and some 50% of the methylated nucleotides were released as low-molecular-weight products. The material which was precipitable by 70% ethanol after nuclease S1 digestion was subjected to further digestion by combined T1 plus pancreatic ribonucleases or by T1 ribonuclease alone, and fingerprints were prepared. It was found that the four sites which are modified late during ribosome maturation, and which contain base modifications, were all accessible to nuclease S1. By contrast fewer than one-half of the sites which are modified early during ribosome maturation, and which contain 2'-O-methyl groups, were accessible to nuclease S1; the remainder were protected, presumably by secondary or tertiary interactions within 18-S rRNA.

Biosynthesis of a hypermodified nucleotide in Saccharomyces carlsbergensis 17S and HeLa-cell 18S ribosomal ribonucleic acid

The biosynthesis of a hypermodified nucleotide, similar to or identical with 3-(3-amino-3-carboxypropyl)-1-methylpseudouridine monophosphate, present in Saccharomyces carlsbergensis 17S and HeLa-cell 18S rRNA, was investigated with respect to the sequence of reactions required for synthesis and their timing in ribosome maturation. In both yeast and HeLa cells methylation precedes attachment of the 3-amino-3-carboxypropyl group. In yeast the methylated precursor nucleotide was tentatively characterized as 1-methylpseudouridine. This precursor nucleotide was demonstrated in both 37S and most of the cytoplasmic 18S pre-rRNA (rRNA precursor) molecules. The synthesis of the hypermodified nucleotide is completed just before the final cleavage of 18S pre-rRNA to give 17S rRNA, so that the final addition of the 3-amino-3-carboxypropyl group is a cytoplasmic event. Comparable experiments with HeLa cells indicated that formation of 1-methylpseudouridine occurs at the level of 45S RNA and addition of the 3-amino-3-carboxypropyl group occurs in the cytoplasm on newly synthesized 18S RNA.

Methylated nucleotide sequences in HeLa-cell ribosomal ribonucleic acid. Correlation between the results from 'fingerprinting' hydrolysates obtained by digestion with T1 ribonuclease and with T1 plus pancreatic ribonuclease

The methylated nucleotide sequences in HeLa-cell rRNA were previously characterized after enzymic digestion of the rRNA by T1 ribonuclease alone or by combined T1 plus pancreatic ribonucleases. For any methylated product occurring in a T1-ribonuclease digest there must be one or more corresponding products in a combined T1-plus-pancreatic-ribonuclease digest. Here we correlate fully the inter-relationship between the methylated products occurring in the two digestion systems. The analysis has led to the resolution of some previous uncertainties and has permitted an almost complete qualitative and quantitative description of the methylated components in HeLa-cell rRNA. The data are compared with those reported by other authors for HeLa-cell rRNA.

Secondary methylation of yeast ribosomal precursor RNA

The timing of methylation of the ribosomal sequences of ribosomal precursor RNA (pre-rRNA) from the yeast Saccharomyces carlsbergensis was investigated by fingerprint analysis of the methylated oligonucleotides derived from the various precursors. From the total of 37 ribose and 6 base-methyl groups found in 26-S rRNA, the two copies of the base-methylated nucleoside m3U as well as the doubly methylated sequence Um-Gm psi are not yet present in 37-S RNA, the predominant common precursor of 26-S and 17-S rRNA. Introduction of these methyl groups into the ribosomal sequences appears to take place at the level of 29-S pre-rRNA, the immediate precursor to 26-S rRNA. From the total of 18 ribose-methylated and 6 base-methylated nucleosides found in 17-S rRNA, the latter group (one copy of m7G, the m62A-m62A- sequence and the hypermodified methylated nucleoside "mX") is completely missing in 37-S pre-rRNA. The methyl group of m7G is introduced into 18-S pre-rRNA, the direct precursor of 17-S rRNA, in the nucleus. The -m62A-m62A- sequence is methylated after transport of the 18-S pre-rRNA to the cytoplasm prior to the final maturation into 17-S rRNA.

Comparison between the ribosomal ribonucleic acids from free and membrane-bound ribosomal fractions of HeLa cells

The rRNA species from the total cytoplasmic, free and membrane-bound fractions of HeLa cells were compared. With the use of T1 ribonuclease and combined T1 ribonuclease plus pancreatic ribonuclease 'fingerprinting' procedures, no significant differences were found between the rRNA species from the different subcellular fractions.