Electrophoretic characterization of shuttling and nonshuttling small nuclear RNAs

Differential behavior of liposome-introduced specific RNAs in living Drosophila cells

We have developed a protocol for efficiently introducing macromolecules into Drosophila tissue culture cells using liposomes. By carefully adjusting the fusion parameters, conditions have been established to routinely encapsulate 15-30% of the starting material into liposomes and to introduce 20-30% of the liposome-encapsulated material into the cells during a 30-minute fusion period. Essentially, all of the cells receive material from the liposomes and 10(9) cells can be fused at once. The fusion does not have any measureable effect on cell viability as assayed by trypan blue exclusion, growth rate, and cell morphology. We have utilized this technique to introduce radioactive RNAs into nonradioactive cells, thus enabling the behavior of the introduced RNAs to be followed unambiguously. Liposome-introduced small nuclear RNAs (snRNAs) are stable in the cell for at least 25 hours (approximately two cell generations), with 80% of the radioactivity remaining trichloroacetic acid (TCA) precipitable and the gel electrophoresis pattern remaining essentially unchanged. This is in contrast to liposome-introduced cytoplasmic RNAs, which are only 20% TCA precipitable after the first hour. In the cell, the introduced snRNAs attain a 10-35-fold higher concentration in the nucleus than the cytoplasm. Nuclear accumulation is not seen with Drosophila tRNA or 5S RNA, both of which attain the same nuclear as cytoplasmic RNA concentration.

High level of complexity of small nuclear RNAs in fungi and plants

The complexity of the trimethylguanosine-capped, small nuclear RNA (snRNA) populations in a number of organisms has been examined using immunoprecipitation and two-dimensional gels. From the fungi Aspergillus nidulans and Schizosaccharomyces pombe, over 30 major snRNAs can be resolved. The most abundant of these correspond to the putative analogues of vertebrate U1, U2, U4 and U5, which have been reported to be precipitated by anti-Sm antibodies, but other snRNAs are little less abundant than the major Sm-precipitable species. A similarly high level of complexity of snRNAs is detected in pea plants. In Candida albicans, the snRNAs are somewhat less numerous (about 22 major species) and are substantially less abundant than those of the above fungi, features shared with another budding yeast, Saccharomyces cerevisiae. Ten species of human snRNA have been reported; on two-dimensional gels, a number of additional snRNAs can be resolved from human cells. Each fungus, as well as pea plants, contains snRNAs substantially larger than any reported from vertebrates or detected in the human RNA used here. It appears that many eukaryotes contain substantially more species of snRNA than was previously believed.

Small nuclear RNAs in the ciliate Tetrahymena

We have isolated and partially characterized a family of small nuclear RNAs (snRNAs) from three different species of the protozoan Tetrahymena. We find six distinct snRNAs ranging in size from 100 to 250 nucleotides. The two largest snRNAs, as well as an abundant, heterogenous group of smaller snRNAs are found in the nucleolar RNA fraction. None of the snRNAs are transcription products of the ribosomal RNA gene or its flanking regions, as shown by hybridization tests. The snRNAs are metabolically stable as determined by pulse/chase experiments and several of them contain a number of modified nuclotides. The snRNAs from Tetrahymena all have slightly different sizes from mammalian snRNAs. The cap structure of the snRNAs from Tetrahymena differs from that of the snRNAs from mammalian cells, but has not yet been fully characterized. The relative amount of snRNAs to total RNA is less in Tetrahymena (greater than 0.1%) than in mammalian cells (2%).

Yeast contains small nuclear RNAs encoded by single copy genes

We have identified a group of RNA molecules in Saccharomyces cerevisiae that appears to be equivalent to the U class of small nuclear RNAs previously described in other eucaryotes, resembling them in size, metabolic stability, 5' cap structure, presence of modified bases, and nuclear localization. However, the yeast snRNAs differ from their counterparts in several potentially important ways. First, they are present in very low abundance, less than 200 copies per cell, as compared to 10(5)-10(6) for mammalian U1-U6. Second, there appear to be more species in yeast than in any cell type previously examined. Finally, we have cloned five yeast snRNA genes, and find that each is present in a single copy per haploid genome, whereas all previously characterized snRNAs are encoded by multiple (5 to 100) gene copies. The presence of single copy genes in yeast will greatly facilitate the genetic analysis of snRNA function.

Small RNAs from Drosophila KC-H cells. Characterization of nuclear and cytoplasmic 7S subspecies

The metabolic characteristics and nuclear and cytoplasmic localization of Drosophila 7 S small RNAs were investigated. It was found that although most Drosophila small RNAs show electrophoretic mobilities similar to those of mammalian cells, distinct metabolic behaviour can be detected in three 7 S subspecies (d1, d3 and d4) which were found not to be class III RNAs as reported for mammalian 7 S RNAs. Drosophila d3 and d4 were found to be localized in the nucleus and to be very rapidly labelled minor subspecies turning over with a half-life of less than 6 h. Although 7 S-d1 migrates close to mammalian 7 S-K, it is a major subspecies in Drosophila, is localized mostly in the nucleus and turns over with a half-life of less than 10 h in contrast with mammalian 7 S-K which has been observed to be quite stable.

Attenuation and modulation of mRNA secondary structure in a feedback control system regulating SV40 gene expression

Alternative secondary structures can be predicted for the initial 94 nucleotides synthesized from the major transcription initiation site of SV40 late RNA: a transcription-termination conformation results in the production of aborted RNA and a readthrough conformation leads to the synthesis of the primary SV40 late RNA. In the cytoplasm similar alternative conformations can be predicted for the initial nucleotides at the 5' ends o both the major 16S and 19S late mRNAs. In one of these alternative conformations the AUG initiation codon of the leader protein (agnoprotein) is sequestered and not available for ribosome binding. In the alternative conformation the same AUG is accessible for ribosome binding. We suggest that these mutually exclusive conformations are fundamental elements in a transcription and translation feedback control mechanism regulating the synthesis of 16S and 19S mRNA in the nucleus and the translation of their encoded proteins in the cytoplasm.

Isolation and characterization of hnRNA-snRNA-protein complexes from Morris hepatoma cells

Of the RNA labelled after incubation of hepatoma cells with radioactive precursors for 20 and 150 min. 35% and 70%, respectively, can be isolated from nuclei by two consecutive extractions with 0.14 M NaCl at pH 8. The isolated RNA is complexed with nuclear proteins forming structures with sedimentation coefficients of less than 30 S to greater than 100 S. Similar complexes from rat liver isolated under the same experimental conditions show coefficients of 30-40 S. The RNA-associated proteins are similar, on the basis of sodium dodecyl sulphate/polyacrylamide gel electrophoresis, to the respective proteins of other cell types. The presence on these RNP complexes of six discrete small nuclear RNAs (snRNA) has been established. Experiments with a reversible inhibitor of RNA synthesis, D-galactosamine, demonstrated, differences in the turnover of hnRNA and snRNA. The half-lives of the six snRNA species has been determined, varying from 32 h for snRNA species a, b and d, to 22 h for snRNA species e and f and to 13 h for snRNA species c. Treatment of the nuclear extracts with 0.7 M and 1 M NaCl results in dissociation of hnRNA from the 'core' and other polypeptides, whereas snRNA remains complexed with polypeptides of Mr 54 000-59 000. Incubation of the nuclear extracts at 0 C with low doses of pancreatic R Nase (up to 1.5 micrograms/ml), which renders approximately 80% of the hnRNA acid-soluble and cleaves most of the snRNA, results in conversion of the high-molecular-weight hnRNPs to 30-S structures, without disrupting the 30-S RNP. Treatment of the nuclear extracts with higher doses of RNase (3 micrograms/ml) leads to disruption of the 30-S RNP and release of the hnRNA-associated proteins, underlining the importance of hnRNA-protein interaction for the retainment of the hnRNP structures.

Characterization of RNP and Sm ribonucleoprotein nuclear antigens

Antibodies to the RNase-sensitive RNP and to the RNase-resistant Sm nuclear antigens were used to affinity purify these antigens from a saline extract of rabbit thymus acetone powder. Determination of the protein subunits recovered by either glycine-HCl, pH 2.8, or 2.5 M MgCl2 elution on gradient sodium dodecyl sulfate-polyacrylamide electrophoresis containing mercaptoethanol revealed that RNP was composed of five proteins with mol. wts from 10,000 to 15,000 whereas Sm contained the same or similar five chains plus six additional subunits with mol. wts from 21,000 to 42,000. RNase treatment of the thymus extract increased the recovery in Sm of the same bands compared to untreated extract. Thus, RNP and Sm appear to have different numbers of protein components and RNP may be a subset of Sm. Sucrose gradient centrifugation of the 125I-labeled, pH 2.8 eluted antigens gave peaks of 3 and 6S for RNP and Sm, respectively. Sucrose gradient centrifugation of the crude untreated thymus extract followed by quantitative single radial immunodiffusion analysis of each fraction produced a broad peak from 16S to the top of the gradient while pretreatment of the extract with RNase resulted in a discrete 6S peak. These results indicate that in rabbit thymus acetone powder native RNP and Sm exist as larger polydisperse complexes with additional material including RNA and that after acid elution or RNase treatment the antigens are found in a smaller monodisperse form.

Small nuclear RNAs from Drosophila KC-H cells; characterization and comparison with mammalian RNAs

Small molecular weight nuclear RNAs were extracted from cultured Drosophila KC-H cells and characterized by their electrophoretic mobilities in 5--15% gradient acrylamide gels or in 10% acrylamide-7 M urea gels. Comparison between the electrophoretic profiles of these SnRNAs with those from human and mouse cells revealed striking similarities and allowed for assignation of band nomenclatures as established for mammalian cells. Comparison of mobilities in the two gel systems also permitted correspondence between the different nomenclatures established by various groups for this class of RNAs, as well as an approximate estimate of their molecular sizes.

Intracellular transport of microinjected 5S and small nuclear RNAs

The mechanism by which some RNAs are segregated in the cell nucleus was analysed by microinjecting 32 P-labelled total RNA from HeLa cells into the cytoplasm of Xenopus oocytes. Small nuclear RNAs (u1, U2, U4, U5 and U6) migrated into the cell nucleus, where they became 30-60 fold more concentrated than in the cytoplasm. Other RNAs, such as tRNA and 7S RNA, remained in the cytoplasm, while 5S RNA became concentrated in the nucleolus. Studies with lupus erythematosus antibodies showed that the migrating RNAs become associated with oocyte RNA-binding proteins.

The structure of ribonucleoprotein particles from rat liver nuclei

During or immediately after transcription of chromatin the high molecular weight pre-mRNA is complexes with proteins and low molecular weight RNA (lmwRNA). In the presence of a cytosolic RNase inhibitor pre-mRNA-protein complexes, designated as polyparticles, can be isolated from rat liver nuclei. The polyparticles are characterized by a maximum of their sedimentation coefficient of around 90 S, a protein to RNA ratio of 4.1, and a density in CsCl of 1.4 g/cm3. A set of 6--10 basic proteins of molecular weights between 30 and 45 kd as well as a multitude of polypeptides of higher molecular weights is associated with the rapidly labeled, polydisperse, high molecular weight RNA and several lmwRNA species. In order to study the structure of these very complex nuclear RNP complexes, the polyparticles were incubated at various concentrations of sodium chloride, urea or proteinases of different specificities (trypsin, chymotrypsin, proteinase K), recentrifuged through a sucrose layer and analyzed with respect to their sedimentation behavior, their protein to RNA ratios and their protein- and RNA components. Rhe results of these experiments led us to the proposal of a structural model which is presented here.

Distribution of proteins between nucleus and cytoplasm of Amoeba proteus

By transplanting nuclei between labeled and unlabeled cells, we determined the localization of the major proteins of amebas and described certain features of their intracellular distributon. We identified approximately 130 cellular proteins by fluorography of one-dimensional polyacrylamide electrophoretic gels and found that slightly less than half of them (designated NP, for nuclear proteins) are almost exclusively nuclear. About 95 percent of the other proteins (designated CP for cytoplamsic proteins) are roughly equally concentrated in nucleus and cytoplasm, but-because the cytoplasm is 50 times larger than the nucleus-about 98 percent of each of the latter is in the cytoplasm. Of the CP, roughly 5 percent are not detectable in the nucleus. Assuming that these are restricted to the cytoplasm only because, for example, they are in structures too large to enter the nucleus and labeled CP readily exit a nucleus introduced into unlabeled cytoplasm, we conclude that the nuclear envelope does not limit the movement of any nonstructural cellular protein in either direction between the two compartments. Some NP are not found in the cytoplasm (although ostensibly synthesized there) presumably because of preferential binding within the nucleus. Almost one half of the protein mass in nuclei in vivo is CP and apparently only proteins of that group are lost from nuclei when cells are lysed. Thus, while an extracellular environment allows CP to exit isolated nuclei, the nuclear binding affinities for NP are retained. Further examination of NP distribution shows that many NP species are, in fact, detectable in the cytoplasm (although at only about 1/300 the nuclear concentration), apparently because the nuclear affinity is relatively low. These proteins are electrophoretically distinguishable from the high-affinity NP not found in the cytoplasm. New experiments show that an earlier suggestion that the nuclear transplantation operation causes an artifactual release of NP to the cytoplasm is largely incorrect. Moreover, we show that cytoplasmic "contamination" of nuclear preparations is not a factor in classifying proteins by these nuclear transplantation experiments. We speculate the no mechanism has evolved to confine most CP to the cytoplasm (where they presumably function exclusively) because the cytoplasm's large volume ensures that CP will be abundant there. Extending Bonner's idea of "quasi-functional nuclear binding sites" for NP, we suggest that a subset of NP usually have a low affinity for available intranuclear sites because their main function(s) occurs at other intranuclear sites to which they bind tightly only when particular metabolic conditions demand. The other NP (those completely absent from cytoplasm) presumable always are bound with high affinity at their primary functional sites.

Small RNA species in maize tissue

The low molecular weight RNA components of maize have been analyzed after labeling callus and leaf tissue with [(3)H]uridine in vitro. Electrophoresis of the isolated RNA on acrylamide slab gels reveals, apart from 5S and transfer RNA, three major and about five minor RNA species with chain lengths between 140 and 280 nucleotides. These RNA molecules are labeled as rapidly as 5S, transfer RNA, and do not represent degradation products of large ribosomal RNA molecules. Furthermore, like 5S and transfer RNA, these small RNA species are stable and show no detectable turnover within forty-eight hours. Fractionation of the tissue into crude subcellular fractions indicates a preferential association of some of the small stable RNA species with the nucleus, while others appear to be located in the cytoplasm. The low molecular weight RNA spectrum from the leaf is similar to that observed in callus, with the major small RNA species equally present in both tissues.

An abundant cytoplasmic 7S RNA is complementary to the dominant interspersed middle repetitive DNA sequence family in the human genome

Evidence is presented that a homogeneous cytoplasmic species known as 7S RNA is the only abundant RNA in uninfected HeLa cells which can form strong hybrids with the dominant family of middle repetitive DNA sequences in the human genome. These DNA sequences are known collectively as the Alu family, because most of them share a common Alu I restriction site. When purified 7S RNA was hybridized to three different genomic clones containing Alu family DNA sequences, a specific region (or regions) comprising at most half the RNA sequence was protected from mild digestion with T1 ribonuclease; moreover, the hybrids between 7S RNA and cloned Alu family DNA wer imperfect, since T1 RNAase was able to nick the protected 7S RNA sequences under conditions where a true RNA: DNA duplex would have been resistant. This suggests that 7S RNA is encoded either by a small subset of the 300,000 Alu family sequences in the human genome or by an entirely different family of genes. The sequence of 7S RNA has been highly conserved through recent evolution, and in both avian and murine cells the RNA is selectively incorporated into oncornavirus particles during productive infection. The cellular function of 7S RNA is unknown.

Reinitiation of synthesis of small cytoplasmic RNA species K and L in isolated HeLa cell nuclei in vitro

Isolated HeLa cell nuclei were used to synthesize low molecular weight RNA species in-vitro. The labelled RNA released from the nuclei during the incubation mainly consists of 5S RNA, pre-tRNA and small cytoplasmic RNA species K and L. All these low molecular weight RNA species are synthesized by RNA polymerase C (or III). The polyanion heparin was applied to study the reinitiation of these RNA molecules in-vitro. A comparison of the kinetics of RNA synthesis in the absence and in the presence of this inhibitor demonstrates a highly efficient in-vitro reinitiation of scRNA species K and L as well as 5S and pre-tRNA by RNA polymerase C. These results indicate a general competence of this enzyme to catalyze the de-novo formation of specific gene products in-vitro.

Stimulation of transcription of chromatin by specific small nuclear RNAs

Small molecular weight nuclear RNAs (SnRNA) purified from the chromatin of SV40-transformed W138 human fibroblasts have been found to stimulate transcription of chromatin in homologous isolated nuclei as well as in nuclei of untransformed human and monkey cells. Stimulation in normal cell nuclei involves an increase in both initiation sites and rate of elongation of RNA chains. Fractionation of the SnRNA in polyacrylamide gradient slab gels revealed that the "active" RNA was 160-175 nucleotides in length.

Low molecular weight RNAs as components of nuclear ribonucleoprotein particles containing heterogeneous nuclear RNA

70-130 S polyparticles as well as 38 S monoparticles were isolated from rat liver nuclei and analyzed in respect to their RNA components by microgel polyacrylamide electrophoresis in formamide. In addition to the high molecular weight polydisperse hnRNA of polyparticles several low molecular weight RNAs (snRNA) were detected. There are at least six distinct snRNA species in polyparticles. Except for one species, which is missing, 38 S monoparticles showed a similar snRNA pattern. From densitometer tracings of microgels the snRNAs were estimated to represent about 11% of the total polyparticle RNA. The number of nucleotides for the various snRNAs were determined from a plot of relative electrophoretic mobility versus log number of nucleotides. The possibility that the snRNAs are degradation products of the hnRNA was excluded on the basis of the following findings. (1) The snRNA pattern was similar in mono- and polyparticles. (2) Whereas the hnRNA of polyparticles incubated at 37 degrees C was extensively degraded, the snRNA did not show a corresponding increase. (3) After a 30 min pulse with [3H]orotate the hn RNA was readily labeled; none of the snRNAs, however, incorporated radioactivity. The snRNAs were still found after treatment of polyparticles with 2 M NaCl excluding contamination by nucleoplasm.

Isolation and characterization of the nuclear matrix from the male Xenopus laevis following estrogen administration: kinetics of [3H] uridine incorporation

At various times following estrogen administration, the nuclear matrix was isolated from the liver of male Xenopus laevis by sucrose gradient centrifugation of nuclei treated with a high-salt buffer and DNase I in the presence of a proteolytic inhibitor (PMSC--phenylmethyl sulfonyl chloride). Electron micrographs of the nuclear matrix demonstrate a sponge-like network attached to a well-defined inner envelope with a ribosome-free outer envelope. Chemical analyses show that the HSB-DNase-treated nuclei consist of 16% DNA, 2% RNA, and 82% protein, a composition that is consistent with that of nuclear matrices isolated from other species. The specific activity of the matrix-associated RNA following estrogen treatment appears to be maximally enhanced after 5 h and decreases until approximately 12 h, when the activity begins to increase again.

Identification of the small nuclear RNAs associated with the mitotic chromosomes of Amoeba proteus

Amebas contain 7 electrophoretically distinct species of small nuclear RNAs (snRNAs), some of which are known to associate in a striking manner with mitotic chromosomes. These RNAs can be divided into 2 classes, one consisting of 4 snRNA species that shuttle in a non-random way between nucleus and cytoplasm during interphase and one consisting of 3 snRNA species that do not leave the nucleus at all during interphase. In the work reported here we sought to determine which class is associated with mitotic chromosomes. Through a series of micromanipulative procedures we arranged for the shuttling snRNAs to be the only radioactive molecules in the cell. Such cells were allowed to enter mitosis, whereupon they were fixed and subjected to autoradiography. In those cells no radioactive snRNAs were found associated with mitotic chromosomes. It is concluded, therefore, that those snRNAs that do associate with mitotic chromosomes must be one or more of the non-shuttling species.--In the Discussion, how the non-shuttling snRNAs may function in cell activities is considered.

Low molecular weight RNAs hydrogen-bonded to nuclear and cytoplasmic poly(A)-terminated RNA from cultured Chinese hamster ovary cells

A group of RNAs 90--100 nucleotides long were isolated by melting them from poly(A)-terminated nuclear or cytoplasmic RNA from cultured Chinese hamster ovary cells. Conditions that favor hydrogen bond formation allowed the reassociation of these low molecular weight RNAs with poly(A)-terminated RNA. The nuclear poly(A)-terminated molecules contained 1.3 moles of the low molecular weight RNAs per mole of poly(A), while the cytoplasmic poly(A)-terminated RNA contained only one seventh as much. These low molecular weight RNAs were also isolated from the total 4S RNA of either the nucleus or cytoplasm by polyacrylamide gel electrophoresis. They formed a prominantly labeled band of RNA in the gels after cells had been labeled with H(3)32PO4 for 4 hr. The low molecular weight RNAs melted from the nuclear poly(A)-terminated RNA were slightly different (although not necessarily in primary nucleotide sequence) from those melted from the cytoplasmic poly(A)-terminated RNA.

Rat liver nuclear skeleton and small molecular weight RNA species

Small molecular weight RNA species (smwRNAs) were studied in rat liver nuclei with and without chromatin as well as with and without nuclear envelope and nucleoplasm. From all the species identified, only two, N5 and 5Sb, were related to ribosomes. The others were localized exclusively in the nuclear skeleton or the spongelike network that was described in the preceding communication. This network or protein matrix contains a less abundant but exclusive set of molecules designated 5Sa, N1, and 4.5S, as well as other more abundant molecules which also exist in rat liver endoplasmic reticulum but not in polysomes or postribosomal RNP complexes. The smwRNAs behave like HnRNA; they remain located in the nuclear skeleton when nuclei are deprived of nucleoplasm and chromatin. With the information presently available, it is not possible to know whetherer both species are in the same or different RNP complexes and whether some of the smwRNAs contribute to the architecture of the nuclear skeleton. Distinct from any other nuclear RNA species, smwRNAs have two unique properties: facility of extraction, and resistance to nuclear ribonuclease digestion.

5' Caps in hnRNA: absence of m32,2,7G and size distribution of capped molecules

In HeLa cells the "small nuclear" RNA has a cap II 5' structure (8)-- m32,2,7G(5') pppXmpYmp-- where X and Y are 2'0 methylated adenosine and uridine. In contrast hnRNA contains only cap I structures were the 2'0 methylated residue may be any base as was earlier reported for cytoplasmic mRNA (8,9,11). With a clear distinction between the source of these two caps an analysis of the size distribution of capped hnRNA could be performed which revealed over 65% of the capped hnRNA molecules were larger than cytoplasmic mRNA.

Small nuclear RNA localization during mitosis. An electron microscope study

The localization of small nuclear ribonucleic acids (snRNAs) during mitosis in Amoeba proteus was studied by high voltage (1,000 kV) electron microscope autoradiography. By suitable micromanipulations, the snRNA's, labeled with [3H]uridine, were made to be the only radioactive molecules in the cell and thus easy to follow autoradiographically. During interphase the snRNA label, which is almost exclusively nuclear, is distributed fairly uniformly through the nucleus with a slightly higher amount of label over chromatin than over nonchromatin areas. During prophase the snRNAs, which continue to be largely nuclear, become highly concentrated in the condensing chromosomes. At metapase, almost all of the snRNAs are cytoplasmic and essentially none are associated with the maximally condensed chromatin. Beginning in early anaphase, the snRNAs resume their association with the chromosomes, with the degree of association increasing throughout anaphase. Most of the snRNAs are back in the nuclei by telophase, but the intranuclear localization is hard to determine. We conclude that snRNAs have a great affinity for the partially condensed chromosomes of prophase and anaphase, but none for the maximally condensed chromosomes of metaphase. A minor amount of snRNA localizations in association with nucleoli and the nuclear envelope are also reported. On the basis of these findings a role of snRNAs in genetic "reprogramming" or chromosome organization is proposed.

Occurrence and properties of low molecular weight RNA components from cells at different taxonomic levels

1. The occurrence and gel electrophoretic properties of low molecular weight RNA components (LMW RNA) have been studied in species at different taxonomic levels. The LMW RNA components apart from tRNA, 5S RNA and 5.5S RNA are called LMW*RNA. 2. The major components of LMW*RNA in mammalian cells are L, A, C and D, accounting for 0.1-0.7% of cellular RNA. The gel electrophoretic migration of components L, C, and D is similar in different mammals but the migration of component A shows differences. 3. Amphibia, reptiles and birds contain L, A, C and D in about the same amounts as mammals but slight differences in migration are seen for L, C and D. Component A is absent from the nucleated red blood cells of the chicken and the frog. 4. Sea urchins contain three LMW*RNA components with migrations different from L, A, C and D. These components account for about 0.1% of the cellular RNA. 5. Insects contain only one LMW*RNA component, migrating as component L. 6. Tetrahymena, Physarum and Mycoplasmas have one component which may be a counterpart to component L in higher cells. Yeast shows no LMW*RNA components. 7. In the multicellular species the occurrence and gel electrophoretic migration of LMW*RNA components are not related to tumorigenicity, developmental stage or origin of tissue.

Effects of alpha-amanitin, cycloheximide, and thioacetamide on low molecular weight nuclear RNA

Studies were made on the effects of alpha-amanitin, cycloheximide, and thioacetamide on synthesis and content of low molecular weight nuclear RNA. Cycloheximide, an inhibitor of protein synthesis and the synthesis of 45S pre-rRNA and 5S RNA, also inhibited synthesis of nuclear U1 and U3 RNAs. alpha-Amanitin, an inhibited the synthesis of U1 and U2 low molecular weight nuclear RNA. Thioacetamide, which induces nucleolar hypertrophy and increased nucleolar RNA polymerase activity, markedly increased synthesis of 5.8S RNA and U3 RNA. These results show that syntheses of individual low molecular weight nuclear (LMWN) RNAs are controlled by different regulatory mechanisms. In particular, there appears to be a specific relationship between U3 RNA and functional states of the nucleolus.

Small RNA species of the HeLa cell: metabolism and subcellular localization

The small molecular weight RNAs of the HeLa cell have been located in specific subcellular fractions. SnA is located in the nucleolus and is partially bonded to nucleolar 28S RNA. SnD, the most abundant of the small nuclear RNAs, is partially released from the nucleus when the nuclear preparation is briefly warmed. SnF is released from the nuclei when chromatin is digested with the micrococcal nuclease and not when pancreatic DNAase is used. The remainder of the small nuclear species remain in the nucleus following the digestion of chromatin and are concluded to be elements of the "nuclear skeleton." SnK is found predominantly in the cytoplasm, but migrates quantitatively to the nuclear fraction in the presence of high levels of actinomycin D. ScL is totally cytoplasmic and is partially bound to cell membranes. It is the 7S RNA found in oncornavirus virions. All the small nuclear RNAs appear initially in the cytoplasmic fraction before fixation in the nucleus. Two short-lived cytoplasmic species behave kinetically as precursors to the stable nuclear RNAs.