Structure of nuclear ribonucleoprotein: identification of proteins in contact with poly(A)+ heterogeneous nuclear RNA in living HeLa cells

The processing of heterogeneous nuclear RNA into messenger RNA takes place in special nuclear ribonucleoprotein particles known as hnRNP. We report here the identification of proteins tightly complexed with poly(A)+ hnRNA in intact HeLa cells, as revealed by a novel in situ RNA-protein cross-linking technique. The set of cross-linked proteins includes the A, B, and C "core" hnRNP proteins, as well as the greater than 42,000 mol wt species previously identified in noncross-linked hnRNP. These proteins are shown to be cross-linked by virtue of remaining bound to the poly(A)+ hnRNA in the presence of 0.5% sodium dodecyl sulfate, 0.5 M NaCl, and 60% formamide, during subsequent oligo(dT)-cellulose chromatography, and in isopycnic banding in Cs2SO4 density gradients. These results establish that poly(A)+ hnRNA is in direct contact with a moderately complex set of nuclear proteins in vivo. This not only eliminates earlier models of hnRNP structure that were based upon the concept of a single protein component but also suggests that these proteins actively participate in modulating hnRNA structure and processing in the cell.

Structural organization of nascent transcripts and hnRNA molecules in amphibian oocytes

Comparisons of relative lengths of lampbrush loops, nascent RNP transcripts and hnRNA molecules from oocytes of amphibia with different C-values show that there is an increasing trend in loop, and transcriptional unit, length with increase in genome size but no increasing trend with respect to RNA contour length. The formation of duplex regions and circles in RNP fibrils indicates that RNA processing may occur within the nascent fibrils. The hnRNA molecules from oocytes of the various amphibia readily form intermolecular duplex structures. These complementary sequences have a low kinetic complexity and are transcribed from highly repetitive sequences distributed throughout the genome. Their possible function is considered.

Altered expression of the calcitonin gene associated with RNA polymorphism

The selective control of gene expression results in diversified morphology and physiological function. Understanding the expression of differentiated function in molecular terms requires detailed characterization of the regulation of mRNA synthesis and catabolism. Although considerable emphasis has been placed on transcriptional control, the discovery of HnRNA and 'split genes' gives rise to the possibility of post-transcriptional regulation at the level of processing of nuclear precursors. A rat calcitonin-producing medullary thyroid carcinoma (MTC) line was used as a model for definition of certain aspects in regulation of gene expression. Serial transplantation of several rat MTC lines containing and secreting large amounts of calcitonin generated tumours in which calcitonin biosynthesis was decreased more than 10-fold. We report here that the conversion from a 'high' to a 'low' calcitonin producing state is associated with specific modifications of the calcitonin mRNA synthetic pathway and a consequence of these changes seems to be the production of a new cytoplasmic mRNA.

hnRNA and its attachment to a nuclear protein matrix

In this study, DNA-depleted nuclear protein matrices are isolated from HeLa S3 cells. These nuclear matrices consist of peripheral laminae, residual nucleoli, and internal fibrillar structures. High molecular weight, heterogeneous nuclear RNA (hnRNA) is quantitatively associated with these structures and can be released intact only by affecting the integrity of the matrices. It is, therefore, concluded that hnRNA is part of a highly organized nuclear structure. By irradiation of intact cells or isolated nuclear matrices with ultraviolet light, proteins tightly associated with hnRNA can be induced to cross-link with the RNA. Performing the cross-linking in vivo is an extra guarantee that only hnRNA-protein (hnRNP) complexes existing in the intact cell are covalently linked. Such hnRNP complexes were isolated and purified under conditions that completely dissociate nonspecific RNA-protein complexes. By comparison of the hnRNP found in nuclear matrices and the published data on the composition of hnRNP particles, it was found that the so-called hnRNP "packaging" proteins (32,000-38,000 mol wt) were not efficiently cross-linked to hnRNA by UV irradiation. They were, however, present in the matrix preparations, bound to hnRNA, because they were released from nuclear matrices after ribonuclease treatment of these structures. On the other hand, two major hnRNPs (41,500 and 43,000 mol wt) were efficiently cross-linked to hnRNA. These proteins were not released by ribonuclease treatment, which suggests that they are involved in the binding of hnRNA to the nuclear matrix.

[Nuclear RNA turnover. 1. Metabolic heterogeneity]

It is usually necessary to compare the kinetics of labelling of nucleoside triphosphates and nuclear RNAs to determine the turnover rate (half-life, T1/2) of nuclear RNAs. It is shown that the widely adopted correction for non-constant specific radioactivity of precursor pool is not correct in general and could be used only for very stable RNAs. The method for T1/2 determination is described which is suitable for any form of UTP labelling kinetics. Besides, the criterion was found for revelation of metabolic heterogeneity of nuclear RNA population. Rat liver nuclear DNA-like RNA appeared to be heterogeneous and consisted of two subpopulations, one rapidly labelled with T1/2 about 30 min and other, three times larger, with no labelling during the experiment.

Large heterogeneous nuclear ribonucleic acid has three times as many 5' caps as polyadenylic acid segments, and most caps do not enter polyribosomes

The rate of synthesis in Chinese hamster cells of 5' cap structures, m7 GpppNmp, in large (greater than 700 bases) heterogeneous nuclear ribonucleic acid (RNA) molecules is two to three times faster than the synthesis of 3'-terminal polyadenylic acid segments. As judged by presence of caps, newly synthesized polysomal messenger RNA, exclusive of messenger RNA the size of histone messenger RNA, is more than 90% in the polyadenylated category. It appears, therefore, that between half and two-thirds of the long capped heterogeneous nuclear RNA molecules do not contribute a capped polysomal derivative to the cytoplasm. There are capped, nonpolysomal, non-polyadenylated molecules with a rapid turnover rate that fractionate with the cytoplasm. These metabolically unstable molecules either could represent leakage into the cytoplasm during fractionation or could truly spend a brief time in the cytoplasm before decay.

The phosphorylation of the proteins of rat liver heterogeneous ribonucleoprotein particles by an endogenous kinase activity

An endogenous protein kinase activity of liver heterogeneous nuclear ribonucleoprotein particles has been characterized and the particle proteins which it phosphorylates in vitro have been fractionated on two-dimensional polyacrylamide gels. The activity is dependent on Mg2+ and is further stimulated by cyclic AMP, polyamines and Mn2+.

Turnover of the major polypeptides of 40-S monomer particles

The pulse-chase experiments with Friend erythroleukemia cells designed to reveal the metabolic properties of the protein complex of 40-S particles showed that the major polypeptides of this complex turn over with half-lives between 19 h and 206 h. the main conclusion from the experiments is that the complex does not degrade as a single unit. Since the individual polypeptides forming the complex live much longer than hnRNA, and in addition degrade at a different rate, we considered the following two modes of degradation as most likely. (1) The complex might not be subjected to a profound degradation at the end of the processing of associated pre-mRNA. In this case it should exist as a long-lived recyclable mosaic of metabolically differing polypeptides whose replacement takes place at a specific rate. (2) Alternatively, the protein complex might be completely degraded at the end of processing, but in a way that liberates free individual polypeptides available for recycling. The further experiments indicate that the 37 000-Mr, 34 000-Mr and 32 000-Mr core proteins in isolated 40-S particles and in particles associated with a nuclear fraction released from chromatin after micrococcal nuclease digestion degrade at different rates. These experiments suggest the existence of at least a metabolic heterogeneity among the population of nuclear particles carrying pre-mRNA.

Arrangement of 30S heterogeneous nuclear ribonucleoprotein on polyoma virus late nuclear transcripts

Heterogeneous nuclear ribonucleic acid (hnRNA) molecules in eucaryotic cell nuclei associate with a well-defined group of abundant, highly conserved proteins to form heterogeneous nuclear ribonucleoproteins (hnRNP). The exact manner in which these 30S complexes assemble on nuclear transcripts, however, has not been well documented. To determine whether any site selectivity in the formation of hnRNP can be detected (e.g., preferential recognition of intervening sequences or of premessage regions), we investigated the distribution of 30S hnRNP on a particular nuclear RNA, the polyoma virus late transcript. Hybridization studies showed not only that the majority of polyoma late nuclear RNA sequences can be isolated in the form of 30S complexes, but that the RNP were located equally on intervening sequences and premessage portions of the transcript. The latter conclusion was confirmed by ribonuclease T1 oligonucleotide fingerprint analysis of polyoma virus-specific RNA recovered from native 30S complexes. However, fingerprint analysis of the small segments of viral RNA in the 30S fraction that survived extensive ribonuclease treatment revealed that oligonucleotides corresponding to intervening sequences were preferentially lost. We discuss these findings in relation to the structure of 30S hnRNP and their function in RNA biogenesis.

Messenger RNA processing and nuclear structure: isolation of nuclear ribonucleoprotein particles containing beta-globin messenger RNA precursors

To explore the relationships between transcription, messenger RNA (mRNA) processing, and nuclear structure, ribonucleoprotein particles containing heterogeneous nuclear RNA (hnRNP) have been purified from globin-producing mouse Friend erythroleukemia cells. These nuclear hnRNP particles sediment at 50S-200S and contain, in addition to high molecular weight hnRNA, a specific set of nuclear proteins predominated by a major component of approximately 38,000 mol wt. The hnRNP particles are free of histones and ribosomal structural proteins, indicating their purification from the two other major nucleoprotein components of the nucleus: chromatin and nucleolar ribosomal precursor RNP particles. Th authenticity of the Friend cell hnRNP particles is demonstrated by the results of reconstruction experiments with deproteinized hnRNA, and by the resistance of the articles to dissociation during isopycnic banding in Cs2SO4 gradients without prior aldehyde fixation. Hybridization analysis with cloned mouse beta-globin DNA demonstrates that hnRNP particles from induced Friend cells contain newly synthesized transcripts of the beta-globin gene. Agarose gel electrophoresis of hnRNP particle-derived RNA denatured in glyoxal followed by "Northern" transfer to diazobenzyloxymethyl paper and hybridization with 32P-labeled cloned mouse beta-globin DNA reveals the presence in hnRNP of two size classes of beta-globin gene transcripts, the larger of which corresponds to the pre-spliced 15S beta-globin mRNA precursor previously identified in whole nuclear RNA, and the smaller of which corresponds to completely processed 9S beta-globin mRNA. These results establish, for the first time, that the nuclear transcripts of a specific, well-defined eukaryotic structural gene can be isolated in an RNP particle form, and that their RNP structure persists throughout mRNA splicing.

Addition of poly(A) to nuclear RNA occurs soon after RNA synthesis

A kinetic analysis of the appearance of [3H]uridine label in RNA sequences that neighbor poly(A), as well as the incorporation of [3H]adenosine label into both the RNA chain and the poly(A) of poly(A)-containing molecules, shows that poly(A) is added within a minute or so after RNA chain synthesis in Chinese hamster ovary cells and HeLa cells. Previous conclusions by several groups (5-7) that poly(A) might be added as long as 20-30 min after RNA synthesis appear to be in error, and the present conclusion seems much more in line with several different types of recent studies with specific mRNAs that suggest prompt poly(A) addition (13-16).

Nuclear processing of viral high-molecular-weight RNA in cells infected with herpes simplex virus type 1

Nuclear RNA isolated from cells infected by herpes simplex virus type 1, strain F, was fractionated on formamide-sucrose gradients into two major classes, greater and less than 45S. These two classes of labeled nuclear RNA were hybridized to viral DNA fragments generated by digestion with the restriction enzymes HindIII and BglII. Early in infection, only a few DNA fragments hybridized to RNA, with slight differences between the two classes. Late in infection, all DNA fragments hybridized, showing that all viral RNA was present in large precursor molecules greater than 14 kilobases. The fragments that correspond to late gene products hybridized more of the small RNA than the large RNA. This suggests that the mRNA corresponding to late genes accumulated after the large precursors have been cleaved. Large (greater than or equal to 45S) and small (< 45S) nuclear RNA and cytoplasmic RNA from cells late in infection were hybridized in excess to in vitro-labeled HindIII M and L fragments. More than 50% of the HIndIII M fragment annealed with the large nuclear RNA, but only 36% of it annealed with the cytoplasmic RNA. The HindIII L fragment hybridized large nuclear RNA and cytoplasmic RNA to the same extent (30% and 26%). These results suggest that RNA complementary to the HindIII M fragment, which is the template for immediate early polypeptides, was regulated in the nuclei at the posttranscriptional level. This seems to suggest that temporal regulation of RNA cleavage occurs in the nucleus.

Developmental shifts in frequency distribution of polysomal mRNA and their posttranscriptional regulation in the sea urchin embryo

The frequency distributions of polyadenylylated RNAs from the polysomes of sea urchin blastulae and gastrulae were estimated from their kinetics of hybridization with complementary DNA. Developmental decreases in complexity were observed among abundant, intermediate, and rare frequency classes. The class of highest abundance in the blastula polysomes had a complexity of 5.6 X 10(4) nucleotides and contained about 30 mRNA species, which divided into subsets according to developmental fate. Studies with purified DNA complementary to this abundant class revealed that five of these mRNA species remained abundant in the gastrula, wherein each comprised 2% of the polyadenylylated RNA in the polysomes. Approximately 5 species decreased to a nearly rare frequency and 20 were absent or at the limits of detection in polyadenylylated RNA of gastrula polysomes. These distinctly different developmental fates suggest distinct modes of regulation of mRNA concentration for different subsets. Focusing on the small number of abundant blastula mRNAs, we ascertained that those which were absent from gastrula polysomes were nevertheless represented in the gastrula nuclear RNA. Therefore, the appearance of abundant mRNA species in polysomes can be regulated by posttranscriptional processes.

Developmental changes in the molecular weight of heterogeneous nuclear RNA

The size distribution of heterogeneous nuclear RNA (hnRNA) in the sea urchin embryo changes markedly during early development. Measurement of cleavage (4.5 h) to late gastrula (40 h) hnRNA by sedimentation in aqueous and denaturing solvent indicates that in the stages tested cleavage (4.5 h) hnRNA is smallest and rotating blastula (13 h) hnRNA is largest. The molecular weight of cleavage hnRNA is calculated to be about one-third that of rotating blastula hnRNA. Sedimentation of early embryo hnRNA in denaturing solvent to disaggregate complexes demonstrated mean S values lower than those obtained in aqueous solvent.

Two-dimensinal gel electrophoresis of rat liver nuclear washes, nuclear matrix, and hnRNA proteins

The proteins of rat liver cytoplasm, nuclear washes, matrix, membrane, heterogeneous nuclear (hn)RNA proteins and chromatin were examined by two-dimensional gel electrophoresis. The inclusion in the gels of six common protein standards of carefully selected molecular weight and isoelectric point allowed us to clearly follow the distribution of specific proteins during nuclear extraction. In the nuclear washes and chromatin, we observed five classes of proteins: (a) Exclusively cytoplasmic proteins, present in the first saline-EDTA wash but rapidly disappearing from subsequent washes; (b) ubiquitous proteins of 75,000, 68,000, 57,000, and 43,000 mol wt, the latter being actin, found in the cytoplasm, all nuclear washes and the final chromatin pellet; (c) proteins of 94,000, 25,000, and 20,500 mol wt specific to the nuclear washes; (d) proteins present in the nuclear washes and final chromatin, represented by species at 62,000, 55,000, 54,000, and 48,000 mol wt, primarily derived from the nuclear matrix; and (e) two proteins of 68,000 mol wt present only in the final chromatin. The major 65,000-75,000-mol wt proteins seen by one-dimensional gel electrophoresis of nuclear matrix were very heterogeneous and contained a major acidic, an intermediate, and a basic group. A single 68,000-mol wt polypeptide constituted the majority of the membrane-lamina fraction, consistent with immunological studies indicating that a distinct subset of matrix proteins occurs, associated with heterochromatin, at the periphery of the nucleus. Actin was the second major nuclear membrane-lamina protein. Two polypeptides at 36,000 and 34,000 mol wt constituted 60% of the hnRNP. Approximately 80% of the mass of the nonhistone chromosomal proteins (NHP) from unwashed nuclei is contributed by nuclear matrix and hnRNPs, and essentially the same patterns were seen with chromatin NHP. The concept of NHP being a distinct set of DNA-bound proteins is unnecessarily limiting. Many are derived from the nuclear matrix or hnRNp particles and vary in the degree to which they share different intracellular compartments.

Immunofluorescent localization of the proteins of nuclear ribonucleoprotein complexes

Antibodies were raised in chickens against heterogeneous nuclear RNA (hnRNA)-binding proteins from 30S ribonucleoprotein (RNP) complexes of mouse Taper hepatoma ascites cell nuclei. The antibody preparations were characterized for immunological specificity and purity by double-diffusion gels, binding to specific bands in SDS polyacrylamide gels, and crossed immunoelectrophoresis. Antibodies raised against either whole 30S RNP complexes or purified RNP core proteins had a strong selective affinity for the four 34,000- to 40,000-dalton polypeptides which comprise the major structural proteins of hnRNP. The intracellular distribution of 30S RNP antigens in mouse ascites cells was determined by indirect immunofluorescence microsacopy. In interphase cells immunofluorescent sites were restricted to the nucleus, and nucleoli were free of fluorescence. The chicken anti-mouse-RNP antibodies were also able to react with cells from many different vertebrate species, showing a similar nucleus-restricted localization of the reacting sites. The antibodies also bound chick 30S RNP-proteins and reacted with the nuclei of chick cells. An exception to this was the failure of the antibody to bind to adult chick erythrocytes, suggesting that these major hnRNA binding proteins may be found only in nuclei capable of RNA synthesis.

Metabolically labile nonhistone proteins of chromatin

In a previous paper in this journal [Djondjurov, L., Ivanova, E. and Tsanev, R. (1979) Eur. J. Biochem. 97, 133-139], we showed that a nuclear fraction released from chromatin under a mild nuclease digestion contained an increased amount of hnRNA and the bulk of nonhistone proteins with a high metabolic rate. The present investigation has revealed that the nonhistone proteins of this fraction could be divided into three distinct metabolic groups. The first group consists of proteins with a fast turnover rate (mean half-life 30 min) which migrate into chromatin immediately after their synthesis. These proteins are predominantly acid-soluble and have relatively high molecular weights. The second group includes proteins which migrate to the nucleus more slowly and metabolize with a moderate turnover rate (mean half-life 5 h). The third group contains proteins with a more conservative metabolic behaviour. In experiments with actinomycin D it was found that the bulk of the nonhistone proteins of this fraction are not real components of the chromatin but belong to the protein moiety of heterogeneous nuclear ribonucleoprotein particles associated with chromatin.

Specific RNA-cleaving activities from HeLa cells

Subcellular fractionation of HeLa cells was carried out under gentle conditions to isolate enzymes that cleave RNA precursors in a specific manner. Four separate activities--cleavage of HeLa cell heterogeneous nuclear RNA, the HeLa cell 45S rRNA precursor, RNA . DNA hybrids (RNase H), and the Escherichia coli tRNATyr precursor (RNase P)--were revealed by these studies. The specificity and limited nature of these cleavages suggest that they are due to eukaryotic RNA-processing enzymes. The virtual absence of random nucleases from these enzymes was demonstrated by their inability to cleave the 8000-base early mRNA precursor of bacteriophage T7, E. coli 30S rRNA precursor, or HeLa cytoplasmic poly(A)-containing RNA.

Ribonucleoprotein structure in nascent hnRNA is nonrandom and sequence-dependent

We have studied the ribonucleoprotein morphology of nascent hnRNA using chromatin spreading methods. This approach allows visualization of multiple transcripts of the same DNA sequence. Thus the RNP structure of hnRNA molecules with the same nucleotide sequence can be compared. We find that RNP particles averaging 240 A in diameter occur on the majority of hnRNA transcription units, but that RNA fibrils are generally not completely covered with particles. Furthermore, the RNP particle location is nonrandom for the transcripts of a given gene. Particle number and arrangement on RNP fibrils vary from one transcription unit to the next, and are not obviously related to transcription unit size or activity. Analysis of particle location with respect to the 5' RNA terminus allows RNP particle localization to homologous sequences of less than 500 ribonucleotides on the different fibrils of a single transcription unit. RNP particles assemble very soon after synthesis of the particular sequence. Sites of particle formation are distinct from sites of double-stranded hairpin loops on the RNA fibrils. A differential, or differentially stable, RNP structure with respect to RNA sequence may bear on mechanisms of specific hnRNA processing.

A mechanism for RNA splicing

The most abundant of the stable small nuclear RNAs of eukaryotic cells, U-1 small nuclear RNA, is exactly complementary to the consensus sequences at RNA splice sites. We propose that this RNA is the recognition component of the nuclear RNA splicing enzyme and forms base pairs with both ends of an intron so as to align them for cutting and splicing.

The detection of messenger ribonucleic acid sequences in heterogeneous nuclear ribonucleic acid fractions of the oestrogen-stimulated rat uterus

cDNA (complementary DNA) complementary to the abundant sequences of mRNA isolated from oestrogen-stimulated uterus was hybridized to polyadenylated and non-polyadenylated uterine hnRNA (heterogeneous nuclear RNA). High-molecular-weight polyadenylated hnRNA, isolated under denaturing conditions, was able to saturate the cDNA, but complementary sequences were low abundance when compared with homologous mRNA. The mRNA sequence content of the polyadenylated hnRNA increased considerably during oestrogen-induced growth of the uterus. Non-polyadenylated hnRNA also contained sequences complementary to the cDNA.

Composition of hnRNA-associated proteins in rat liver is specifically altered after cycloheximide treatment

Rats were treated with non-lethal doses of alpha-amanitin or cycloheximide. Nuclei were prepared and the particles carrying heterogeneous nuclear RNA (30--40 S-particles) isolated by density gradient centrifugation. In the case of alpha-amanitin the yield of particles was reduced to about 45%. Cycloheximide affected the composition of proteins associated with the nuclear RNA. In particular, the concentration of a 110 000 molecular weight protein as determined by sodium dodecylsulfate gel electrophoresis was reduced to 20--30% after 2 h and then increased to 150--180% of the control before it approached the normal level after 30 h. Possible mechanisms underlying these changes are discussed.

Two-dimensional analysis of proteins associated with heterogenous nuclear RNA in various animal cell lines

The protein complement of heterogenous nuclear RNA . protein particles from human HeLA, mouse L and Chinese hamster (CHO) cells has been analysed by two-dimensional gel electrophoresis using the two techniques described by O'Farrell [J. Biol. Chem. (1975) 250, 4007--4021 and Cell (1977) 12, 1133--1142]. Over a hundred individual spots habe been reproducibly detected both L-[35S]methionine. Large similarities, especially in the 25 000--40 000 Mr cluster of basic protein, were found among these three mammalian species. As far as phosphoproteins are concerned, it was observed that the bands already described by one-dimensional gels [Eur. J. Biochem. (1978) 86, 301--310] with Mr values of 28 000, 30 000, 37 000 and 52 000 are resolved into about 15 individual spots, suggesting a corresponding number of distinct states of phosphorylation. It was also clearly demonstrated that phosphoproteins are unrelated to the major basic protein species. Particles of different size classes were analysed with respect to their content of individual proteins, both non-phosphorylated and phosphorylated. The most salient feature observed was that phosphoproteins become progressively more abundant with particles of increasing size. This raises the possibility that at least some of these phosphoproteins might belong to a nuclear structure to which hnRNA is normally bound.

Kinetics of hybridization to human DNA of heterogeneous nuclear RNA isolated from normal human lymphoblasts and acute leukemia blast cells

Heterogeneous nuclear RNA was extracted from normal PHA-stimulated human lymphocytes and acute myeloid leukemia blast cells. Experiments were performed to determine the hybridization kinetics of these RNA's to human DNA. The best least squares solutions indicate in the hybridization reaction of both normal and leukemic RNA two main components. For leukemic cell RNA the rate constants of both components were significantly different from that of normal cell RNA. In particular, the difference between the rate constants of the second lower component suggests that the slowly hybridizing sequences in leukemic cell RNA have a degree of repetition higher than of the corresponding sequences of normal cell RNA.

Transcription of spacer sequences in genes coding for ribosomal RNA in Xenopus cells

Untreated Xenopus cells synthesize RNA is increased and some sequences complementary to those of the nontranscribed spacer are found in the heavy shoulder of the 40S precursor rRNA peak. In such events transcription initiation seems to take place within the spacer because its middle and right BamHI endonuclease fragments are preferentially transcribed, whereas only few RNA sequences complementary to the left spacer fragment are found. It is concluded that at least some spacer regions contain promoters for transcription and can be transcribed either into a special class of "spacer transcripts" or into molecules covalently linked to rRNA precursor.

Two chromatin fractions with different metabolic properties of non-histone proteins and of newly synthesized RNA

Digestion of chromatin with micrococcal nuclease under mild conditions results in the release of a minor chromatin fraction showing an increased RNA and non-histone protein content, a fast turnover of the non-histone proteins and the presence of rapidly labelled heterogeneous nuclear RNA (hnRNA) with half-life of about 20 min. Further digestion of the chromatin leads to the elimination of about 19% of the initial chromosomal DNA, thus leaving a second chromatin fraction relatively resistant to nuclease attack. This fraction has a low protein and RNA content and contains only metabolically stable non-histone proteins. No differences in the histone complement of the two fractions was found except for a 40% deficiency of H1 in the minor fraction.

On pre-messenger RNA and transcriptions. A review

From the present review integrating old and new data emerge a few principles of gene expression in eukaryotes, and an infinite variety of possible mechanistic details generating the overal pattern. The few principles, most of which are not fundamentally new, may thus be summarized. 1) The eukaryotic genome is subdivided into transcriptional units: into transcriptons which are subject to individual activation controlled at DNA level. 2) Viral genomes may contain one or a few transcriptons, while cells of multicellular organisms contain from 3 x 10(3) in diptera up to an estimated 2 x 10(5) in birds and mammals. 3) Transcriptons may include one or several coding sequences. 4) Transcriptons vary considerably in size: in mammals and birds their size spectrum falls into the 2,000 to 20,000 bp range. 5) Units of coding information constituting one message (genes) and, possibly, units of regulative information are frequently broken up and stored within the transcripton in sub-genic blocks (of so far unknown significance) in general located at a certain distance from the 5' and 3' transcript terminals which are determined by the promotor and terminator signals. 6) The gene, in its specific definition as the functional unit underlying the phenotype, is in general constituted posttranscriptionally by the processing mechanisms from the mosaic of its genomic subunits in the transcripton; segments of coding, service and regulative sequences are recombined within themselves and with each other, polygenic transcripts separate into their unit messages. 7) Activated transcriptons produce pre-mRNA; these primary transcripts are colinear with the DNA of the transcriptional unit. 8) Primary pre-mRNA is processed into secondary pre-mRNA's by extragenic cleavage and intragenic ("splicing") processing, giving rise stepwise to functional mRNA. During this process chemical modifications as methylation, 5'-terminal capping and 3'-terminal polyadenylation take place. 9) Translation yields either potentially functional polypeptides or polycistronic polyproteins subject to further processing. 10) Processing is a regulated process; it involves many of the possible phases and mechanisms of post-transcriptional regulation (cf. 39, 40).

HnRNP core proteins: synthesis, turnover and intracellular distribution

Our present data indicate that the Mr 34-40,000 polypeptides which are involved in the binding of a large fraction of hnRNA sequences, including mRNA, are for the most part metabolically stable species in mouse ascites tumor cells. An exception to this generalization is the smallest of 30S RNP core polypeptides, the Mr 34,000 protein, which has a relatively high turnover rate. The relationship of the various synthesis and degradation rates to the physiological state of mammalian cells remains to be determined, as does the pathway of assembly and disassembly of RNP substructures during re-utilization of the proteins and during their turnover. Immunofluorescent studies, which have confirmed the expected nucleoplasmic or euchromatic localization of the RNP core proteins, have also indicated that these species are stable during mitosis, at which time they are dispersed through the cell away from the condensed chromosomes. The proteins appear to relocate in the nucleus as soon as the nuclear envelope is reformed.

[Studies on the kinetics of the RNA metabolism in the prostate of normal and castrated rats (author's transl)]

In the prostate of adult Wistar rats the RNA/DNA quotient of the whole organ as well as the amount of RNA and DNA in the nucleus was measured at different times after castration. Furthermore the half-life time for the turnover of the RNA in the nucleus and the cytoplasm was determined for normal and castrated rats with the aid of pulse labelling using [5(-3)H]uridine. A mathematical model was developed to analyze the experimental results. This model enabled us to make differentiated statements on the heterogeneous nuclear RNA (hmRNA) and the remaining RNA in the nucleus. The evaluation of the experimental values gave the following results: 1. By deprivation of androgens the uptake of [3H]uridine into the prostate is lowered. 2. The amount of DNA in the morphologically intact nucleus remains constant at least up to the 12th day after castration. 3. 6 days after castration the amount of hmRNA decreases to 1/10 and that of cytoplasmic RNA to 1/4. 4. The half-life time for the decrease of the whole nuclear RNA is 3.7 d and that of the cytoplasmic RNA 1.7 d. 5. The half-life time for the turnover of hmRNA is 16 min and that of cytoplasmic RNA about 2 days. 6 days after castration the half-life times are unchanged. The experimental results suggest that the observed decrease of nuclear RNA following castration can mainly be attributed to a reduced synthesis of hnRNA, while the decrease of cytoplasmic RNA is first of all caused by an increase in RNA degradation.

Effect of 5-bromodeoxyuridine on heterogeneous nuclear RNA in rat hepatoma cells

Heterogeneous nuclear RNA HnRNA) was isolated from untreated and 5-bromodeoxyuridine (BrdUrd) treated hepatoma tissue culture (HTC) cells. analysis of this RNA by either electrophoresis on polyacrylamide-agarose gels or centrifugation in sucrose gradients demonstrated that BrdUrd caused a shift in the labeled HnRNA population toward a smaller size distribution. This effect was produced by concentrations of BrdUrd which specifically lower the level of the differentiated enzyme tyrosine aminotransferase, but do not greatly affect cell growth. Differential binding to oligo(dT) cellulose was used to fractionate HnRNA further into classes containing poly(A) (alpha), oligo(A) (beta) or neither category of A-rich sequences (gamma). BrdUrd did not alter the relative rates of uridine incorporation into the three classes. The shift in the labeled HnRNA population due to BrdUrd was observed in all three subclasses of HnRNA.