Globin RNA synthesis in vitro by isolated erythroleukemic cell nuclei: direct evidence for increased transcription during erythroid differentiation

Transcription factor EKLF (KLF1) recruitment of the histone chaperone HIRA is essential for β-globin gene expression

The binding of chromatin-associated proteins and incorporation of histone variants correlates with alterations in gene expression. These changes have been particularly well analyzed at the mammalian β-globin locus, where transcription factors such as erythroid Krüppel-like factor (EKLF), which is also known as Krüppel-like factor 1 (KLF1), play a coordinating role in establishing the proper chromatin structure and inducing high-level expression of adult β-globin. We had previously shown that EKLF preferentially interacts with histone H3 and that the H3.3 variant is differentially recruited to the β-globin promoter. We now find that a novel interaction between EKLF and the histone cell cycle regulation defective homolog A (HIRA) histone chaperone accounts for these effects. HIRA is not only critical for β-globin expression but is also required for activation of the erythropoietic regulators EKLF and GATA binding protein 1 (GATA1). Our results provide a mechanism by which transcription factor-directed recruitment of a generally expressed histone chaperone can lead to tissue-restricted changes in chromatin components, structure, and transcription at specific genomic sites during differentiation.

Nuclear sphingomyelin pathway in serum deprivation-induced apoptosis of embryonic hippocampal cells

Sphingomyelin (SM) cycle has been involved in the regulation of proliferation, differentiation, and apoptosis. Increases in ceramide have been found after a larger number of apoptotic stimuli including cytokines, cytotoxic drugs, and environmental stresses. Accumulating evidence suggest that the subcellular localization of ceramide generation is a critical factor in determining the cellular behavior. Since recently enzymes involved in ceramide metabolism such as sphingomyelinase, SM synthase, sphingosine kinase and ceramidase have been found in the nucleus of hepatocyte cells, we have studied first the presence and the physicochemical characteristics of SM metabolism enzymes in nuclei isolated from embryonic hippocampal cells (cell line HN9.10e). The activities of sphingomyelinase and SM-synthase have been assayed and the ceramide production evaluated at different times after serum deprivation in these neurones cultivated in serum-deficient medium. We report that both enzymes are present in the nucleus of embryonic hippocampal cells and differ from those present in the homogenate in optimum pH. After serum deprivation, that induces a time-dependent decrease in cell viability and increase of the cell percentage in G1 phase of the cell cycle, a nuclear sphingomyelinase activation together with SM-synthase inhibition and a consequent increase of nuclear ceramide pool have been demonstrated. No similar enzyme activity modifications in homogenate have been identified. The possible role of nuclear sphingomyelinase/sphingomyelin-synthase balance in serum deprivation-induced apoptosis in the embryonic hippocampal cell is discussed.

Nuclear sphingomyelin-synthase and protein kinase C δ in melanoma cells

The aim of this research is to study the influence of protein kinase C delta on the nuclear phospholipids metabolism. Murine and human melanoma cells, in which overexpression of protein kinase delta was induced, were used. After purification of the nuclei, the phosphatidylcholine-dependent phospholipase C, sphingomyelin-synthase, and sphingomyelinase activities were measured. The results showed that the nuclear sphingomyelin-synthase activity increased and sphingomyelinase activity decreased in the protein kinase C delta overexpressive cells with respect to the controls. As a consequence, the ceramide pool decreased and diacylglycerol pool increased; this effect was not due to the phosphatidylcholine-dependent phospholipase C activity that did not change. The inhibition of sphingomyelinase could be due to protein kinase C delta as well as to existence of a sort of nuclear self-regulation between sphingomyelin-synthase and sphingomyelinase. The possible role of nuclear sphingomyelin-synthase in cell proliferation is discussed.

Nuclear organization of splicing snRNPs during differentiation of murine erythroleukemia cells in vitro

Murine erythroleukemia (MEL) cells are erythroid progenitors that can be induced to undergo terminal erythroid differentiation in culture. We have used MEL cells here as a model system to study the nuclear organization of splicing snRNPs during the physiological changes in gene expression which accompany differentiation. In uninduced MEL cells, snRNPs are widely distributed throughout the nucleoplasm and show an elevated concentration in coiled bodies. Within the first two days after induction of terminal erythroid differentiation, the pattern of gene expression changes, erythroid-specific transcription is activated and transcription of many other genes is repressed. During this early stage splicing snRNPs remain widely distributed through the nucleoplasm and continue to associate with coiled bodies. At later stages of differentiation (four to six days), when total transcription levels have greatly decreased, splicing snRNPs are redistributed. By six days postinduction snRNPs were concentrated in large clusters of interchromatin granules and no longer associated with coiled bodies. At the end-point of erythroid differentiation, just before enucleation, we observe a dramatic segregation of splicing snRNPs from the condensed chromatin. Analysis by EM shows that the snRNPs are packaged into a membrane-associated structure at the nuclear periphery which we term the "SCIM" domain (i.e., SnRNP Clusters Inside a Membrane).

Beta globin gene transcripts originating in the promoter region during early hexamethylene bisacetamide and dimethylsulfoxide induction of Friend erythroleukemia cells

The beta-globin transcripts which are induced by dimethylsulfoxide (DMSO) and hexamethylene bisacetamide (HMBA) have been characterized in order to assess potential differences in their mechanisms of induction. Transcripts which initiate in the 5' flanking promoter region are likely indicators of promoter accessibility and were therefore characterized during the time course of induction with each inducer in Friend Erythroleukemia cells. S1 analysis with probes labeled at - 12 or +82 relative to the (+1) cap site showed no major differences between 5' ends of the upstream initiated transcripts in cells induced by DMSO or HMBA. We detected several upstream bands with each inducer corresponding to beta-globin transcripts with 5' ends between - 190 and -55 relative to the cap site and found that cells induced with DMSO and HMBA show a similar transcription response as measured by initiation in the 5' flanking region of the beta-globin gene. Interestingly, the upstream initiated transcripts reach their peak concentration levels much earlier in the time course of induction than do the mRNA transcripts with 5' ends at the major (+1) cap site. Northern blot analysis detected the upstream initiated transcripts as early as 16 hours after induction with DMSO, primarily in unprocessed large transcripts. We find that the promoter region containing transcripts constitute a higher percent of total beta-globin transcripts at the start of the induction and may therefore have an early function in the multistep induction process.

Specific pattern of gene expression during induction of mouse erythroleukemia cells

We have studied the expression of several characterized genes during induction of mouse erythroleukemia (MEL) cells with dimethyl sulfoxide (DMSO) and have observed a specific pattern of changes in transcriptional activity and steady-state RNA levels associated with erythroid differentiation. During induction there is a gradual, steady decrease in total transcriptional activity and RNA content per cell, which by day 3 of DMSO treatment amounts to less than 50% of the level in the uninduced cell. During this time we observe increases in transcriptional activity for 5-aminolevulinic acid synthase, carbonic anhydrase form II, and band 3 coordinate with the large increase in beta-globin gene transcription. The results also demonstrate an early decrease in transcription for carbonic anhydrase form I, which precedes decreases in transcription for glyceraldehyde phosphate dehydrogenase and rRNA genes. Changes in steady-state RNA levels reflected changes in transcriptional activity during induction except for carbonic anhydrase II mRNA. These results represent the first report characterizing the regulated expression at transcriptional and posttranscriptional levels of several known genes that are characteristically expressed in the erythrocyte. The results demonstrate that coordinate gene expression in erythroid differentiation occurs primarily at the level of transcription.

Cell growth and hemoglobin synthesis in murine erythroleukemic cells propagated in high density microcapsule culture

A new microencapsulation technology, developed for the encapsulation of living cells, has been demonstrated to be useful for the study of growth and differential gene expression using Friend erythroleukemic cells cultured at high cell densities. Using this technology, cultures of FL Clone 745 cells were encapsulated within semipermeable membranes composed of cross-linked alginic acid and poly-l-lysine. Cell growth studies measuring total cell number demonstrated an average generation time of 8.5 h in 5% (vol/vol) microcapsule cultures vs. 8.0 h in suspension cultures. Similar microcapsule cultures were serially propagated for more than 90 cell generations (13 sequential passages) with no significant change in this growth rate. In addition, final culture densities of greater than 1.0 X 10(8) cells/ml of intracapsular volume were attained using a 3% (vol/vol) microcapsule culture in conjunction with a standard refeeding schedule. Comparison of the level of dimethyl sulfoxide-induced hemoglobin production in suspension and microcapsule cultures demonstrated that the total amount of hemoglobin produced on a per cell basis was comparable in both systems. Due to the retention characteristics of the semipermeable membrane, the concentration of detergent-released hemoglobin, relative to other released protein, was approximately twofold higher in microcapsule cultures than in control suspension cultures.

Linker scanning mutagenesis of the 5'-flanking region of the mouse beta-major-globin gene: sequence requirements for transcription in erythroid and nonerythroid cells

We analyzed the sequences required for transcription of the mouse beta-major-globin gene by introducing deletion and linker scanning mutations into the 5'-flanking region and then studying the effects of these mutations on beta-globin gene transcription in a HeLa cell transient expression assay or after stable introduction into mouse erythroleukemia cells. Consistent with earlier studies, we found that three distinct regions upstream from the RNA capping site are required for efficient beta-globin gene transcription in HeLa cells: the ATA box located 30 base pairs upstream from the mRNA capping site (-30), the CCAAT box located at -75, and the distal sequence element CCACACCC located at -90. In the ATA and CAAT box regions, the sequences necessary for efficient transcription extend beyond the limits of the canonical sequences. Mutations in the sequences located between the three transcriptional control elements do not significantly affect transcription in HeLa cells. Although the promoter defined in HeLa cell transfection experiments is also required for efficient transcription in mouse erythroleukemia cells, none of the mutations tested affects the regulation of beta-globin gene transcription during mouse erythroleukemia cell differentiation. Thus, DNA sequences downstream from the mRNA cap site appear to be sufficient for the regulation of beta-globin gene expression during the differentiation of mouse erythroleukemia cells in culture.

Linker scanning mutagenesis of the 5'-flanking region of the mouse beta-major-globin gene: sequence requirements for transcription in erythroid and nonerythroid cells

We analyzed the sequences required for transcription of the mouse beta-major-globin gene by introducing deletion and linker scanning mutations into the 5'-flanking region and then studying the effects of these mutations on beta-globin gene transcription in a HeLa cell transient expression assay or after stable introduction into mouse erythroleukemia cells. Consistent with earlier studies, we found that three distinct regions upstream from the RNA capping site are required for efficient beta-globin gene transcription in HeLa cells: the ATA box located 30 base pairs upstream from the mRNA capping site (-30), the CCAAT box located at -75, and the distal sequence element CCACACCC located at -90. In the ATA and CAAT box regions, the sequences necessary for efficient transcription extend beyond the limits of the canonical sequences. Mutations in the sequences located between the three transcriptional control elements do not significantly affect transcription in HeLa cells. Although the promoter defined in HeLa cell transfection experiments is also required for efficient transcription in mouse erythroleukemia cells, none of the mutations tested affects the regulation of beta-globin gene transcription during mouse erythroleukemia cell differentiation. Thus, DNA sequences downstream from the mRNA cap site appear to be sufficient for the regulation of beta-globin gene expression during the differentiation of mouse erythroleukemia cells in culture.

A third type of murine T-cell receptor gene

By subtractive cDNA hybridizations, we have isolated a new species of T-cell receptor cDNA clone whose predicted amino acid sequence has homology to variable, constant, joining and diversity segments of immunoglobulins and T-cell receptors. The corresponding genomic sequence is also rearranged in several T-cell DNAs. The four potential N-linked glycosylation sites, frequency of expression and predicted molecular weight (27,800) of this molecule make it a likely candidate for the alpha-chain of the T-cell receptor. Expression data also indicate that this gene may be activated at a later stage of T-cell differentiation than the beta-chain.

Differentiation of murine erythroleukemia cells results in the rapid repression of vimentin gene expression

We show that vimentin filaments are present in undifferentiated Friend murine erythroleukemia cells, but are lost progressively to undetectable levels by 96 h of dimethyl sulfoxide-mediated differentiation. The amount of newly synthesized cytoskeletal vimentin is decreased dramatically by 24 h of induction, and is paralleled by a rapid loss of vimentin mRNA (approximately 25-fold reduction at 96 h). Hence, disappearance of vimentin filaments in these cells appears to be regulated at the level of vimentin mRNA abundance. On the other hand, the levels of actin synthesis and actin mRNA remain essentially unchanged. The kinetics of vimentin mRNA reduction during dimethyl sulfoxide-mediated differentiation, and the levels of vimentin mRNA observed in the presence of hexamethylene-bisacetamide or hemin as inducers suggest that the cessation of vimentin expression precedes, but may be associated with commitment to terminal differentiation. Our results demonstrate the dynamic regulation of vimentin expression in mammalian erythropoiesis.

Tissue-specific formation of transcription-initiation complexes at the 5' end of the mouse beta major globin gene

Accurate initiation of transcription in vitro requires, in addition to RNA polymerase II, factors present in soluble extracts of cultured cells. We have developed transcription system in vitro, which permits us to visualize the transcription-initiation complexes on a mouse beta globin restricted fragment from a recombinant beta globin bacteriophage DNA. Using the lambda fragments as internal controls this system has allowed us to assess the specificity of transcription with RNA polymerase II. Comparing extracts from cells and tissues expressing the globin genes (Friend cells, spleen and blood from phenylhydrazine-induced anemic mice) with those which do not (thymus, liver, PCC3 cells), we observed that specific initiation of transcription on the beta globin gene occurs only with soluble extracts from erythroid tissues. This tissue-specific transcription is partially sensitive to alpha-amanitin and occurs at the 5' end of the globin gene.

The isolation and measurement of tRNAmeti using RNA/DNA hybridization

A pBR322 plasmid containing the initiator tRNAmet gene of Xenopus (pt145 - donated by Stuart Clarkson) will specifically bind to mouse initiator tRNAmet (tRNAmeti) when total mouse tRNA, extracted from uninduced Friend erythroleukemia cells, is hybridized to the gene probe. One dimensional electrophoresis of the hybridizing tRNA in 20% polyacrylamide reveals one major band (95%) and a minor band. The hybridizing tRNA has been identified as initiator tRNAmet by RNA sequencing. Hybridization of tRNAtotal to another plasmid containing the Xenopus gene for tRNAasn results in two bound species with different electrophoretic mobilities than the tRNA bound to the initiator tRNAmet gene. pt145 has been used to measure the steady state concentration of initiator tRNAmet in the uninduced and erythroid Friend cell, and in the unfertilized egg and 21 h blastula of the sea urchin. Initiator tRNAmet represents 0.91% and 0.52% of the tRNA populations extracted from uninduced and erythroid Friend cells, respectively. Based upon the total tRNA content per cell, there is a 3.8 fold decrease in initiator tRNAmet per cell during erythroid differentiation. tRNA extracted from unfertilized eggs and 21 h blastula of the sea urchin both have 0.5% of total tRNA as initiator tRNAmet (approximately 1.5 pg).

Preferential transcription and nuclear transport of globin gene sequences, as control steps leading to final differentiation of murine erythroleukemic cells

Murine erythroleukemic (MEL) cells undergo a specific program of differentation in vitro, which is mainly characterized by accumulation of globin mRNA. These cells serve as a model system to study in detail the expression of a specific gene product at the transcriptional and post-transcriptional level. In this report we describe experiments in which the transcription rate of globin and non-globin genes, as well as their cytoplasmic appearance, was measured during the differentiation process. Two independent steps for regulating the abundance of globin mRNA were observed. On the transcriptional level we have observed that, in contrast to the transcription of globin genes, the transcription rate of non-globin genes is dramatically reduced throughout the period of induction. When the rate of cytoplasmic appearance was measured newly synthesized globin RNA molecules were found to be preferentially transported into the cytoplasm. It was shown that the reduction in cytoplasmic appearance of nonglobin genes is not a result of a shut-off in their transcriptional activity. In cells treated with 12-O-tetradecanoylphorbol 13-acetate the transcription rate remains constant while a continuous reduction in the cytoplasmic appearance is observed. These two independent phenomena which affect the non-globin genes, i.e. the suppression of their transcription and reduced cytoplasmic appearance, lead to the reduction in the relative amounts of the stable poly(A)-rich mRNA population and to the accumulation of globin sequences in the cytoplasm of the differentiated erythroid cells. These observations are in agreement with our previous model, which claimed that disappearance of the stable poly(A)-rich mRNA population is an obligatory process leading to the final differentiation of MEL cells.

Simian virus 40 transcriptional complexes incorporate mercurated nucleotides into RNA in vitro

Simian virus 40 Sarkosyl transcriptional complexes incorporated mercurated nucleotide precursors into preinitiated RNA chains. Synthesis with mercurated precursors was three- to fivefold slower than with nonmercurated nucleotides (12 to 20 pmol per 10(6) cells per h at 25 degrees C), and 50 to 70% of the RNA product bound specifically to sulfhydryl-agarose. The amount of mercurated CMP incorporated into RNA was measured by specific binding of [35S]cysteine to the mercury residues. More than 90% of the mercurated RNA hybridized to simian virus 40 DNA.

RNA synthesis in rabbit endometrial nuclei. Hormonal regulation of transcription of the uteroglobin gene

Endometrical nuclei, prepared from rabbits subjected to different hormonal treatments, were used for the cell-free synthesis of RNA. Optimal conditions for the incorporation of [3H]UMP into RNA are described, leading to the synthesis of relatively undegraded RNA molecules. Under these conditions there is virtually no initiation of new RNA chains in vitro, and RNA chain elongation is inhibited up to 60% by low concentrations of alpha-amanitin and up to 90% by actinomycin D. The synthesis of RNA is slightly inhibited in the presence of Hg-CTP and monothioglycerol, but newly synthesized mercurated RNA can be efficiently separated from endogenous RNA upon chromatography on sulfhydryl-Sepharose under stringent conditions. The RNA synthesized in vitro by endometrial nuclei from pseudopregnant rabbits contains RNA sequences transcribed from the uteroglobin gene, as demonstrated by hybridization to an excess of purified preuteroglobin cDNA. In endometrial cells from pseudopregnant animals the number of RNA polymerase II molecules transcribing the uteroglobin gene is 12-fold higher than in control animals, demonstrating that at least part of the hormonally induced accumulation of preuteroglobin mRNA is due to an increased rate of transcription of the uteroglobin gene.

Role of prolactin in the transcription of beta-casein and 28-S ribosomal genes in the rabbit mammary gland

Isolated mammary nuclei were incubated in the presence of HgCTP and the neosynthesized RNA was isolated with a SH-Sepharose column. The concentration of beta-casein mRNA and 28-S ribosomal RNA in the neosynthesized RNA fractions was evaluated using [3H]DNA probes complementary to beta-casein mRNA and 28-S rRNA respectively. In the unstimulated pseudopregnant rabbit, the transcription of both genes was easily detectable. Injections of prolactin progressively enhanced the transcription rate of both genes and preferentially the beta-casein gene. A comparison between the transcription rates and the accumulation of the corresponding gene products in the cell revealed that there is a good correlation between these two parameters for the 28-S rRNA gene. By contrast, the acceleration of beta-casein gene transcription by prolactin is unable to account for the simultaneous accumulation of beta-casein mRNA, indicating that prolactin is a potent stabilizer of casein mRNA. Injections of CB154 into lactating rabbits (a drug which suppresses the secretion of prolactin by hypophysis), induced a rapid drop of beta-casein mRNA concentration and a slow decline of beta-casein gene transcription. Simultaneously the drug was responsible for a marked and rapid decrease of 28-S rRNA gene transcription, while the concentration of the rRNA remained elevated. During weaning the transcription of the beta-casein gene and, to a lower degree, the transcripton of the 28-S rRNA gene proceeded more slowly and this phenomenon was accompanied by a progressive decline of the RNA concentrations.

Transcription of Friend virus proviral sequences in isolated nuclei

Transcription studies using isolated Friend nuclei and Escherichia coli polymerase are presented. Combination of the techniques of thiol-Sepharose chromatography and cDNA-Sepharose hybridisation has resulted in a system in which the transcription of the Friend virus proviral sequences with endogenous and E. coli polymerase can be examined. The results show that the percentage of Friend viral-specific sequences in RNA transcribed by E. coli RNA polymerase and by endogenous RNA polymerase in isolated nuclei are similar. The percentage of viral-specific sequences synthesized in isolated nuclei is similar to that found in Friend cell nuclear RNA.

Transcriptional activity of nuclei from WI-38 cells at various passages

Transcriptional activity of nuclei from human diploid fibroblast (WI-38) cells at different passage levels was studied with endogenous RNA polymerase. The rate and extent of the RNA synthesis decreased when the nuclei were prepared from senescent cells. The decreased activity of RNA synthesis in senescent cell nuclei was demonstrated from both the transcriptional assay and the estimation of the processing activity, i.e., polyadenylation of the transcripts.

In vitro DNA dependent synthesis of globin RNA sequences from erythroleukemic cell chromatin

Murine erythroleukemic cells in culture accumulate cytoplasmic globin mRNA during differentiation induced by dimethyl sulfoxide (DMSO)1. Chromatin was prepared from DMSO induced erythroleukemic cells that were transcribing globin RNA in order to determine whether in vitro synthesis of globin RNA sequences was possible from chromatin. RNA was synthesized in vitro using 5-mercuriuridine triphosphate and exogenous Escheria coli RNA polymerase. Newly synthesized mercurated RNA was purified from endogenous chromatin associated RNA by affinity chromatography on a sepharose sulfhydryl column, and the globin RNA sequence content of the mercurated RNA was assayed by hybridization to cDNA globin. The synthesis of globin RNA sequences was shown to occur and to be sensitive to actinomycin and rifampicin and insensitive to alpha-amanitin. In contrast, synthesis of globin RNA sequence synthesis was not detected in significant amounts from chromatin prepared from uninduced erythroleukemic cells, nor from uninduced cell chromatin to which globin RNA was added prior to transcription. Isolated RNA:cDNA globin hybrids were shown to contain mercurated RNA by affinity chromatography. These results indicated that synthesis of globin RNA sequences from chromatin can be performed by E. coli RNA polymerase.

Partial purification, characterization and translation in vitro of rat liver metallothionein messenger ribonucleic acid

Poly(A)+ (polyadenylated) mRNA coding for metallothioneins was purified 13-fold from rat liver polyribosomes and was identified by its ability to direct the biosynthesis of these proteins in a wheat-germ cell-free system. The carboxymethylated products of the protein-synthesizing system in vitro were analysed with sodium dodecyl sulphate/20% polyacrylamide-gel electrophoresis. The labelled compounds [3H]serine and [35S]cysteine were incorporated at high specific radioactivity into proteins that co-migrated with authentic metallothioneins. No [3H]leucine incorporation was found, in agreement with the amino acid composition of the metallothioneins. Metallothionein mRNA had a sedimentation coefficient of 9 S and carried a maximum of four ribosomes. At 5 h after a subcutaneous injection of ZnCl2 or CdCl2 (10 mumol/kg body wt.), the amount of this mRNA increased approx. 2- and 4-fold respectively, on the basis of translation in vitro. The increase in metallothionein mRNA (defined by translation in the wheat-germ system) was transient and, after CdCl2 treatment, fell back to control values by 17 h. Metallothioneins constituted a maximum of 0.8% of the total protein products synthesized in the wheat-germ system by total mRNA isolated from rat liver after CdCl2 treatment.

RNA synthesis in isolated nuclei of lactating mammary cells in presence of unmodified and mercury-labeled CTP

Isolated nuclei of lactating mouse mammary gland were capable of supporting DNA-dependent RNA synthesis in vitro in presence of unmodified and mercurated CTP (Hg-CTP) at high ionic condition at 25 degrees C. In presence of unmodified CTP, [3H]UMP incorporation into RNA increased linearly upto 180 min. The kinetic pattern of the reaction and the rate of RNA synthesis were essentially similar when CTP was replaced by Hg-CTP. Both in unmodified and Hg-CTP containing reactions, 70-80% of RNA synthesis was inhibited by alpha-amanitin. Presence of poly(A) in a small portion of the in vitro synthesized messenger-like RNA was detectable by oligo(dT) cellulose chromatography. Both poly(A)+ and poly(A)- RNAs sedimented with a clear peak around 15S region in a formamide-sucrose denaturing gradient. The Hg-RNA after separation from endogenous nuclear RNA by SH-agarose affinity column chromatography also sedimented around 15S region in a formamide-sucrose gradient. The Hg-RNA synthesized in the isolated mammary cell nuclei in vitro should now permit monitoring hormonal regulation of specific gene (casein) transcription in the mammary cells by molecular hybridization of the Hg-RNA with cDNA to casein mRNA.

Frequency distribution of mRNA and pre-mRNA in growing and differentiated Friend cells

The frequency distribution of poly(A)+-mRNA in growing and in differentiated Friend cells has been measured by mRNA-cDNA hybridization and their differences established by heterologous hybridization of mRNA of one type and cDNA of the other. It was shown that induction of Friend cells involves an increase in abundance of a small number of mRNAs, while no specific pattern of messenger disappearance could be detected. The frequency distribution of pre-mRNA was determined by hybridizing nuclear RNA with the cDNA probes complementary to mRNA. In uninduced Friend cells, it was shown that most precursor messenger sequences are present at a single frequency of about 3 molecules per nucleus, independently of their final frequency in polysomal mRNA. In induced Friend cells, the frequency distribution of pre-mRNA is more heterogeneous and correlated to some extent with the corresponding mRNA frequency distribution.

Is there specific transcription from isolated chromatin?

Hg-UMP-containing transcripts made from chick erythroid chromatins with E. coli RNA polymerase hybridize to chick globin cDNA. Contamination with endogenous globin RNA has been largely removed by purification on SH-agarose columns at 55 degrees C. Some endogenous globin mRNA sequences remain, probably as hybrids with "anti-sense" Hg-transcripts produced by RNA-dependent RNA synthesis. Heating to 115 degrees C before SH-agarose chromatography eliminates these contaminants. Hg-transcripts from adult and embryonic erythroid chromatins purified by this method are hybridized to globin cDNA; they contain a 4- to 6-fold higher proportion of globin-specific sequences (10-13 PPM) than do transcripts from brain chromatin. Dissociation of erythroid chromatins in salt and urea, followed by reconstitution using standard methods, destroys even this low degree of specificity.

In vitro RNA synthesis and expression of vitellogenin gene in isolated chicken liver nuclei

Optimal conditions for prolonged in vitro synthesis of RNA in isolated chicken liver nuclei have been described. It is shown by incorporation of gamma32P-GTP into RNA, analysis of the product on sucrose density gradient, and digestion with alkaline phosphatase and ribonuclease A that there is reinitiation of RNA synthesis. Polynucleotide kinase activity has been ruled out as explanation for the incorporation of gamma32P-GTP. alpha-Amanitin inhibits RNA synthesis by about 50%. Nuclei prepared from estradiol-treated chicks have twice the RNA synthesis activity as the controls. RNA is synthesized in the presence of Hg-UTP and the mercurated product separated by affinity chromatography on sulfhydryl-Sepharose column under stringent conditions. Vitellogenin mRNA sequences are measured by hybridization with DNA complementary to vitellogenin mRNA. Estradiol treatment leads to a 10-fold increase in vitellogenin mRNA sequences.

Mapping of adenovirus late promoters with nascent mercurated RNA

Nascent RNA molecules were labeled in vivo and elongated in vitro by incubation of the isolated nuclei in the presence of mercurated nucleotides. The RNA molecules initiated and labeled in vivo and elongated in vitro were then selectively purified on a thiopropyl 6-B Sepharose affinity column. The procedure was shown to be free of artifacts since the addition of mercurated nucleotides and the retention on the affinity column is mediated by the endogenous RNA polymerase II (nucleoside triphosphate:RNA nucleotidyltransferase; EC 2.7.7.6), is sensitive to actinomycin D, and is dependent on the presence of all four ribonucleotide triphosphates. This general procedure was applied to the mapping of viral promoters late after adenovirus 2 infection of HeLa cells. RNA purified as described above was hybridized to restriction enzyme fragments attached to nitrocellulose filters. The 5' ends of the nascent RNA chains are located in coordinates 9.5-17 for a rightward transcript, 0-25 for a leftward transcript, and possibly 60-70 for a second rightward transcript. These locations clearly differ from locations of the early promoters and therefore suggest that the transition from early to late functions is controlled at the transcriptional level.

Differentiation of murine erythroleukemic (Friend) cells: an in vitro model of erythropoiesis

Normal erythropoiesis involves differentiation of uncommitted stem cells through committed erythroid precursors into cells specialized for hemoglobin synthesis. Several aspects of this developmental sequence may be studied in murine erythroleukemic cells infected with Friend virus complex. These cells are arrested at the proerythroblast stage, yet capable of continuous growth in vitro. Maturation along an erythroid pathway is induced after treatment with a variety of agents (e.g. dimethylsulfoxide, butyric acid, hemin, ouabain). Following induction, the cells morphologically resemble normoblasts, accumulate globin mRNAs and strain-specific globins, increase heme synthesis and acquire erythrocyte membrane antigens. Cloned populations of erythroleukemic cells mature in a nonhomogeneous fashion upon induction, indicative of a stochastic response in the inductive process. This "probability of differentiation" phenotype is formally analogous to stem cell development in which hematopoietic precursor cells form a constant, dividing population from which cells are continuously maturing. Although the sequence of events involved in triggering differentiation is uncertain, cloning and cell hybridization experiments demonstrate that this phenotype is under rather stable genetic (or epigenetic) control. Recent molecular analysis shows that induced differentiation is accompanied by transcriptional activation of the globin genes rather than posttranscriptional stabilization of the globin RNAs. Further application of cellular, molecular and genetic approaches in this system may help to define specific control mechanisms in erythroid development.