Post-transcriptional suppression of globin gene expression in cells transformed by avian erythroblastosis virus

Primary transcripts: From the discovery of RNA processing to current concepts of gene expression - Review

The main purpose of this review is to recall for investigators - and in particular students -, some of the early data and concepts in molecular genetics and biology that are rarely cited in the current literature and are thus invariably overlooked. There is a growing tendency among editors and reviewers to consider that only data produced in the last 10-20 years or so are pertinent. However this is not the case. In exact science, sound data and lucid interpretation never become obsolete, and even if forgotten, will resurface sooner or later. In the field of gene expression, covered in the present review, recent post-genomic data have indeed confirmed many of the earlier results and concepts developed in the mid-seventies, well before the start of the recombinant DNA revolution. Human brains and even the most powerful computers, have difficulty in handling and making sense of the overwhelming flow of data generated by recent high-throughput technologies. This was easier when low throughput, more integrative methods based on biochemistry and microscopy dominated biological research. Nowadays, the need for organising concepts is ever more important, otherwise the mass of available data can generate only "building ruins" - the bricks without an architect. Concepts such as pervasive transcription of genomes, large genomic domains, full domain transcripts (FDTs) up to 100 kb long, the prevalence of post-transcriptional events in regulating eukaryotic gene expression, and the 3D-genome architecture, were all developed and discussed before 1990, and are only now coming back into vogue. Thus, to review the impact of earlier concepts on later developments in the field, I will confront former and current data and ideas, including a discussion of old and new methods. Whenever useful, I shall first briefly report post-genomic developments before addressing former results and interpretations. Equally important, some of the terms often used sloppily in scientific discussions will be clearly defined. As a basis for the ensuing discussion, some of the issues and facts related to eukaryotic gene expression will first be introduced. In chapter 2 the evolution in perception of biology over the last 60 years and the impact of the recombinant DNA revolution will be considered. Then, in chapter 3 data and theory concerning the genome, gene expression and genetics will be reviewed. The experimental and theoretical definition of the gene will be discussed before considering the 3 different types of genetic information - the "Triad" - and the importance of post-transcriptional regulation of gene expression in the light of the recent finding that 90% of genomic DNA seems to be transcribed. Some previous attempts to provide a conceptual framework for these observations will be recalled, in particular the "Cascade Regulation Hypothesis" (CRH) developed in 1967-85, and the "Gene and Genon" concept proposed in 2007. A knowledge of the size of primary transcripts is of prime importance, both for experimental and theoretical reasons, since these molecules represent the primary units of the "RNA genome" on which most of the post-transcriptional regulation of gene expression occurs. In chapter 4, I will first discuss some current post-genomic topics before summarising the discovery of the high Mr-RNA transcripts, and the investigation of their processing spanning the last 50 years. Since even today, a consensus concerning the real form of primary transcripts in eukaryotic cells has not yet been reached, I will refer to the viral and specialized cellular models which helped early on to understand the mechanisms of RNA processing and differential splicing which operate in cells and tissues. As a well-studied example of expression and regulation of a specific cellular gene in relation to differentiation and pathology, I will discuss the early and recent work on expression of the globin genes in nucleated avian erythroblasts. An important concept is that the primary transcript not only embodies protein-coding information and regulation of its expression, but also the 3D-structure of the genomic DNA from which it was derived. The wealth of recent post-genomic data published in this field emphasises the importance of a fundamental principle of genome organisation and expression that has been overlooked for years even though it was already discussed in the 1970-80ties. These issues are addressed in chapter 5 which focuses on the involvement of the nuclear matrix and nuclear architecture in DNA and RNA biology. This section will make reference to the Unified Matrix Hypothesis (UMH), which was the first molecular model of the 3D organisation of DNA and RNA. The chapter on the "RNA-genome and peripheral memories" discusses experimental data on the ribonucleoprotein complexes containing pre-mRNA (pre-mRNPs) and mRNA (mRNPs) which are organised in nuclear and cytoplasmic spaces respectively. Finally, "Outlook " will enumerate currently unresolved questions in the field, and will propose some ideas that may encourage further investigation, and comprehension of available experimental data still in need of interpretation. In chapter 8, some propositions and paradigms basic to the authors own analysis are discussed. "In conclusion" the raison d'être of this review is recalled and positioned within the overall framework of scientific endeavour.

The 33 kb transcript of the chicken ?-globin gene domain is part of the nuclear matrix

Giant nuclear transcripts, and in particular the RNAs of the globin gene domains which are much larger than their canonical pre-mRNAs, have been an enigma for many years. We show here that in avian erythroblastosis virus (AEV)-transformed chicken erythroleukaemic cells, where globin gene expression is abortive, the whole domain of alpha-globin genes is transcribed for about 33 kb in the globin direction and that this RNA is part of the nuclear matrix. Northern blot hybridisation with strand-specific riboprobes, recognising genes and intergenic sequences, and RT-PCR with downstream primers, show that the continuous full domain transcript (FDT) starts in the vicinity of a putative LCR and includes all the genes as well as known regulatory sites, the replication origin, and the DNA loop anchorage region in the upstream area. Absent in chicken fibroblasts, the globin FDT overlaps the major part of the ggPRX housekeeping gene that is transcribed in the opposite direction. RT-PCR and in situ hybridisation with genic and extra-genic globin probes demonstrated that the globin FDT is a component of the nuclear matrix. We suggest that the globin FDTs keep the domain in an active state, and the globin RNAs on the processing pathway are a component of the nuclear matrix. They may take part in the dynamic nuclear architecture when productively processed, or turn over slowly when globins are not synthesised.

Genetic test for involvement of intervening sequences in transport of nuclear RNA

The construction of a recombinant virus in the late region of simian virus 40 is presented. The small intervening sequence of late 19S RNA (0.760 to 0.765 map unit) was cloned and inserted into the EcoRI site (1.0 map unit) in the late region of simian virus 40. This is a mutant virus that now has two intervening sequences, one at the normal position (0.760 map unit) and another out of the context of its flanking sequence and now at 1.0 map unit. The recombinant appears poisonous, as repeated attempts to plaque it as a virus with a standard helper virus were unsuccessful. The transcription of this recombinant was, therefore, studied after direct DNA transfection onto CV-1 cells. Nuclease S1 analysis of mutant RNA indicates that the major nuclear transcript was a spliced but nuclear 16S RNA species. Normally, 16S RNA is not found in the nucleus. This result was shown to be an artifact of the DNA transfection protocol. When the glycerol shock was done after infection with virus, a similar alteration in the makeup of nuclear RNA was seen. A transient stock of this double-intron mutant was finally obtained, using a nonrevertable helper virus. The transcriptional analysis of this mutant showed that unspliced 19S RNA was not transported and remained within the nucleus, whereas spliced 19S and 16S RNAs were transported. We conclude that the retention of nuclear transcripts within the nucleus is not simply due to the presence of intronic sequences, as spliced 19S and 16S RNAs which contain the second intron were efficiently transported.

Genetic test for involvement of intervening sequences in transport of nuclear RNA

The construction of a recombinant virus in the late region of simian virus 40 is presented. The small intervening sequence of late 19S RNA (0.760 to 0.765 map unit) was cloned and inserted into the EcoRI site (1.0 map unit) in the late region of simian virus 40. This is a mutant virus that now has two intervening sequences, one at the normal position (0.760 map unit) and another out of the context of its flanking sequence and now at 1.0 map unit. The recombinant appears poisonous, as repeated attempts to plaque it as a virus with a standard helper virus were unsuccessful. The transcription of this recombinant was, therefore, studied after direct DNA transfection onto CV-1 cells. Nuclease S1 analysis of mutant RNA indicates that the major nuclear transcript was a spliced but nuclear 16S RNA species. Normally, 16S RNA is not found in the nucleus. This result was shown to be an artifact of the DNA transfection protocol. When the glycerol shock was done after infection with virus, a similar alteration in the makeup of nuclear RNA was seen. A transient stock of this double-intron mutant was finally obtained, using a nonrevertable helper virus. The transcriptional analysis of this mutant showed that unspliced 19S RNA was not transported and remained within the nucleus, whereas spliced 19S and 16S RNAs were transported. We conclude that the retention of nuclear transcripts within the nucleus is not simply due to the presence of intronic sequences, as spliced 19S and 16S RNAs which contain the second intron were efficiently transported.

In chicken leukemia cells globin genes are fully transcribed but their rnas are retained in the perinucleolar area

Using hybridization in situ with a ribo-probe recognizing transcripts of the chicken alpha A globin gene, we show here that in proliferating AEV-transformed erythroblasts this gene is strongly transcribed, but the corresponding transcripts are retained in the nuclei. Most surprisingly, this globin RNA accumulates in the perinucleolar areas in a pattern never observed before. Upon induction of cells to differentiate, leading to productive expression of the hemoglobins, the transcripts of the alpha A globin gene were found for the most part in the cytoplasm. In the nuclei of differentiated cells, the globin RNA is concentrated in one or two specific spots, which are likely to represent the "processing centers" (PCs) of the globin RNA. The results presented indicate that posttranscriptional steps of regulation involving in particular the perinuclear areas are of major importance for erythroid differentiation.

HIV-1 expression is posttranscriptionally repressed in Drosophila cells

The long terminal repeats (LTRs) of the human immunodeficiency virus (HIV) the Rous sarcoma virus (RSV) and the copia Drosophila retrotransposon were compared in their capacity to direct expression of the bacterial cat (chloramphenicol acetyltransferase) gene in human, murine, and Drosophila cell lines. The results indicate that HIV and RSV LTR expression is post transcriptionally repressed in the Drosophila cells while copia LTR expression is post-transcriptionally repressed in the human and murine cells.

AEV-transformed erythroleukemia cell induced differentiation: expression of specific cell membrane antigenic molecules

A simultaneous decay of the expression of Im 140 kDa, Im 150 kDa and Im 160 kDa high MW membrane antigens, concomitant with the cell proliferation arrest, was observed during erythropoietin induced differentiation of ts 34 AEV-transformed erythroid cells cultivated at the restrictive temperature. Expression of embryo-immature antigens was maintained during induced differentiation of erythroleukemia cells, but their MW shifted from 50 to 48 kDa, which corresponds to the MW of embryo-immature antigens detected on normal erythroid cells. In the absence of erythropoietin at the restrictive temperature, conditions under which the ts 34 AEV-transformed erythroid cells fail to differentiate and maintain their capacity to proliferate, the expression of high MW antigens as well as the expression of embryo-immature antigens remained unaffected. Therefore, it is shown that the expression of specific membrane antigens is modulated under conditions rendering the erythroleukemia cell differentiation process possible.

Transcription of the alpha globin gene domain in normal and AEV-transformed chicken erythroblasts: mapping of giant globin-specific RNA including embryonic and adult genes

The genomic domain of about 20 kbp of the chicken alpha-type globin genes, framed by AT-rich linkers (ATRLs; Moreau et al. 1982) and repetitive sequences (Broders et al. 1986), was cut into 13 fragments and subcloned. The in vitro labelled individual restriction fragments were used to test the extent of the transcribed domain by blot-hybridization of nuclear RNA in large excess from normal adult chicken and Avian Erythroblastosis Virus (AEV)-transformed erythroblasts. In both these types of cells, the AT-rich segments situated 6 kbp upstream of the first gene as well as all the domain including the embryonic pi and the adult alpha D and alpha A genes down to the AT-rich segment placed 3 kbp downstream were found to be transcribed. Electrophoresis of nuclear RNA, Northern blotting and hybridization with most of the nick-translated DNA probes revealed in all cases the presence of heterogeneous globin RNA molecules in the 3-12 kb range, as well as some distinct RNA bands. Single-stranded RNA probes of some genomic segments indicated asymmetrical transcription of the minus strand. A 12 kb globin-specific RNA including the pi and alpha A genes but not the intervening alpha D gene was observed in AEV-transformed cells: it includes sequences located far upstream and downstream from the alpha globin genes and might represent a processing product of a full length transcript spanning the whole domain. Reverse transcription by extension of primers placed in the first exon of each of the three globin genes confirmed the presence of continuous transcripts of the domain including the two adult and the embryonic globin genes.

Expression of a human cytomegalovirus late gene is posttranscriptionally regulated by a 3'-end-processing event occurring exclusively late after infection

A phenomenon of posttranscriptional regulation has been previously identified in cytomegalovirus-infected human fibroblast cells (Wathen and Stinski, J. Virol. 41:462, 1982). A region typifying this phenomenon has been located within the large unique component of the viral genome (map units 0.408 to 0.423). Even though this transcriptional unit was highly transcribed at early times after infection, mRNAs from this region were only detectable on the polyribosomes after viral DNA replication. Thus, this region is believed to code for a late gene. Single-strand-specific nuclease mapping experiments of viral transcripts established that the transcriptional initiation sites and the 5' ends of a downstream exon were identical at early and late times. However, the late transcripts differed from the early transcripts by the processing of the 3' end of the viral RNAs. This involved either the removal of a distinct region of the transcript by the selection of an upstream cleavage and polyadenylation site or the differential splicing of the RNA molecule. The upstream cleavage and polyadenylation site was identified by nuclease mapping analyses and DNA sequencing. The 3'-end processing of these transcripts is necessary for the detection of these viral RNAs within the cytoplasm of the infected cell. We propose that human cytomegalovirus either codes for a factor(s) that is involved in the 3'-end-processing event at late times after infection or stimulates the synthesis of a host cell factor(s) involved in this complex regulatory event. This level of regulation may have an influence on the types of cells that permit productive cytomegalovirus replication.

Expression of a human cytomegalovirus late gene is posttranscriptionally regulated by a 3'-end-processing event occurring exclusively late after infection

A phenomenon of posttranscriptional regulation has been previously identified in cytomegalovirus-infected human fibroblast cells (Wathen and Stinski, J. Virol. 41:462, 1982). A region typifying this phenomenon has been located within the large unique component of the viral genome (map units 0.408 to 0.423). Even though this transcriptional unit was highly transcribed at early times after infection, mRNAs from this region were only detectable on the polyribosomes after viral DNA replication. Thus, this region is believed to code for a late gene. Single-strand-specific nuclease mapping experiments of viral transcripts established that the transcriptional initiation sites and the 5' ends of a downstream exon were identical at early and late times. However, the late transcripts differed from the early transcripts by the processing of the 3' end of the viral RNAs. This involved either the removal of a distinct region of the transcript by the selection of an upstream cleavage and polyadenylation site or the differential splicing of the RNA molecule. The upstream cleavage and polyadenylation site was identified by nuclease mapping analyses and DNA sequencing. The 3'-end processing of these transcripts is necessary for the detection of these viral RNAs within the cytoplasm of the infected cell. We propose that human cytomegalovirus either codes for a factor(s) that is involved in the 3'-end-processing event at late times after infection or stimulates the synthesis of a host cell factor(s) involved in this complex regulatory event. This level of regulation may have an influence on the types of cells that permit productive cytomegalovirus replication.

Subcellular localization of RNAs in transfected cells: role of sequences at the 5' terminus

We have investigated the intracellular location of RNAs transcribed from transfected DNA. COS cells transfected with a clone containing the human adult beta globin gene contain three classes of globin RNAs. Their 3' termini and splice sites are indistinguishable from those of mature reticulocyte beta globin mRNA, and they are polyadenylated. However, as determined by S1 mapping, their 5' sequences are different. The 5' terminus of one is the same as that of mature beta globin mRNA (+1, cap site). The presumed 5' terminus of the second is located 30 nucleotides downstream from the cap site (+30). The third class contains additional nucleotides transcribed from sequences located 5' to the cap site (5' upstream RNA). The 5' upstream RNA molecules are restricted to the nucleus and are more stable than heterogeneous nuclear RNA. The +30 and +1 RNAs are located primarily in the cytoplasm. The data support the notion that nucleotide sequences and/or secondary modifications in the 5' region determine if an RNA is to be transported.

Relationship between the transport from isolated nuclei of two abundant cytoplasmic messengers and the source of a messenger RNA transport factor

Messenger RNA transport from isolated nuclei requires a 35 X 10(3) dalton cytoplasmic protein(s) which is present in both the cytosol and polyribosome fractions. Recombinant DNA probes containing cDNA inserts were used to quantitate the transport of rat liver-specific albumin and male rat liver-specific alpha 2U-globulin messenger RNA (mRNA) from male rat liver nuclei in response to the mRNA transport factors from homologous and heterologous tissues. No mRNA transport occurs in the absence of the transport factor(s). Both messengers are transported proportionately in response to the factor(s) from male or female rat liver cytosol, or from the polyribosomes (messenger ribonucleoprotein) of male or female rat liver, or brain. The transport factor(s) do not, therefore, appear to differentiate between the coding sequences of two unrelated hepatic messenger RNA's.

An assessment of the evidence for the role of ribonucleoprotein particles in the maturation of eukaryote mRNA

This article has sought to draw together, on the one hand, what is known of mRNA processing and its control and, on the other hand, what is known of the structure and validity of hnRNP and snRNP particles. At the same time, it has attempted to synthesize these two themes into a critical assessment of the evidence which suggests that the particles are intimately involved in processing. It cannot be said that the case is proven. The evidence is compelling but circumstantial. The last few years have seen the development of the first in vitro splicing systems (Weingartner and Keller, 1981; Goldenberg and Raskus, 1981; Kole and Weissman, 1982), the isolation of monoclonal antibodies to defined snRNP (Lerner et al., 1981a; Billings et al., 1982) and hnRNP proteins (Hugle et al., 1982), and the ability to use artificial lipid vesicles to transfer antisera (Lenk et al., 1982) and radioactive snRNA (Gross and Cetron, 1982) into cells. It is to be hoped that further refinements of these and other techniques will allow us to solve this, one of the major outstanding problems of molecular biology.

In vitro transformation by avian erythroblastosis virus recruits cells of erythroid lineage with varying degree of differentiation

We have previously shown that AEV infection of leukosis-free chickens provokes an erythroleukemia in the infected birds, and that the target cell for virus transformation is most likely an early pro-erythroblast comparable to the BFU-E. The virus infection correlates with a block in differentiation which was studied at a specific cellular gene level, namely that of the globin genes. In live birds, the viral transformation has been found to affect the transcriptional activity to two out of three adult globin genes and the phenotypic expression of the third. Indeed, the alpha A gene that is transcribed is silent as well, since its transcription product is fully eliminated in the nucleus following an "abortive processing". Chick bone marrow cells can also be transformed in vitro by AEV in which case the virus recruits, as targets, cells of the erythroid lineage belonging to a wider spectrum and hence in varying degrees of differentiation. In contrast to the results obtained with in vivo transformed cells, molecular hybridization with globin cDNA probes showed that in the erythroblasts transformed in vitro under tissue culture conditions all the three adult globin genes are transcriptionally active. In fact, low but detectable amounts of globin mRNA sequences are present in both the nuclear and cytoplasmic compartment of the cell prior to exposure to any chemical inducers of red cell differentiation. Judging from our present results, we may propose that AEV transformation in vitro does not shut off out only modulates the already functioning erythroid cell differentiation program.

No evidence for post-transcriptional control of albumin and alpha-fetoprotein gene expression in developing rat liver neoplasia

Rot analysis of hybridization data using highly labeled alpha-fetoprotein (AFP) and albumin (32P)cDNA probes has been used to quantitate AFP and albumin mRNA sequences in RNA preparations from different subcellular fractions of developing rat liver and Morris hepatoma 7777. In addition, size analysis of these mRNA sequences has been carried out by electrophoretic fractionation on agarose gels containing methylmercury hydroxyde and hybridization to radioactive cloned albumin and AFP cDNA probes. In all the tissues examined (fetal, newborn and adult rat liver, and hepatoma 7777) most of the albumin and AFP mRNA sequences were found associated with the polysomes as mature mRNA molecules; less than 2% of these sequences were present in the nuclear or the non polysomal cytoplasmic compartments. The number of AFP mRNA molecules was found to decrease in parallel in all the cellular compartments during rat liver development. In Morris hepatoma 7777 the content of albumin mRNA was considerably decreased in all the cellular fractions as compared to normal liver. These results demonstrate that post-transcriptional control mechanisms leading to an accumulation of non-functional mRNA molecules are not implicated in the changes of expression of albumin and AFP genes during rat liver development and neoplasia.

Cloning of Xenopus laevis nuclear poly(A)-rich RNA sequences. Evidence for post-transcriptional control

cDNA/RNA hybridization experiments of polysomal and nuclear poly(A)-rich RNA from early tadpole stages of Xenopus laevis revealed that part of the nuclear poly(A)-rich RNA sequences are not present within the polysomal polyadenylated RNA. For a more detailed analysis of these sequences we have cloned double-stranded cDNA derived from tadpole nuclear poly(A)-rich RNA in the PstI cleavage site of pBR 322. By colony screening with 32P-labelled cDNA from polysomal and nuclear poly(A)-rich RNA of the tadpole stage we could identify and isolate some of the cloned sequences, which are present only within the nuclear RNA. However, hybridization with cDNA from polysomal poly(A)-rich RNA of the gastrula stage indicated that at least one of those sequences which are confined to the nucleus at tadpole stage may serve as mRNA at gastrula stage. We present evidence that nuclear and polysomal poly(A)-rich RNA molecules containing the same nucleotide sequence differ in size and that size reduction at the level of processing precedes and may enable cytoplasmic export. We conclude that, besides stage-specific regulation of transcription, post-transcriptional control mechanisms are also involved in gene expression during embryonic development.

Variation of expression of histocompatibility antigens on tumor cells: absence of H-2Kk-gene products from a gross-virus-induced leukemia in BALB.K

The antigenic profile of the K-GV tumor of BALB.K origin, induced by Gross virus and maintained in vitro and in vivo, was investigated by serological and immunochemical methods and techniques of cell-mediated immunity. The H-2Kk-gene products were absent by several criteria: (1) monoclonal antibody and conventional alloantisera directed against eh H-2Kk antigenic specificities were nonreactive by direct testing and by absorptions. (2) H-2Kk products could not be precipitated from glycoprotein or protein extracts of the radiolabeled K-GV tumor. (3) Cytotoxic effectors against H-2Kk produced by sensitization in vitro and in vivo failed to kill K-GV target cells. (4) The tumor could neither stimulate BALB.B congenic mice to produce cytotoxic effectors nor specific cytotoxic antibody against H-2Kk-gene products. In contrast, the H-2Dk antigen was readily detectable by all these criteria. These findings therefore describe a tumor which has selectively lost the H-2K-gene products. The K-GV tumor was able to generate Gross-virus specific CTL, but had greatly reduced susceptibility to lysis by Gross-virus specific CTL generated by H-2K expressing AKR (H-2k) tumors. These findings have important implications for the associative recognition of tumor antigens and the immune surveillance of virally induced tumors.

RNA metabolism in isolated nuclei: processing and transport of immunoglobulin light chain sequences

Transport of prelabeled RNA from isolated myeloma nuclei is studied using conditions that permit RNA synthesis. Cytosol and spermidine are not required to maintain nuclear stability and inhibited RNA release. Omission of ATP or GTP decreased release 25 to 40%. The stimulatory effect of ATP or GTP is not due to hydrolysis of the triphosphates by the nuclear envelope NTPase, since addition of quercetin (an inhibitor of this NTPase) has no effect on the quantity of RNA released. The size distribution and percentage of poly A-containing species released from nuclei incubated with or without ATP or the other rNTPs are identical. Hybridization analysis of nuclear RNA before the transport assay revealed mature and precursor k light chain mRNA sequences. Following the transport assay, a significant fraction of k mRNA precursors is chased into mature k mRNA which is found both in nuclear-retained and released RNA.

Genetic variation in rates of antipyrine metabolite formation: a study in uninduced twins

Adult, male, unmedicated twins received antipyrine orally under carefully controlled environmental conditions. Relative contributions of genetic and environmental factors to 2-fold interindividual variations in rate constants for formation of the three main antipyrine metabolites were compared. Heritabilities for rate constants for formation of 4-hydroxyantipyrine, N-demethylantipyrine, and 3-hydroxymethylantipyrine were 0.88, 0.85, and 0.70, respectively. These results suggest that each molecular form of cytochrome P-450 that converts antipyrine to a different metabolite exhibits genetically controlled interindividual variations in activity. Unrelated adult male subjects whose environments were also carefully controlled exhibited highly reproducible rate constants for formation of antipyrine metabolites. Because the rate constant for metabolite formation sensitively detects certain variations in the gene product, it should be used in future pharmacogenetic studies on rates of production of multiple metabolites from a single parent drug.

Modification of the methylation pattern in the vicinity of the chicken globin genes in avian erythroblastosis virus transformed cells

Having previously found a reduced transcription of globin genes and an abortive processing of the already transcribed globin pre-mRNA in Avian Erythroblastosis Virus (AEV) transformed cells (1), we compared the genomic DNA of these cells with that of normal chicken erythroblasts, using 32-P-labelled cDNA probes specific for the beta, alpha A and alpha D globin sequences. Restriction endonuclease digestion, electrophoresis of digests in agarose gels, Southern blotting and hybridization were carried out. Our results show that the overall genome organization is not disturbed in the immediate neighbourhood of the adult globin genes; the observed restriction fragments are identical for both DNAs after EcoRI, HindIII, BamHI and XbaI digestion, using the beta, alpha A and alpha D globin cDNA probes. However, we observe specific modifications at some methylation sites in the beta, beta-like and alpha D regions: after HpaII or MspI digestion in the alpha D region and after HhaI digestion in the beta and beta-like region, heavier bands appear in the transformed cell DNA in addition to the ones observed in normal DNA. This implies that, at some specific sites, the transformed cell DNA is more methylated than the normal erythroblast DNA. The possible significance of this observation is discussed.

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).

Distinct single-copy sequence sets in sea urchin nuclear RNAs

The purpose of this study was to determine whether nuclear RNAs (nRNA) of sea urchin embryos and adult tissues contain identical or partially distinct sets of single-copy sequence transcripts. A DNA tracer was prepared consisting mainly of sequences absent from gastrula nRNA; 3.6% of this tracer reacted with adult intestine nRNA but not with gastrula nRNA. The existence of a differentially transcribed DNA fraction was verified by its partial purification and rehybridization to intestine and gastrula nRNAs. About one-third of the genomic single-copy sequence is represented in both nRNAs, or about 2 x 10(8) nucleotides. The differentially transcribed portion of the single-copy genome identified in this work includes about 3.5 x 10(7) nucleotides.