Showing papers on "Regulation of gene expression published in 1985"

Amplification, enhanced expression and possible rearrangement of EGF receptor gene in primary human brain tumours of glial origin

Abstract: Epidermal growth factor (EGF), through interaction with specific cell surface receptors, generates a pleiotropic response that, by a poorly defined mechanism, can induce proliferation of target cells. Subversion of the EGF mitogenic signal through expression of a truncated receptor may be involved in transformation by the avian erythroblastosis virus (AEV) oncogene v-erb-B, suggesting that similar EGF receptor defects may be found in human neoplasias. Overexpression of EGF receptors has been reported on the epidermoid carcinoma cell line A431, in various primary brain tumours and in squamous carcinomas. In A431 cells the receptor gene is amplified. Here we show that 4 of 10 primary brain tumours of glial origin which express levels of EGF receptors that are higher than normal also have amplified EGF receptor genes. Amplified receptor genes were not detected in the other brain tumours examined. Further analysis of EGF receptor defects may show that such altered expression and amplification is a particular feature of certain human tumours.

Isolation and characterization of genomic and cDNA clones of human erythropoietin

Abstract: The glycoprotein hormone erythropoietin regulates the level of oxygen in the blood by modulating the number of circulating erythrocytes, and is produced in the kidney or liver of adult and the liver of fetal or neonatal mammals. Neither the precise cell types that produce erythropoietin nor the mechanisms by which the same or different cells measure the circulating oxygen concentration and consequently regulate erythropoietin production are known. Cells responsive to erythropoietin have been identified in the adult bone marrow, fetal liver or adult spleen. In cultures of erythropoietic progenitors, erythropoietin stimulates proliferation and differentiation to more mature red blood cells. Detailed molecular studies have been hampered, however, by the impurity and heterogeneity of target cell populations and the difficulty of obtaining significant quantities of the purified hormone. Highly purified erythropoietin may be useful in the treatment of various forms of anaemia, particularly in chronic renal failure. Here we describe the cloning of the human erythropoietin gene and the expression of an erythropoietin cDNA clone in a transient mammalian expression system to yield a secreted product with biological activity.

Production of transgenic rabbits, sheep and pigs by microinjection

Abstract: Direct microinjection has been used to introduce foreign DNA into a number of terminally differentiated cell types as well as embryos of several species including sea urchin, Candida elegans, Xenopus, Drosophila and mice. Various genes have been successfully introduced into mice including constructs consisting of the mouse metallothionein-I (MT) promoter/regulator region fused to either the rat or human growth hormone (hGH) structural genes. Transgenic mice harbouring such genes commonly exhibit high, metal-inducible levels of the fusion messenger RNA in several organs, substantial quantities of the foreign growth hormone in serum and enhanced growth. In addition, the gene is stably incorporated into the germ line, making the phenotype heritable. Because of the scientific importance and potential economic value of transgenic livestock containing foreign genes, we initiated studies on large animals by microinjecting the fusion gene, MT-hGH, into the pronuclei or nuclei of eggs from superovulated rabbits, sheep and pigs. We report here integration of the gene in all three species and expression of the gene in transgenic rabbits and pigs.

Plasticity of the differentiated state.

Abstract: Heterokaryons provide a model system in which to examine how tissue-specific phenotypes arise and are maintained. When muscle cells are fused with nonmuscle cells, muscle gene expression is activated in the nonmuscle cell type. Gene expression was studied either at a single cell level with monoclonal antibodies or in mass cultures at a biochemical and molecular level. In all of the nonmuscle cell types tested, including representatives of different embryonic lineages, phenotypes, and developmental stages, muscle gene expression was induced. Differences among cell types in the kinetics, frequency, and gene dosage requirements for gene expression provide clues to the underlying regulatory mechanisms. These results show that the expression of genes in the nuclei of differentiated cells is remarkably plastic and susceptible to modulation by the cytoplasm. The isolation of the genes encoding the tissue-specific trans-acting regulators responsible for muscle gene activation should now be possible.

Isolation and characterization of deletion mutants of herpes simplex virus type 1 in the gene encoding immediate-early regulatory protein ICP4.

Abstract: Using Vero cells transformed with the wild-type gene for ICP4 as the permissive host cell, we isolated herpes simplex virus type 1 (HSV-1) mutants containing deletions in both copies of the ICP4 gene The mutants, d120 and d202, contained deletions of 41 and 05 kilobases, respectively, in each copy of ICP4 ICP4 mRNA synthesized in d202-infected Vero cells was 05 kilobases smaller than that synthesized in cells infected with the wild-type virus No ICP4 mRNA was detected in d120-infected Vero cells d120 and d202 specified polypeptides that reacted with ICP4 antiserum and were smaller than the wild-type ICP4 polypeptide by 130 and 30 kilodaltons, respectively The only other HSV-1 gene products detectable on infection of Vero cells with d120 and d202 were ICP6 (of the early kinetic class of HSV-1 polypeptides), ICP0 (immediate early), ICP22 (immediate early), and ICP27 (immediate early) Immediate-early polypeptides ICP0 and ICP27 were expressed to a higher level in Vero cells infected with an ICP4 temperature-sensitive (ts) mutant (tsB32) at 39 degrees C, indicating immediate-early stimulatory activity associated with the ts ICP4 polypeptide In addition, the patterns of complementation of d120, d202, and tsB32 in ICP4-transformed cells also demonstrated inhibitory activity associated with the ts form of the ICP4 polypeptide

Expression in brain of a messenger RNA encoding a novel neuropeptide homologous to calcitonin gene-related peptide.

Abstract: As a consequence of alternative RNA processing events, a single rat gene can generate messenger RNA's (mRNA's) encoding either calcitonin or a neuropeptide referred to as alpha-type calcitonin gene-related peptide (alpha-CGRP). An mRNA product of a related gene has been identified in rat brain and thyroid encoding the protein precursor of a peptide differing from alpha-CGRP by only a single amino acid. The RNA encoding this peptide, which is referred to as beta-CGRP, appears to be the only mature transcript of the beta-CGRP gene. Hybridization histochemistry reveals a similar distribution of alpha- and beta-CGRP mRNA's, but their relative levels of expression vary in different cranial nerve nuclei. Thus beta-CGRP is a new member of a family of related genes with potential functions in regulating the transduction of sensory and motor information.

Genes regulating HLA class I antigen expression in T-B lymphoblast hybrids.

Abstract: Regulation of HLA class I and class II antigen expression was studied in hybrids of human T and B lymphoblastoid cell lines (LCL). The T-LCL CEMR.3 expresses no HLA class II antigens. It expresses little total HLA class I antigen and no HLA-B antigens. The B-LCL 721.174 is a radiation-induced variant immunoselected for loss of class II antigen expression. In addition to showing a deletion of all HLA-DR and DQ structural genes, 721.174 expresses no HLA-B antigens and a decreased level of HLA-A antigen compared with the parental cell line. A hybrid of 721.174 and CEMR.3 expresses class II antigens encoded by CEMR.3. Increased expression of HLA class I antigens encoded by both 721.174 and CEMR.3 was also observed. Specifically, the previously undetectable HLA-B5 and HLA-Bw6 antigens encoded by 721.174 and CEMR.3, respectively, were present on the hybrid. Increased expression of the HLA-A2 antigen encoded by 721.174 was also observed. An immunoselected variant of the hybrid lacking both CEMR.3-derived copies of chromosome 6 lost expression of the HLA-B5 antigen encoded by 721.174 and expressed a decreased amount of HLA-A2. From these data, we infer that two complementary trans-acting factors mediate enhanced expression of HLA class I antigens in the hybrid. One of these factors is provided by a gene located on chromosome 6, derived from CEMR.3. The second factor, introduced by 721.174, is the gene previously postulated to induce expression of CEMR.3-encoded class I antigens in hybrids of CEMR.3 with B-LCL.

The "beta-like-globin" gene domain in human erythroid cells

Abstract: We have mapped the distribution of the major and minor DNase I-hypersensitive sites in the human "beta-like-globin" gene domain. The minor DNase I-hypersensitive sites map close to the 5' end of each of the beta-like-globin genes. Their presence is specifically associated with the transcription of the immediate downstream beta-like-globin genes. The major DNase I-hypersensitive sites map in what appear to be the 5' and 3' boundary areas of the human beta-like-globin gene domain, a region estimated to span at least 90 kilobases of DNA. These major sites are present in various erythroid cells, which express predominantly either the embryonic, the fetal, or the adult beta-like-globin genes, and seem to be involved in defining the active beta-like-globin genes domain in cells of erythroid lineage. The four major DNase I-hypersensitive sites in the 5' boundary area, when correlated with sequencing data, are shown to be located in DNA regions containing enhancer core-like sequences and alternating purine and pyrimidine bases.

Light Regulation of Gene Expression in Higher Plants

Abstract: In this review areas of currently active research are considered which have demonstrated that a plant's response to light involves changes in the expression of specific genes at the level of RNA. The regulation of gene expression by phytochrome and the UV-sensitive photoreceptor have been studied most extensively at the molecular level, and this review particularly focuses on such studies in higher plants. Some of the observations made on the differences in gene expression between light-grown and dark-grown plants are also included, although the photoreceptor(s) responsible for the differences may not have been ascertained. In some of these cases, phytochrome involvement has been or may be demonstrated in later studies, while in others the observed differences may be a result of the action of other photoreceptors or of multiple light-affected processes. One such process is the development of chloroplasts, a major developmental step triggered by light in angiosperms. In addition, many of the genes whose expression is changed by light and which have been studied at a molecular level encode chloroplast proteins. 156 references.

Herpes simplex virus type 1 ICP27 is an essential regulatory protein.

Abstract: The five immediate-early genes of herpes simplex virus are expressed during the initial stages of the infectious cycle, and certain immediate-early proteins have been shown to play a regulatory role in subsequent viral gene expression. Until recently, the functional properties of only one immediate-early protein, ICP4, had been examined in any detail, primarily because mutants had been isolated only in the gene for ICP4. We report herein the genetic and phenotypic characterization of four temperature-sensitive mutants of herpes simplex virus type 1 (tsY46, tsE5, tsE6, and tsLG4) that have begun to elucidate the function(s) of a second immediate-early protein, ICP27. The four mutants complemented each other inefficiently or not at all, indicating that they are defective in the same function. Marker rescue tests placed the mutations in tsY46 and tsE5 in sequences that encode the transcript for ICP27; the mutations in tsE6 and tsLG4 lie in or near these sequences. The ability of wild-type ICP27 expressed from a cloned gene to complement tsY46 and tsLG4 constitutes additional evidence that these mutants are defective in an ICP27-associated function. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of mutant-infected cell polypeptides showed that certain immediate-early (alpha) polypeptides were overproduced, whereas significant levels of early (beta) and drastically reduced levels of several late (gamma) proteins were synthesized at the nonpermissive temperature. Interestingly, the mutants were observed to form a spectrum with regard to their relative abilities to induce the expression of a number of polypeptides, especially those of the delayed-early (beta gamma) class. Consistent with their ability to induce expression of early polypeptides, all of the mutants induced the synthesis of substantial levels of viral DNA at the nonpermissive temperature. Taken together, the results of these studies demonstrate that ICP27 plays an essential regulatory function in virus replication, that this function is required after the onset of early gene expression and viral DNA synthesis, and that the inability of the mutants to induce the synthesis of late proteins is independent of viral DNA synthesis.

Structure and expression of the human gene encoding major heat shock protein HSP70.

Abstract: We have cloned a human gene encoding the 70,000-dalton heat shock protein (HSP70) from a human genomic library, using the Drosophila HSP70 gene as a heterologous hybridization probe. The human recombinant clone hybridized to a 2.6-kilobase polyadenylated mRNA from HeLa cells exposed to 43 degrees C for 2 h. The 2.6-kilobase mRNA was shown to direct the translation in vitro of a 70,000-dalton protein similar in electrophoretic mobility to the HSP70 synthesized in vivo. From the analysis of S1 nuclease-resistant mRNA-DNA hybrids, the HSP70 gene appears to be transcribed as an uninterrupted mRNA of 2.3 kilobases. We show that the cloned HSP70 gene contains the sequences necessary for heat shock-induced expression by two criteria. First, hamster cells transfected with a subclone containing the HSP70 gene and flanking sequences synthesized a HSP70-like protein upon heat shock. Second, human cells transfected with a chimeric gene containing the 5' flanking sequences of the HSP70 gene and the coding sequences of the bacterial chloramphenicol acetyltransferase gene transcribed the chimeric gene upon heat shock. We show that the HSP70 mRNA transcribed in an adenovirus 5 transformed human cell line (293 cells) is identical to the HSP70 mRNA induced by heat shock.

Naturally occurring poly(dA-dT) sequences are upstream promoter elements for constitutive transcription in yeast

Abstract: pet56, his3, and ded1 are adjacent but unrelated genes located on chromosome XV of the yeast Saccharomyces cerevisiae. his3 and pet56 are transcribed in opposite directions from initiation sites separated by approximately equal to 200 base pairs. Under normal growth conditions, both genes are transcribed at a similar basal level. Deletion analysis of the his3 gene indicates that the upstream promoter element for constitutive expression is defined by a 17-base-pair region that contains 15 thymidine residues in the coding strand. Sequential deletions of the pet56 gene indicate that this same region is required for wild-type transcription levels. Thus, this poly(dA-dT) sequence acts bidirectionally to activate transcription of two unrelated genes. Transcription of the ded1 gene is initiated approximately equal to 300 base pairs downstream from the his3 gene, and it occurs at a 5-fold higher level. This gene contains a 34-base-pair region containing 28 thymidine residues in the coding strand located upstream from the ded1 TATA box. Deletion of this dA-dT stretch significantly reduces transcription below the wild-type level. Thus, for at least three different yeast genes, naturally occurring stretches of poly(dA-dT) serve as upstream promoter elements for constitutive expression. In addition, it appears that longer stretches of poly(dA-dT) are more effective upstream promoter elements. These transcriptional effects may be due to exclusion of nucleosomes from poly(dA-dT) regions.

Major deletions in the gene encoding the p53 tumor antigen cause lack of p53 expression in HL-60 cells

Abstract: The tumor antigen p53 is overproduced in transformed cells of various species, including man. HL-60 is an exceptional human tumor cell line that does not express this protein. Hybridization of polyadenylylated mRNA of these cells with a human p53 cDNA probe (p53-H14), which we cloned, had indicated a total absence of the mature-size (3.0 kilobases) or any aberrant p53 mRNA species. Analysis of the genomic HL-60 DNA indicated that the p53 gene in these cells was significantly altered. Most of the gene was deleted, and the residual p53 sequences of these cells, which show weak homology, mapped to the corresponding 5' region of the p53 gene. In agreement with previously documented results, we found that HL-60 cells have an amplified c-myc gene. We suggest that the deficiency of the p53 protein in HL-60 cells could have been overcome by using an alternative metabolic pathway. The c-myc product is a candidate for such an alternative protein.

Pretranslational modulation of acute phase hepatic protein synthesis by murine recombinant interleukin 1 (IL-1) and purified human IL-1.

Abstract: During the acute phase response to tissue injury or inflammation, the concentration of several plasma proteins change. Previous work (29-34) suggested a role for interleukin 1 (IL-1) in the acute phase response. The availability of recombinant-generated mouse IL-1 prompted a study designed to directly test the function of IL-1 and its mechanism of action on hepatic synthesis of two positive acute phase proteins (serum amyloid A [SAA] and complement factor B), and a negative acute phase reactant (albumin). Intravenous injection of purified recombinant-generated murine-IL-1 into C3H/HeJ endotoxin-resistant mice induced a dose-dependent increase in SAA-specific hepatic messenger RNA (mRNA), and an increase in SAA plasma protein concentration. In primary murine hepatocyte cultures, both the recombinant IL-1 and highly purified human IL-1 induced a dose- and time-dependent, reversible increase in expression of the SAA and factor B genes, and a decrease in albumin gene expression. This regulation is pretranslational, since the kinetics and direction of change in specific mRNA for SAA, factor B, and albumin correspond to the changes in synthesis of the respective proteins. Moreover, the effect of IL-1 was specific, since actin gene expression was unaffected, and the IL-1 response was inhibited by antibody specific for IL-1. These data provide direct evidence that a single mediator, IL-1, can effect the positive and negative changes in specific hepatic gene expression characteristic of the acute phase response.

Levels of c-myc oncogene mRNA are invariant throughout the cell cycle

Abstract: The steady-state messenger RNA levels of several genes increase when cells are stimulated to proliferate. The transcripts from one such gene, the proto-oncogene c-myc, increase approximately 20-fold shortly after cells are stimulated to proliferate and then decline before the onset of DNA synthesis. It has been inferred from these data that expression of c-myc may be specific to the G1 portion of the cell cycle. Alternatively, this transient increase in c-myc mRNA following the stimulation of quiescent cells could be the result of an activational event that renders the cells competent to enter the cell cycle. To distinguish between these possibilities, we performed experiments to determine whether the amount of c-myc mRNA fluctuates during the cell cycle in cells that are under constant stimulation to proliferate. Although c-myc mRNA does undergo a transient increase within 2 h of serum stimulation of quiescent serum-deprived cells, our results show that the level of c-myc mRNA is constant throughout the cell cycle and does not diminish in density-arrested cells maintained in the presence of serum growth factors. In contrast to c-myc, the mRNA levels of two other genes whose expression has been associated with cellular proliferation do show consistent variations within the cell cycle. Both thymidine kinase (TK) and histone 2b (H2b) mRNA levels increase during S phase in continuously growing cells and decrease when cell replication ceases in density-arrested cultures. Therefore, the transient increase in c-myc transcription following the activation of quiescent cells is not due to the type of cell cycle-dependent regulation characteristic of the TK and H2b genes.

Cloning, sequence, and expression of a human granulocyte/macrophage colony-stimulating factor.

Abstract: Human granulocyte/macrophage colony-stimulating factor (GM-CSF) is a glycoprotein that is essential for the in vitro proliferation and differentiation of precursor cells into mature granulocytes and macrophages. In this report we have used a mouse GM-CSF cDNA clone to isolate human GM-CSF clones from libraries made from HUT-102 messenger RNA and mitogen-stimulated T-lymphocyte messenger RNA. The human cDNA clones contained a single open-reading frame encoding a protein of 144 amino acids with a predicted molecular mass of 16,293 daltons and showed 69% nucleotide homology and 54% amino acid homology to mouse GM-CSF. One of these cDNA clones was shown to direct the synthesis of biologically active GM-CSF using a yeast expression system. The gene for human GM-CSF appears to exist as a single-copy gene.

Identification of multiple metal regulatory elements in mouse metallothionein-I promoter by assaying synthetic sequences.

Abstract: Metallothionein genes are transcriptionally regulated by a number of inducers including heavy metals. Previous mutational analyses of the mouse metallothionein-I gene (mMTI) promoter have delineated a heavy-metal regulatory region between -60 and -42 relative to the transcription start site. A synthetic copy of a 12-base-pair (bp) conserved sequence located within this region was subsequently shown to confer heavy-metal regulation on a heterologous gene. However, specific disruption of this metal regulatory element (MRE) within a wild-type mMTI promoter reduced but did not eliminate the heavy-metal response. The additional metal regulatory activity was localized to an upstream region containing four sequences homologous to the identified MRE. Similar sequences were also found in multiple copies in metallothionein genes from other species. Here we test synthetic copies of all five mMTI MRE homologues for metal regulatory activity. At least four of these sequences are able to confer heavy-metal regulation on a heterologous promoter.

Close link between reduction of c-myc expression by interferon and, G0/G1 arrest.

Abstract: It has recently been reported that c-myc is an inducible gene, regulated directly by growth signals which promote proliferation and expressed in a cell-cycle dependent manner Because various leukaemic cell lines express high levels of c-myc messenger RNA, it was of interest to discover whether the gene could be down-regulated in these cells by a growth inhibitor such as interferon (IFN) We show here that in Daudi Burkitt's lymphoma cells, IFN-alpha produces a five- to sevenfold reduction in c-myc mRNA through a decreased rate of c-myc gene transcription By isolating a growth-resistant Daudi cell variant that had escaped from this down-regulation, we provide the first clear link between reduction of c-myc mRNA and the IFN-mediated G0/G1 arrest characteristic of Daudi cells Furthermore, by screening other cell lines, we demonstrate the heterogeneity of human leukaemic cells with respect to these criteria Thus, IFN-alpha fails to reduce the c-myc mRNA and to change the cell-cycle distribution in HL-60 and U937 cells, although normal induction of other IFN-regulated activities takes place The latter group of cells shows a decline in c-myc gene expression when they become arrested in the G0/G1 phase as part of their terminal differentiation

Evidence for a direct role for both the 175,000- and 110,000-molecular-weight immediate-early proteins of herpes simplex virus in the transactivation of delayed-early promoters.

Abstract: We reconstructed the regulated induction of delayed-early (DE) transcription that occurs during herpes simplex virus (HSV) infection by using a transient expression system in which recombinant target genes were cotransfected into Vero cells together with intact activating genes. Plasmids containing cloned HSV-1 or HSV-2 immediate-early (IE) genes stimulated by up to 100-fold the expression from recombinant constructs containing the bacterial chloramphenicol acetyltransferase (CAT) gene under the control of the DE promoter/regulatory region from the genes for an HSV-2 38,000-molecular-weight (38K) protein and the HSV-1 thymidine kinase. This activation was specific to hybrid genes containing DE regulatory regions since no significant increases in expression were observed in cotransfection experiments with the CAT gene without any promoter region or under the control of a number of other regulatory regions, including an HSV-1 IE regulatory region, the complete or enhancerless early regulatory region of simian virus 40, and an inducible cellular promoter/regulatory region. By using a variety of cotransfected plasmids containing individual or different combinations of HSV-1 or HSV-2 IE genes, we show that of the five known IE genes, two, those coding for the 175K and 110K polypeptides, each possessed the ability to stimulate expression from both DE promoters. Cleavage of the input plasmids within the known coding regions for the 175K and 110K proteins abolished stimulation of DE/CAT gene expression, whereas cleavage outside the coding regions had no effect on stimulation. We conclude that stimulation of CAT expression occurred exclusively by a transactivation mechanism in which the products encoded by these IE genes acted on the DE hybrid constructs at the transcription level. No transcriptional stimulatory function was demonstrated for the IE 68K and 63K proteins, although our results indicate that the IE 12K protein may augment the DE stimulatory activity of the 175K and 110K proteins.

A Transcriptional Activator Protein Encoded by the X-lor Region of the Human T-Cell Leukemia Virus

Abstract: Human T-cell leukemia viruses type I and II (HTLV-I and -II) exhibit several features characteristic of this retroviral family: the presence of an x-lor gene encoding a nuclear protein, transformation properties suggesting the involvement of a virus-associated trans-acting factor, and transcriptional trans-activation of the long terminal repeat (LTR) in infected cells. In the study described here the HTL x-lor products, in the absence of other viral proteins, were able to activate gene expression in trans directed by HTLV LTR. The regulation of the expression of particular genes in trans by HTLV x-lor products suggests that they play a role in viral replication and possibly in transformation of T lymphocytes.

Induction of the proto-oncogene fos by nerve growth factor

Abstract: Nerve growth factor (NGF) causes the differentiation of PC12 cells to sympathetic neuron-like cells and also induces a rapid but transient expression of fos mRNA and protein. fos mRNA transcripts can be detected 5 min after the addition of NGF, are maximally abundant after 30 min, and then their levels decrease. fos protein synthesis parallels the expression of fos mRNA, and the induced fos proteins are located in the nucleus. cAMP, epidermal growth factor, the phorbol ester phorbol 12-myristate 13-acetate, and K+ depolarization also induce the fos gene. Growth of PC12 cells in the presence of dexamethasone, which induces differentiation into chromaffin-like cells, is not accompanied by fos expression. We propose that while fos gene induction is associated with the differentiation of PC12 cells to sympathetic nerve, its enhanced expression is primarily involved in the anabolic responses induced by NGF and many growth factors.

Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature

Abstract: We have investigated the developmental transitions of myosin heavy chain (MHC) gene expression in the rat extraocular musculature (EOM) at the mRNA level using S1-nuclease mapping techniques and at the protein level by polypeptide mapping and immunochemistry. We have isolated a genomic clone, designated lambda 10B3, corresponding to an MHC gene which is expressed in the EOM fibers (recti and oblique muscles) of the adult rat but not in hind limb muscles. Using cDNA and genomic probes for MHC genes expressed in skeletal (embryonic, neonatal, fast oxidative, fast glycolytic, and slow/cardiac beta-MHC), cardiac (alpha-MHC), and EOM (lambda 10B3) muscles, we demonstrate the concomitant expression at the mRNA level of at least six different MHC genes in adult EOM. Protein and immunochemical analyses confirm the presence of at least four different MHC types in EOM. Immunocytochemistry demonstrates that different myosin isozymes tend to segregate into individual myofibers, although some fibers seem to contain more than one MHC type. The results also show that the EOM fibers exhibit multiple patterns of MHC gene regulation. One set of fibers undergoes a sequence of isoform transitions similar to the one described for limb skeletal muscles, whereas other EOM myofiber populations arrest the MHC transition at the embryonic, neonatal/adult, or adult EOM-specific stage. Thus, the MHC gene family is not under the control of a strict developmental clock, but the individual genes can modify their expression by tissue-specific and/or environmental factors.

Expression of T-cell antigen receptor genes during fetal development in the thymus

Abstract: The T-cell antigen receptor is a heterodimeric molecule composed of alpha- and beta-subunits of relative molecular mass 40,000-50,000 (refs 1-6). Recently, the genes encoding both the beta- and alpha- chains have been cloned. By comparing amino-acid and nucleic-acid sequences, it is clear that these genes encode the alpha- and beta-proteins of the T-cell receptor. In addition, a third receptor-like gene, the gamma-chain gene, has been identified, which has many structural and sequence characteristics in common with the alpha- and beta-chain genes. The role of the gamma-chain gene in T-cell development is unknown. We have reported recently that the beta-chain genes are transcriptionally turned on in the thymus at about day 17 of fetal development. Here we report that the alpha-chain also is transcriptionally activated during this time, but that the gamma-chain gene is active in the thymus at day 14, reaches a peak steady-state level at day 15 and rapidly declines thereafter. If the gamma-chain gene has a functional role, it would seem to be involved very early in T-cell development, before the mature T-cell receptor is expressed.