Control of specific gene expression in higher organisms. Expression of mammalian genes may be controlled by repressors acting on the translation of messenger RNA

The activities of uroporphyrinogen synthetase and cosynthetase in congenital erythropoietic porphyria (CEP)

Normal or increased amounts of series III porphyrins with greater amounts of series I were observed on incubation of PBG in hemolysates of congenital erythropoietic porphyria vs. normal erythrocytes, human or bovine. Correlation with reticulocyte percentage was poor, in the aggregate a general trend toward increased values of both isomers I and III was noted with increasing reticulocytes. When the percent of type III was low the net amount was increased as compared with normal. Hemolysates of non-porphyric, reticulocyte-rich red cells (hemolytic or posthemorrhagic anemia) formed only minute amounts of type I porphyrin but at the same time no more, or even less type III than the porphyric hemolysates, although representing red cells of greater reticulocyte content. No evidence of deficient heme synthesis was observed in porphyric hemolysates incubayed with [14C]-porphobilinogen or 59Fe. Other studies of porphyric hemolysates incubated with and without added mouse spleen synthetase failed to reveal evidence of an absolute UPG-III cosynthetase (Co-S) deficiency. The large increases of type I porphyrin with normal or increased formation of type III, both in the disease and in the hemolysates, are believed due to a primary increase of ALA-S or UPG-S activity rather than a decrease of Co-S. Possible mutations which might be responsible for this increase are considered.

Turnover of polyadenylated messenger RNA in fission yeast. Evidence for the control of protein synthesis at the translational level

Polyadenylated RNA was isolated from fission yeast (Schizosaccharomyces pombe) total RNA using oligo(dT)-cellulose, and was studied as a model for messenger RNA. The half-life of poly adenylated RNA was measured by two independent methods. (a) The rate of labelling of polyadenylated RNA during incubation of cells with [5-3H]uridine was measured. A half-life of 40-45 min was found by comparing the experimental data with theoretical curves calculated for labelling of RNAs with various half-lives. The influence of precursor-pool specific activity on RNA labelling kinetics is considered. (b) Cells were labelled with [5-3H]uridine then further RNA synthesis was inhibited by addition of 8-hydroxyquinoline. The rate of loos of radioactivity from polyadenylated RNA indicated a half-life of 50 min. The half-life found by these two methods is about one-third of the cell doubling time, and is much longer than previous estimates by indirect methods of yeast messenger RNA half-life. Both experimental methods provided evidence for the existence of tas a half-life of 40-50 min; a much smaller population is probably turning over more rapidly. After inhibition of RNA synthesis by 8-hydroxyquinoline, the rate of total protein synthesis declined much more rapidly than the polyadenylated RNA content of the cells. However, 60 min after inhibition of RNA synthesis there was a small rise in the rate of portein synthesis. These data are interpreted as evidence for mechanisms controlling protein synthesis which operate at the level of messenger RNA translation.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

Functional level of rat liver tryptophan 2,3-dixoygenase messenger RNA during superinduction of enzyme with actinomycin D

Tryptophan 2,3-dioxygenase [EC 1.13.11.11; L-tryptophan:oxygen 2,3-oxidoreductase (decyclizing)] activity is induced by glucocorticoid hormones and superinduced by actinomycin D. Previous experiments had shown that hormonal induction of the enzyme activity is accompanied by parallel increases in tryptophan 2,3-dioxygenase mRNA level. In this study, we measured the tryptophan 2,3-dioxygenase mRNA levels during superinduction as well as hormonal induction, to determine whether superinduction of the enzyme activity is also mediated through changes in mRNA concentration. Tryptophan 2,3-dioxygenase mRNA was measured in a Krebs ascites cell-free protein synthesizing system supplemented with rabbit reticulocyte initiation factors. We found that during superinduction of the enzyme activity by actinomycin D, the mRNA level is identical to that of the actinomycin D-free controls. Our results do not, therefore, support the hypothesis that hormonal induction and/or superinduction of tryptophan 2,3-dioxygenase mRNA are regulated by a rapidly turning over repressor.

The effects of metabolic inhibitors on the synthesis of inducible tyrosine aminotransferase in cultured hepatoma cells

The effects of actinomycin-D and 3'-deoxyadenosine (cordycepin) on the steroid-mediated induction of tyrosine aminotransferase (TAT) synthesis have been reexamined in view of recent reports that the primary inhibitory action of these compounds may affect synthesis of proteins as well as RNA. The present results confirm that cordycepin blocks the steroid-mediated induction of TAT in rat hepatoma cells (HTC), but unlike actinomycin-D, cordycepin neither increases nor maintains the levels of TAT found in HTC cells preinduced with dexamethasone. Indeed, cordycepin added to preinduced cells, either in the presence or absence of steroid, causes a prompt decline in TAT activity. These data also confirm that both actinomycin-D and cordycepin have an early inhibitory effect on protein synthesis, but the cordycepin effect is observed sooner and the extent of inhibition is greater. When actinomycin-D and cordycepin are added simultaneously to preinduced cells with the steroid removed, the actinomycin-td produced maintenance of preinduced levels of TAT persists. Also, the inhibition of protein synthesis in cultures with both inhibitors approaches that for the cells treated with actinomycin-D alone instead of cordycepin alone. These data suggest that cordycepin inhibits TAT synthesis in preinduced cells by its inhibition of protein synthesis, and this inhibitory effect of cordycepin is blocked by actinomycin-D. It is possible that actinomycin-D does this by preventing the incorporation of cordycepin into RNA. However, regardless of the correctness of this speculation, the multiple effects of cordycepin indicate that this inhibitor cannot be used either to prove or rule out the post-transcriptional model for regulation of gene expression. Also, this requirement that protein synthesis must continue in order to maintain pre-induced levels of TAT is inconsistent with the assumption that the maintenance of these induced TAT levels by actinomycin-D is due to inhibition of TAT degradation.

Some aspects of the mechanism of action of steroid hormones

Events related to the very early steps of steroid hormone action are reviewed, after a brief exposé of the methods used for the measurement and calculation of protein-steroid interactions. A list of steroid binding plasma proteins is given, and their physiological variations and possible role(s) are discussed. The mechanism of entry of steroids into target cells is then considered especially the possibility of a protein mediated step. The metabolism of steroid hormone in target cells and its physiological significance are discussed, especially with reference to androgens in prostate, levator ani muscle and hypothalamus. No receptor of steroid hormone has yet been discovered if defined as the last entity with which the active steroid interacts before action is initiated. However, in the target tissues of steroid hormones, specific proteins have been detected (termed receptors) with properties that are compatible with an essential if not an obligatory role in steroid action. The properties ascribed to the cytosol receptors of different steroid hormones in various tissues, their binding properties, and their quantitative variations are reported. The transformation or activation of receptor after binding of the hormone is related to its transfer to the nucleus. The question posed by the recent observations on Non Histone Chromatin estrogen binding protein and of the "insoluble" nuclear receptor are discussed. Finally, a series of experiments are reviewed, dealing more specifically with the increase of protein and RNA synthesis in tissues in which steroids and RNA synthesis in tissues in which steroids promote growth (i.e. estradiol in uterus). The concept of a cascade phenomenon is reported, including the early synthesis of KIP (Key Intermediary Protein(s)) which would responsible for the secondary amplification of the response. It is discussed in connection with the present (and insufficient) knowledge of receptors and transcription machinery.

Blood group antigens A and B in ameloblastomas, odontogenic keratocysts and non-keratinizing cysts

The expression of blood group antigens A and B has been studied in 8 ameloblastomas, 16 odontogenic keratocysts from patients with basal cell nevus syndrome, 11 odontogenic keratocysts from patients without the syndrome, and 12 non-keratinizing odontogenic cysts, using a double layer immunofluorescence staining technique. The amount of antigen in the lesions was compared with the content of antigen in normal buccal mucosa from each patient. All ameloblastomas reacted negatively, three cysts from the patients with the basal cell nevus syndrome reacted negatively, and the odontogenic keratocysts from patients without the syndrome as well as the non-keratinizing odontogenic cysts all gave a positive reaction.

Different sensitivities of avian- and mammalian-haemoglobin synthesis to elevated temperatures

Avian- and mammalian-haemoglobin synthesis show different sensitivities to elevated temperatures. Temperature-dependent, reversible polyribosome disaggregation in avian cells occurs only at 45 degrees C, which is 3 degrees higher than the temperature for mammalian cells, and seems to be due to a block in the initiation of new polypeptide chains. The implications of these findings are discussed.

Control of the mRNA for hepatic tryptophan oxygenase during hormonal and substrate induction

Glucocorticoid hormones increase the level of hepatic tryptophan oxygenase (EC 1.13.11.11; L-tryptophan:oxygen 2,3-oxidoreductase (decyclizing) by increasing its rate of synthesis. Studies were performed to determine whether this induction is mediated by controlling the level of the mRNA for tryptophan oxygenase of by changing the translational efficiency of a fixed level of mRNA. Activity of tryptophan oxygenase mRNA was quantitated in a Krebs ascites cell-free, protein-synthesizing system, supplemented with tRNA and rabbit reticulocyte initiation factors. De novo synthesis of the protomeric unit(s) of the enzyme was a linear function of the amount of mRNA added. Time course and dose-response studies in which the enzyme level and mRNA activity in livers from rats injected with inducing doses of hydrocortisone were compared indicate that the induction of this enzyme is accompanied by a proportional increase in the level of its mRNA. This was true for mRNA isolated from total liver as well as from cytoplasmic polysomes. Induction of the enzyme by its substrate, tryptophan, however, was not accompanied by a parallel increase in mRNA activity.

Interferon induction: tool for establishing interactions among homopolyribonucleotides

Hitherto unrecognized interactions between homopolyribonucleotides and complexes thereof are suggested by interferon induction data obtained in a highly sensitive assay system of primary rabbit kidney cell cultures superinduced by metabolic inhibitors.

Mechanism of stimulation of murine type-C RNA tumor virus production by glucocorticoids: post-transcriptional effects

We have previously shown that dexamethasone stimulates production of type-C virus from seemingly normal murine fibroblasts (BALB/3T3) and from transformed (Kirsten sarcoma-leukemia virus) nonproducing cells (BALB/K3T3) induced by 5-iododeoxyuridine. In this report, we further examine the mechanism of this effect by using BALB/K3T3 cells. Several observations suggest that this effect is post-transcriptional. The optimal stimulation by dexamethasone is obtained when dexamethasone is given 24 to 48 h after 5-iododeoxyuridine induction. Although this effect is late, time course experiments suggest that dexamethasone does not act to promote release of preformed virions. The stimulation by dexamethasone is blocked when cells are treated with cordycepin (3'-deoxyadenosine) during the first 24 h of induction, but not when cordycepin is added later. Conversely, interferon, which inhibits virus production, interferes with dexamethasone when it is added late or after removal of the steroid. The results of molecular hybridization experiments show that there is no detectable increase in Kirsten sarcoma-leukemia virus-specific RNA in dexamethasone-treated cells (with or without 5-iododeoxyuridine). The results of the time course studies, and the cordycepin, interferon, and hybridization experiments, suggest that the effect of dexamethasone on type-C virus production in this system is post-transcriptional.

Growth control of differentiated fetal rat hepatocytes in primary monolayer culture. VII. Hormonal control of DNA synthesis and its possible significance to the problem of liver regeneration

The initiation of DNA synthesis has been studied in quiescent primary cultures of fetal rat hepatocytes using defined hormones and chemically defined medium. Preparations of crystalline insulin (0.01-10 microg/ml) or 20,000-fold purified somatomedin (0.01-1 microg/ml) are stimulatory. Growth hormone (0.025 microg/ml) and hydroxycortisone (0.025 microg/ml), 3':5'-GMP! (10(-5) M) fail by themselves to initiate DNA synthesis but added together with insulin, enhance the stimulatory response by 50-100%. Thyroid hormones (L-T(3), L-T(4), 7.5-30 ng/ml) are by themselves without effect, but when they are added to thyroid hormone-depleted serum, the reconstituted mixtures show an enhanced capacity to initiate DNA synthesis. In contrast, glucagon (0.01 microg/ml) inhibits the insulin-stimulated response by about 50% without reducing basal DNA synthesis rates. The results described here and in the accompanying two reports indicate that environmental control over the initiation of DNA synthesis is complex, and can involve the participation of many factors. The in vitro findings are consistent with the concept that liver regeneration is hormonally controlled and raise the possibility that alterations of the intrahepatic ratio of insulin to glucagon are growth regulatory.

Stimulation of polyphenol oxidase (monophenolase) activity in wheat endosperm by gibberellic acid, cycloheximide and actinomycin D

Embryoless wheat (Triticum aestivum L.) half-seeds on incubation with gibberellic acid (GA3) showed a 2- to 2.5fold stimulation of monophenolase activity. The enzyme activity was not released into the incubation medium in GA3-treated half-seeds. The effect of GA3 was counteracted by the addition of abscisic acid (ABA) to the half-seeds. Adenosine-3',5'-cyclic monophosphate and its structural analogues were ineffective in increasing the monophenolase activity. Actinomycin D and cycloheximide showed no inhibitory effecton the monophenolase activity in controls as well as in GA3-treated half-seeds, but on the contrary caused a 2- to 3fold stimulation of enzyme activity similar to that observed in endosperm treated with GA3 alone. However, there was no additive or synergistic enhancement of monophenolase activity when GA3 was tested in combination with cycloheximide or actinomycin D. GA3- or cyclic AMP-treated half-seeds showed no stimulation of o-diphenolase activity.