Regulation of protein synthesis in mammalian cells. V. Further studies on the effect of actinomycin D on translation control in HeLa cells

Complexes of vesicular stomatitis virus matrix protein with host Rae1 and Nup98 involved in inhibition of host transcription

Vesicular stomatitis virus (VSV) suppresses antiviral responses in infected cells by inhibiting host gene expression at multiple levels, including transcription, nuclear cytoplasmic transport, and translation. The inhibition of host gene expression is due to the activity of the viral matrix (M) protein. Previous studies have shown that M protein interacts with host proteins Rae1 and Nup98 that have been implicated in regulating nuclear-cytoplasmic transport. However, Rae1 function is not essential for host mRNA transport, raising the question of how interaction of a viral protein with a host protein that is not essential for gene expression causes a global inhibition at multiple levels. We tested the hypothesis that there may be multiple M protein-Rae1 complexes involved in inhibiting host gene expression at multiple levels. Using size exclusion chromatography and sedimentation velocity analysis, it was determined that Rae1 exists in high, intermediate, and low molecular weight complexes. The intermediate molecular weight complexes containing Nup98 interacted most efficiently with M protein. The low molecular weight form also interacted with M protein in cells that overexpress Rae1 or cells in which Nup98 expression was silenced. Silencing Rae1 expression had little if any effect on nuclear accumulation of host mRNA in VSV-infected cells, nor did it affect VSV's ability to inhibit host translation. Instead, silencing Rae1 expression reduced the ability of VSV to inhibit host transcription. M protein interacted efficiently with Rae1-Nup98 complexes associated with the chromatin fraction of host nuclei, consistent with an effect on host transcription. These results support the idea that M protein-Rae1 complexes serve as platforms to promote the interaction of M protein with other factors involved in host transcription. They also support the idea that Rae1-Nup98 complexes play a previously under-appreciated role in regulation of transcription.

All viruses have mechanisms to suppress or evade host antiviral responses. These mechanisms are critical for viral pathogenicity. Vesicular stomatitis virus (VSV) suppresses antiviral responses by global inhibition of host gene expression mediated by the viral matrix (M) protein. M protein interacts with the host protein Rae1 in a complex with the nucleoporin Nup98. It had been thought that interaction of M protein with Rae1 blocks nuclear-cytoplasmic mRNA transport. However, other data show that Rae1 is not essential for mRNA transport. With this discrepancy in mind, we re-examined the interaction of M protein with Rae1 and Nup98 and the level of host gene expression in which they are involved. A key result was that silencing Rae1 expression did not affect host gene expression, but instead increased cellular resistance to inhibition by M protein. Furthermore, silencing Rae1 expression primarily affected the inhibition of host transcription with no significant effect on nuclear accumulation of mRNA. These results support a model in which Rae1 serves as a “platform” to promote interaction of M protein with cellular targets involved in host transcription. This illustrates a general principle that viral proteins can have multiple cellular effects by interacting with host proteins that are themselves multi-functional.

Plaque Formation by Rickettsia conori in WI-38, DBS-FRhL-2, L-929, HeLa, and Chicken Embryo Cells

Mammalian cells particularly suitable for the study of specialized aspects of rickettsial biology were tested for their ability to support plaque formation by Rickettsia conori. The detection of plaques was substantially influenced by the combination of growth medium and cell type used. Large plaques (2.0 to 3.0 mm in diameter) occurred by 8 days postinfection in WI-38 and DBS-FRhL-2 cells supported by medium 199. Smaller plaques (0.5 to 1.0 mm in diameter) were seen in L-929 and HeLa cells at 8 to 11 days postinfection and were more discernible in cells supported with Eagle minimal essential medium. Chicken embryo cells maintained in Dulbecco's modified Eagle medium exhibited large spherical plaques with a diameter of approximately 1.5 mm by 8 days postinfection.

New mRNAs are preferentially translated during vesicular stomatitis virus infection

During vesicular stomatitis virus (VSV) infection, host protein synthesis is inhibited, while synthesis of viral proteins increases. VSV infection causes inhibition of host transcription and RNA transport. Therefore, most host mRNAs in the cytoplasm of infected cells were synthesized before infection. However, viral mRNAs are synthesized throughout infection and are newer than preexisting host mRNAs. To determine if the timing of appearance of mRNAs in the cytoplasm affected their translation during VSV infection, we transfected reporter mRNAs into cells at various times relative to the time of infection and measured their rate of translation in mock- and VSV-infected cells. We found that translation of mRNAs transfected during infection was not inhibited but that translation of mRNAs transfected prior to infection was inhibited during VSV infection. Based on these data, we conclude that the timing of viral mRNA appearance in the cytoplasm is responsible, at least in part, for the preferential translation of VSV mRNAs. A time course measuring translation efficiencies of viral and host mRNAs showed that the translation efficiencies of viral mRNAs increased between 4 and 8 h postinfection, while translation efficiencies of host mRNAs decreased. The increased translation efficiency of viral mRNAs occurred in cells infected with an M protein mutant virus that is defective in host shutoff, demonstrating that the enhanced translation of viral mRNA is genetically separable from inhibition of translation of host mRNA.

In mouse myoblasts nuclear prosomes are associated with the nuclear matrix and accumulate preferentially in the perinucleolar areas

Prosomes are the core of 26S proteasomes, although they were originally observed as 20S particles associated with cytoplasmic mRNPs. Here we show for the first time that prosomes are also genuine constituents of the nuclear matrix, chromatin and the nuclear RNP networks. Using mouse myoblasts we tested three monoclonal antibodies recognising the prosomal subunits p23K, p27K and p30K, and found that the corresponding prosome subclasses are characterised by a variable distribution pattern within the nuclei. Their presence on the nuclear matrix, and most abundantly in the perinucleolar area, is of particular importance. When myoblasts fuse into myotubes, the distribution pattern of certain types of prosomes on the nuclear matrix changes drastically. Surprisingly, DNA strongly interferes with the detection of prosomal antigens by immunofluorescence methods, whereas RNA, histones and other proteins soluble in 2 M NaCl have no such effect. This ‘masking’ of prosomes can be completely overcome by extensive or even mild digestion with DNase I or restriction enzymes. Many nuclear prosomes can be solubilized by combined treatment with 0.5% Triton X-100 and 2 M NaCl, and others can be released by digestion of DNA and/or RNA, and about 10–20% of nuclear prosomes remain tightly bound to the protein-based nuclear matrix.

Translational control during heat shock

Regulation of translation during heat shock of Drosophila and mammalian cells is reviewed. Protein synthesis is severely inhibited by elevated temperatures but synthesis of heat shock proteins (HSPs) is resistant to this inhibition. The primary site of regulation is polypeptide chain initiation. The activities of two initiation factors, eIF-2 and eIF-4F, are modulated during heat shock. A protein kinase which modulates eIF-2 activity appears to be associated with heat shock proteins (HSPs). Evidence is emerging that HSP70 acts as a heat sensor by detecting the presence of accumulating denatured proteins. In the rabbit reticulocyte lysate denatured proteins bind HSP70 releasing an eIF-2 kinase to shut down protein synthesis. It appears highly likely that a similar mechanism is acting in heat shocked cells. Cell-free protein synthesizing systems prepared from heat shocked cells are deficient in eIF-4F. Modulation of eIF-4F can explain in part the apparent preferential translation of HSP mRNAs.

Transcriptional control of delta-crystallin gene expression in the chicken embryo lens: demonstration by a new method for measuring mRNA metabolism

Crystallins are proteins that accumulate to very high concentrations in the fiber cells of the lens of the eye. Crystallins are responsible for the transparency and high refractive index that are essential for lens function. In the chicken embryo, delta-crystallin accounts for more than 70% of the newly synthesized lens proteins. We used density labeling and gene-specific polymerase chain reaction (PCR) to determine the mechanism regulating the expression of the two very similar delta-crystallin genes. Newly synthesized RNA was separated from preexisting RNA by incubating the lenses with 15N- and 13C-labeled ribonucleosides and then separating newly synthesized, density-labeled RNA from the bulk of light RNA by equilibrium density centrifugation in NaI-KI gradients. The relative abundances of the two crystallin mRNAs in the separated fractions were then determined by PCR. This method permitted the quantitation of newly synthesized processed and unprocessed delta-crystallin mRNAs. Additional studies used intron- and gene-specific PCR primers to determine the relative expression of the two delta-crystallin genes in processed RNA and unprocessed RNA extracted from different regions of the embryonic lens. Results of these tests indicated that the differential expression of the delta-crystallin genes was regulated primarily at the level of transcription. This outcome was not expected on the basis of the results of previous studies, which used in vitro transcription and transfection methods to evaluate the relative strengths of delta-crystallin promoter and enhancer sequences. Our data suggest that the cultured cells used in these earlier studies may not have provided an accurate view of delta-crystallin regulation in the intact lens.

Transcriptional control of delta-crystallin gene expression in the chicken embryo lens: demonstration by a new method for measuring mRNA metabolism

Crystallins are proteins that accumulate to very high concentrations in the fiber cells of the lens of the eye. Crystallins are responsible for the transparency and high refractive index that are essential for lens function. In the chicken embryo, delta-crystallin accounts for more than 70% of the newly synthesized lens proteins. We used density labeling and gene-specific polymerase chain reaction (PCR) to determine the mechanism regulating the expression of the two very similar delta-crystallin genes. Newly synthesized RNA was separated from preexisting RNA by incubating the lenses with 15N- and 13C-labeled ribonucleosides and then separating newly synthesized, density-labeled RNA from the bulk of light RNA by equilibrium density centrifugation in NaI-KI gradients. The relative abundances of the two crystallin mRNAs in the separated fractions were then determined by PCR. This method permitted the quantitation of newly synthesized processed and unprocessed delta-crystallin mRNAs. Additional studies used intron- and gene-specific PCR primers to determine the relative expression of the two delta-crystallin genes in processed RNA and unprocessed RNA extracted from different regions of the embryonic lens. Results of these tests indicated that the differential expression of the delta-crystallin genes was regulated primarily at the level of transcription. This outcome was not expected on the basis of the results of previous studies, which used in vitro transcription and transfection methods to evaluate the relative strengths of delta-crystallin promoter and enhancer sequences. Our data suggest that the cultured cells used in these earlier studies may not have provided an accurate view of delta-crystallin regulation in the intact lens.

Involvement of rRNA synthesis in the enhanced survival and recovery of protein synthesis seen in thermotolerance

Although acquired thermotolerance has been linked to the induction of heat shock proteins, the molecular mechanism(s) by which cells become resistant to heat is unknown. The present study shows a strong correlation between the survival of cells following heat shock and the rate of recovery of protein, total RNA, and rRNA synthesis. Increasing exposure of CHO cells to 45 degrees C was found to decrease survival and cause a lengthening delay in these synthetic processes. The same reciprocal correlation was seen in thermotolerant cells. As thermotolerance develops, more cells survive a heat challenge and the delay in synthesis decreases. These data argue that enhanced recovery of protein and RNA synthesis is one factor which plays a key role in thermotolerance. The involvement of rRNA synthesis was further investigated by using actinomycin D at 0.1 microgram m1(-1), a concentration at which rRNA synthesis is selectively inhibited. When the drug was present during the recovery from a challenge heat treatment, the survival of thermotolerant cells was approximately 3-fold lower than expected from the mild toxicity of the drug. As this could not be accounted for by an interaction of the drug with the response of cells to single heat treatments, it is concluded that the drug inhibits the expression of thermotolerance in cells which would otherwise express a full degree of thermotolerance. The time and concentration dependence of this effect indicates that the drug acts though inhibition of rRNA synthesis. Therefore, enhanced recovery of RNA synthesis, presumably rRNA synthesis, is identified as one of the mechanisms responsible for enhanced survival of thermotolerant cells following heat shock.

Glucose regulates preproinsulin messenger RNA levels in a clonal cell line of simian virus 40-transformed B cells

In HIT-T15 cells grown in the absence of glucose, Northern blot analysis of total RNA revealed a major 0.5 kb preproinsulin (ppI) mRNA transcript which co-migrated with the mature transcript from a human insulinoma. In 4 h tissue cultures, glucose (2-20 mM) stimulated HIT cell ppI mRNA levels in a markedly dose-dependent manner. Glucose-stimulated ppI mRNA was (i) inhibited by actinomycin D, suggesting that regulation may be in part transcriptional, and (ii) potentiated by agents known to activate B cell protein kinases. HIT cells represent a unique model for investigating long term regulation of insulin gene expression and biosynthesis.

Differential inhibition of cellular RNAs by photosensitized trioxalen

Treatment of mouse cell cultures with 5 X 10(-6)M trioxalen (4,5',8-trimethylpsoralen, or TMP) followed by long wavelength ("near") UV-irradiation (NUV) resulted in immediate cessation of most of the cellular transcriptional activity. The effective inhibitory TMP concentrations for ribosomal RNA were lower than those affecting poly RNA containing RNA. Low molecular RNA species were only partially inhibited. Side effects, commonly caused by other drugs, were not detected. These properties, and the simplicity of their use, make psoralens attractive inhibitors of RNA biosynthesis.

Heat shock alters nuclear ribonucleoprotein assembly in Drosophila cells

Heterogeneous nuclear RNA is normally complexed with a specific set of proteins, forming ribonucleoprotein particles termed hnRNP. These particles are likely to be involved in mRNA processing. We have found that the structure of hnRNP is profoundly altered during the heat shock response in Drosophila cultured cells. Although hnRNA continues to be synthesized at a near-normal rate during heat shock, its assembly into hnRNP is incomplete, as evidenced by a greatly decreased protein content of the particles in Cs2SO4 density gradients. RNA-protein cross-linking conducted in vivo (Mayrand and Pederson, Proc. Natl. Acad. Sci. U.S.A. 78:2208-2212, 1981) also reveals that hnRNA made during heat shock is complexed with greatly reduced amounts of protein. The block of hnRNP assembly occurs immediately upon heat shock, even before the onset of heat shock protein synthesis. Additional experiments reveal that hnRNP assembled normally at 25 degrees C subsequently disassembles during heat shock. The capacity for normal hnRNP assembly is gradually restored after heat-shocked cells are returned to 25 degrees C. Heat-shocked mammalian cells also show a similar block in hnRNP assembly. We suggest that incomplete assembly of hnRNP during heat shock leads to abortive processing of most mRNA precursors and favors the processing or export (or both) of others whose pathway of nuclear maturation is less dependent on, or even independent of, normal hnRNP particle structure. This hypothesis is compatible with a large number of previous observations.

Heat shock alters nuclear ribonucleoprotein assembly in Drosophila cells

Heterogeneous nuclear RNA is normally complexed with a specific set of proteins, forming ribonucleoprotein particles termed hnRNP. These particles are likely to be involved in mRNA processing. We have found that the structure of hnRNP is profoundly altered during the heat shock response in Drosophila cultured cells. Although hnRNA continues to be synthesized at a near-normal rate during heat shock, its assembly into hnRNP is incomplete, as evidenced by a greatly decreased protein content of the particles in Cs2SO4 density gradients. RNA-protein cross-linking conducted in vivo (Mayrand and Pederson, Proc. Natl. Acad. Sci. U.S.A. 78:2208-2212, 1981) also reveals that hnRNA made during heat shock is complexed with greatly reduced amounts of protein. The block of hnRNP assembly occurs immediately upon heat shock, even before the onset of heat shock protein synthesis. Additional experiments reveal that hnRNP assembled normally at 25 degrees C subsequently disassembles during heat shock. The capacity for normal hnRNP assembly is gradually restored after heat-shocked cells are returned to 25 degrees C. Heat-shocked mammalian cells also show a similar block in hnRNP assembly. We suggest that incomplete assembly of hnRNP during heat shock leads to abortive processing of most mRNA precursors and favors the processing or export (or both) of others whose pathway of nuclear maturation is less dependent on, or even independent of, normal hnRNP particle structure. This hypothesis is compatible with a large number of previous observations.

Herpes simplex virus types 1 and 2 induce shutoff of host protein synthesis by different mechanisms in Friend erythroleukemia cells

Herpes simplex virus type 1 (HSV-1) and HSV-2 disrupt host protein synthesis after viral infection. We have treated both viral types with agents which prevent transcription of the viral genome and used these treated viruses to infect induced Friend erythroleukemia cells. By measuring the changes in globin synthesis after infection, we have determined whether expression of the viral genome precedes the shutoff of host protein synthesis or whether the inhibitor molecule enters the cells as part of the virion. HSV-2-induced shutoff of host protein synthesis was insensitive to the effects of shortwave (254-nm) UV light and actinomycin D. Both of the treatments inhibited HSV-1-induced host protein shutoff. Likewise, treatment of HSV-1 with the cross-linking agent 4,5',8-trimethylpsoralen and longwave (360-nm) UV light prevented HSV-1 from inhibiting cellular protein synthesis. Treatment of HSV-2 with 4,5',8-trimethylpsoralen did not affect the ability of the virus to interfere with host protein synthesis, except at the highest doses of longwave UV light. It was determined that the highest longwave UV dosage damaged the HSV-2 virion as well as cross-linking the viral DNA. The results suggest that HSV-2 uses a virion-associated component to inhibit host protein synthesis and that HSV-1 requires the expression of the viral genome to cause cellular protein synthesis shutoff.

Hyperthermia and the heat-shock proteins of HeLa cells

When HeLa cells are subject to hyperthermia, the synthesis of specific heat-shock proteins (HSP) is induced under a variety of thermal conditions. HSP synthesis does not occur at temperatures above 43 degrees C but requires return to a culture temperature of 37 degrees C. Maximal induction appears to be achieved if a brief hyperthermia treatment (10 min, 45 degrees -46 degrees C) is followed by 2 h "development" at 37 degrees C. The induction process requires transcription but not DNA replication, and general cell metabolism is probably also required, as induction does not occur if the heat-treated cells are returned to 4 degrees (rather than 37 degrees C) for development. A small proportion of the HSPs of 72-74 Kd are found in nuclei, but do not appear to bind to DNA. The bulk of these proteins, as well as those at 100 Kd, are cytoplasmic, but none are preferentially associated with mitochondria. Increased synthesis of the 100Kd and 72-74 Kd HSPs was also triggered by pretreatment of the cells with 5 x 10(-5) M sodium arsenite.

Effect of microtubule-disrupting drugs on protein and RNA synthesis in Physarum polycephalum amoebae

The effects of the microtubule-disrupting drugs, colchicine, vinblastine, podophyllotoxin, griseofulvin, and lumicolchicine (10(-5) M), on protein and RNA synthesis were studied in Physarum polycephalum amoebae in culture. All, except lumicolchicine, were found to simultaneously reduce the rate of protein synthesis and stimulate RNA synthesis. These results parallel the effects seen in cells exposed to heat shock. Treatment of cells with a microfilament-disrupting drug, cytochalasin B (10 micrograms/ml in ethanol), resulted in a reduced rate of protein synthesis after 2 h compared to a similar effect by vinblastine in 5--15 min. A morphological abnormality, microtubule paracrystals, were seen associated with centrioles in vinblastine-treated cells in which protein synthesis had been reduced by 50%. Vinblastine and podophyllotoxin were shown to interfere with the recovery of protein synthesis after inhibition by low or elevated temperatures. The possible role of microtubules in regulating the translational response of a cell to an external environmental stimulus is discussed.

Cell differentiation and flagellar elongation in Naegleria gruberi. Dependence on transcription and translation

This paper presents evidence that the phenotypic transformation of Naegleria gruberi from amebae to flagellates that occurs when cells are placed in a nutrient-free aqueous environment is dependent on transcription and translation. RNA and protein are synthesized during the hour-long differentiation. Actinomycin D and daunomycin selectively inhibit RNA synthesis, and cycloheximide selectively inhibits protein synthesis, throughout the time required for differentiation. These inhibitors prevent differentiation if added soon after the cells are transferred to nonnutrient buffer but cease to block specific differentiation events at subsequent, reproducible times, the transition points. After each transition point, morphogenesis can occur in the presence of the inhibitor and in the virtual absence of transcription or translation. A map of the transition points indicates that RNA synthesis is required until halfway through the temporal process from initiation to flagellum assembly, and that protein synthesis is required until three-fourths of the way through. Even when flagellum outgrowth can occur in the presence of cycloheximide, the length of the flagella formed is determined by the extent of synthesis of an unknown "limiting precursor." The transition points for formation of flagella and for formation of the streamlined flagellate body shape are temporally separate. These results indicate that differentiation in Naegleria involves a redirection of cell metabolism to produce new RNA and protein molecules that are essential for morphogenesis.

Inhibitory effect of cyclic adenosine 2',3'-monophosphate on leucine incorporation by L5178Y cells

The specificity of the reported stimulation of protein synthesis affected by dibutyryl adenosine cyclic 3',5'-monophosphate in L5178Y cells was tested by assaying the effect of adenosine cyclic 2',3'-monophosphate. Protein synthesis was assayed as total acid precipitable 3H-leucine. Leucine incorporation was inhibited by adenosine cyclic 2',3'-monophosphate. Alone, and in combination with actinomycin, 2',3'-cAMP inhibited leucine incorporation more rapidly and to a greater degree than did actinomycin alone. Since a similar potentiating effect was not observed with cordycepin, the hypothesis has been suggested that adenosine cyclic 2',3'-monophosphate inhibits protein synthesis by interfering with some post transcriptional event, perhaps modification of existing RNA. The results further suggest that 2,3 cyclic AMP may be antagonistic of the effects of 3,5 cyclic AMP, and as such may be useful as an inhibitor of that nucleotide's action.

The initiation and elongation steps in protein synthesis: relative rates in Chinese hamster ovary cells during and after hyperthermic and hypothermic shocks

The relative rates of the initiation and elongation phases of protein synthesis have been determined in heat- and cold-shocked CHO cells from measurements of the incorporation of 35S-methionine into N-terminal and internal positions of growing peptides by a modified Edman degradation. When the cells are shifted from 37 degrees C to temperatures between 10 degrees C and 34 degrees C, the rate of initiation is at first reduced more extensively than that of elongation. After 20 to 30 minutes at the lower temperature, however, the cells undergo a metabolic adjustment which includes increasing the rate of initiation until it corresponds to the rate of elongation at that temperature. Calculated apparent energies of activation for initiation and elongation are in reasonable agreement with those determined in other mammalian cells. When the cooled cells are returned to 37 degrees C, the rates of initiation and elongation recover immediately but do not exceed the control values. Exposure to elevated temperatures (43 degrees C) causes an immediate cessation of initiation and thus a delayed inhibition of elongation; upon return to 37 degrees C, the rate of initiation is transiently elevated above the control rate, and the rate of elongation returns to the control rate after a 2- to 3-minute delay. Hence, a factor which leads to supranormal rates of initiation may accumulate at high but not at low temperatures.

Heat inhibition of reticulocyte protein synthesis. Evidence for a mechanism independent of the hemin-controlled repressor

Incubation of rabbit reticulocytes at 45 degrees C results in a prompt but reversible decrease in protein synthesis and a concomitant conversion of polyribosomes to smaller aggregates. These effects occur even in the presence of 100 micrometer hemin in the incubation medium. There is also inhibition of heme synthesis but this occurs at a later time than the effect on protein synthesis. The inhibtion of heme synthesis results from a decrease in activity of beta-aminolevulinic acid synthetase. This decrease of heme synthesis appears to be secondary to the inhibition of protein synthesis with resultant accumulation of intramitochondrial heme (which will decrease beta-aminolevulinic acid synthetase activity). An inhibitor of reticulocyte cell-free protein synthesis formed in the postribosomal supernatants of cells incubated at both 45 and 37 degrees C but not at 0 degrees C. No temporal or quantitative differences in the amount of this inhibitor from cells treated at either 37 or 45 degrees C was apparent. The inhibitor was not found in the fraction where the hemin-controlled repressor is isolated. It is concluded that heat inactivation of intact reticulocyte protein synthesis does not depend upon a decrease in heme synthesis, heme concentration or generation of the hemin-controlled repressor. Furthermore, it appears that the inhibitor formed in the post-ribosomal supernatant cannot be the sole cause of the heat inhibition of protein synthesis.

Maintenance of the ratio of alpha and beta globin synthesis in rabbit reticulocytes

The synthesis of rabbit alpha and beta globins under various conditions was studied using intact reticulocytes and reticulocyte cell-free systems. Raising salt concentration of media in which reticulocytes were incubated with radioactive amino acids reduced the total protein synthesis but did not affect the ratio of alpha to beta globins produced. Using a reticulocyte lysate which had been incubated with micrococcal nuclease to remove the endogenous globin messenger RNA activity, it was found that unlike the intact cell or the untreated lysate very little alpha globin was synthesised on adding purified globin mRNA. These results are discussed in terms of their compatibility with some proposed models of coordination of alpha and beta globin production.

Stimulation of in vitro translation of messenger RNA by actinomycin D and cordycepin

Actinomycin D and cordycepin were tested for their effect on translation in the wheat germ embryo extract and reticulocyte lysate assays for in vitro protein synthesis. Both drugs were found to stimulate the incorporation of 35S-labeled methionine into protein up to threefold as compared to control assays. This was true for synthesis directed by murine myeloma polyadenylate-containing RNA, tobacco mosaic virus RNA, and endogenous reticulocyte messenger RNA.

Coordinate regulation of protein synthesis and messenger RNA content during growth arrest of suspension Chinese hamster ovary cells

We have found Chinese Hamster Ovary cells, cultured in suspension, are subject to growth control by serum. When suspended in medium containing 0.5% serum the cells becomes reversibly arrested in the beginning of the G1 phase of the cell cycle and can be maintained in this viable, nonproliferating state for several days. This system was used to examine the regulation of protein synthesis with growth rate. In particular, the experiments addressed the question whether mRNA content is the principal controlling factor determining the rate of protein synthesis. The rate of leucine incorporation in resting cells in low serum is 2-to 2.5-fold lower than that of cells growing in 10% serum. The steady-state number of cytoplasmic poly A (+) RNA molecules shows a proportional decrease, consistent with it being a determining factor controlling the rate of protein synthesis. Furthermore, the rate of production of poly A (+) and poly A (-) RNA appears to be regulated coordinately. Regulation of the rate of initiation of translation would result in fewer ribosomes bound per active message and/or a lower proportion of total mRNA's being active. Our measurements indicate that the fraction of cytoplasmic poly A (+) mRNA in polyribosomes and the relative degree of loading of each active poly A(+) mRNA with ribosomes is the same in resting and growing cells. Thus these cells resemble 3T6 and translational control does not appear to be an important part of the change in protein synthetic rate with teh state of growth.

Actinomycin D inhibits hemoglobin synthesis in rabbit reticulocyte lysates and binds to a 15 S cytoplasmic particle involved in the initiation step of translation

Actinomycin D inhibits hemoglobin synthesis in rabbit reticulocyte lysates by affecting initiation of globin polypeptide chains. To obtain information on the distribution of actinomycin-sensitive structures among the components of the translation machinery, a reticulocyte lysate is incubated with [3H]actinomycin. The tritium label is found associated with ribosomes actively engaged in protein synthesis and with a 15 S cytoplasmic particle. During cell-free incubation or when initiation is inhibited by aurintricarboxylic acid, the labeling of the 15 S particles increases, suggesting that they are relased from ribosomes into the cytoplasm and play a role in the initiation process. The 15 S cytoplasmic particles contain a DNAase-sensitive component, probably complexed with ribonucleic acids and proteins. A possible role of these DNA-containing particles in the initiation step of translation is discussed.

Fate of histone messenger RNA in synchronized HeLa cells in the absence of initiation of protein synthesis

The fate of cytoplasmic histone mRNA was studied under conditions in which initiation of protein synthesis in synchronized HeLa cells is S phase was blocked by increasing the osmolarity of the growth medium with NaCl. In contrast to the interruption of DNA replication with hydroxyurea, which results in an exponential degradation of translatable histone mRNA with a half-life of about 10-13 min, blocking the initiation of protein synthesis leads to only a marginal loss of biologically active histone mRNA in the cytoplasm. When the initiation of protein synthesis was interrupted by treating cells with 150 mM NaCl, 40-50% of the total cytoplasmic histone mRNA previously translated in polyribosomes appears in the cytoplasm integrated into mRNA-protein particle(s) sedimenting between 15 S and 30 S. On the other hand, in untreated S-phase cells or in cells blocked with hydroxyurea only 3-6% of the total translatable histone mRNA is found in the cytoplasm not bound to ribosomes or their subunits. In addition, the degradation of histone mRNA in hydroxyurea-blocked S-phase cells is prevented when the initiation of protein synthesis is inhibited with NaCl. These studies clearly indicate that the inhibition of initiation of protein synthesis per se is not the cause for the rapid degradation of cytoplasmic histone mRNA observed when DNA replication is turned off and that the inactivation of these mRNAs is a process dependent on continuous protein synthesis.

Stimulation of cell proliferation in rat liver by alpha-hexachlorocyclohexane or partial hepatectomy and end points during G1 of the inhibitory action of beta-diethyl-aminoethylphenyldiallyl acetate-HC1 (CFT 1201), beta-diethylaminoethyldiphenylpropyl acetate. HC1 (SKF 525-A) and actinomycin D

The present work was designed to study the nature, sequence and temporal position of some inhibitor-sensitive events of the replicative cycle in rat liver. Hepatocyte proliferation was induced by alpha-hexachlorocyclohexane and by partial hepatectomy; the onset of DNA synthesis and of mitotic activity were determined and used as reference points in the cell cycle. Inhibition of cell proliferation was achieved by CFT 1201, SKF 525-A, and actinomycin D. It was found that the inhibitory action of the three agents ends at the same stage of the replicative cycle, 0--2 h before the G1/S transition, in both alpha-hexachlorocyclohexane-stimulated and regenerating rat liver. It is concluded that the molecular events sensitive to CFT 1201, SKF 525-A or actinomycin D are either identical or temporally closely associated; they do not figure in the metabolic activation of alpha-hexachlorocyclohexane.

Selective decrease in the rate of cleavage of an intracellular precursor to Rauscher leukemia virus p30 by treatment of infected cells with actinomycin D

The cleavage of an intracellular 67,000- to 70,000-dalton precursor, termed Pr4 to Rauscher leukemia virus (RLV) p30 protein proceeded at a slower rate when virus-producing cells were treated with actinomycin D (AMD). Treatment with AMD also caused a slight accumulation of Pr4 in purified early virus particles produced by a cell line which usually produces virions that contain little Pr4. The cleavage of other intracellular viral precursor polypeptides was not affected by treatment with AMD. Treatment of infected cells with cycloheximide, on the other hand, allowed the cleavage of Pr4 to proceed at the usual rate for a short period of time before further cleavage was drastically slowed or prevented. The cleavage of several other viral precursor polypeptides was also inhibited by treatment with cycloheximide. Different lines of evidence suggest that the mechanism of action of AMD is not due to a possible indirect effect on protein synthesis. Thus, the rate of cleavage of Pr4 was not affected by the length of pretreatment with AMD between 1 to 8 h. In addition, the combined effect of AMD and cycloheximide, at their maximal inhibitory concentrations, was greater than the effect of either drug alone, indicating the involvement of two at least partially different mechanisms in the action of AMD and cycloheximide. Furthermore, AMD did not affect the pulse labeling of viral precursor polypeptides. These results suggest that the interaction with viral RNA, whose production is inhibited by AMD, accelerates the cleavage of Pr4 to p30 during virus assembly. A hypothetical model is presented to illustrate th possible advantages of having a step in virus assembly in which genomic RNA interacts with a precursor to capsid proteins before the cleavage of that precursor.

Increased synthesis of ribonucleic acids in mouse liver after treatment with a nonionic detergent

A single injection of Tween 40 (polyoxyethylene(20)sorbitan monohexanoate) in the dose range of 600--800 mg/kg body weight induced an increased short-term labeling especially of poly(A)-containing mRNAs in mouse liver (using either [3H]orotic acid or [32P]orthophosphate as RNA precursors), and apparently increased the turnover rates of both rRNAs and mRNAs in this organ over a period of 24 h. In the early period (4 h) after the injection of Tween 40, there was also a significant increase in the content of 32P-labeled adenylic acid in microsomal poly(A)-mRNA fraction. The activity of DNA-dependent RNA polymerases in the nuclei of treated animals was stimulated up to 60% over that in control nuclei in the same period after detergent injection.

Effects of actinomycin D on generation time and morphogenesis in Paramecium

The sensitivity of Paramecium tetraurelia (=P. aurelia syngen 4) cells to pulse treatments with various doses of Actinomycin D (AMD) was estimated by comparing the generation times of treated and untreated sister cells. It was found that the delay of division in treated cells depended on the concentration of AMD, on their "age" at the time of the pulse treatment, and on their individual sensitivity. Sensitivity of Paramecium to AMD changes during the cell cycle in a predictable way. About 3 1/2 hr before the normally expected cell fission (total generation time approximately 5 1/2 hr) there is a decrease of sensitivity. Thereafter, the cell enters a new stage with a progressive increase of sensitivity. This 2nd phase ends at the "transition point" (approximately 2 hr before cell division), when sensitivity drops abruptly. The division process itself may be altered and slowed down by high concentrations of AMD, even if the drug is applied after the transition point, but this process can never be completely annulled. The impairment of the division mechanism may lead to morphologic anomalies in the offspring. Resorption of oral anlagen in P. tetraurelia probably never occurs during the cell cycle after AMD treatment. The reason for individual variability of the cells, mechanisms controlling development, and the question of an obligate sequence of gene action in each cell cycle are discussed.

Subcellular localization of a lesion in protein synthesis in rabbit reticulocytes incubated at elevated temperatures

When rabbit reticulocytes are incubated at 43-45 degrees C their rate of protein synthesis rapidly decreases, compared to a contol 37 degrees C incubation. Lysates prepared from cells incubated at this supra-optimal temperature have an equally decreased capacity for endogenous, but not poly(uridylic acid)-directed, protein synthesis. Subcellular fractionation traced the lesion to the crude ribosomal pellet, 0.5 M KCl ribosomal wash and postribosomal supernatant of the temperature-shocked cells. Preparation of purified ribosomal subparticles showed, however, that they were as active as the control in protein synthesis. In this paper we present evidence that the decreased activity of the heated lysate, 0.5 mM KCl wash and postribosomal supernatant is due to an inhibitor and can be overcome by the addition of 0.5 M KCl or supernatant from control cells. The results are discussed in terms of the inactivation of a component, essential for initiation of endogenous protein synthesis, which is probably partitioned between ribosomes and supernatant. We also suggest that the decreased protein synthetic activity of the heated cells may be related to their decreased synthesis of haem.

Temperature sensitivity of protein synthesis initiation in the reticulocyte lysate system. Reduced formation of the 40 S ribosomal subunit - Met-tRNAf complex at an elevated temperature

Analysis of protein synthesis in the rabbit reticulocyte lysate system revealed the existence of a temperature-sensitive step in chain initiation which became irreversibly inactivated in the incubation at 42 degrees C. This inactivation of initiation was accompanied by a marked reduction in formation of the 40 S ribosomal subunit - Met-tRNAf complex. Decreased protein synthesis and a decrease in formation of the 40 S complex were also evident in unfortified lysates which had been prewarmed at 42 degrees C prior to protein synthesis. Hemin did not protect such lysates. The addition of supernatant fraction of a fresh lysate did not promote recovery of the reduced protein synthesis by such prewarmed lysates. Moreover, the addition of supernatant fraction prewarmed at 42 degrees C in the presence of added hemin caused little inhibition of protein synthesis by fresh lysate. The results indicate that the supernatant fraction is not involved in the inactivation.

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.

Study on the reinitiation of transcription in 37 RC cells after actinomycin D inhibition. Spectrum of major RNA species resynthesized after maximal suppression by the drug

The reinitiation of the synthesis of major RNA species has been studied in 37 RC cells after maximal inhibition of RNA synthesis by actinomycin D (AMD). During the period of recovery from AMD, resynthesized RNA (rec-RNA) is initially composed of almost exclusively light (4-14S) heterogeneous RNA species. All normal species of RNA can be detected in the rec-RNA spectrum as early as 3 h after AMD removal. The synthesis of low molecular weight methylated RNA species increases slightly during the early period after AMD removal, while the increase of low molecular weight unmethylated species is more significant during the same period. Much of the radioactivity in the polyribosomal fraction is EDTA and puromycin sensitive. Since polysomal, puromycin-sensitive RNA is polyadenylated (as evidenced by the binding of poly-U filters), and is heterogenous in size, it belongs to the m-RNA class. The synthesis of m-RNA increases immediately after AMD removal, whereas the reinitiation of the r-RNA synthesis occurs after a lag period of about 2 h. The kinetics of recovery of the synthesis of major RNA species from AMD inhibition show a size dependency comparable to the size-related sensitivity to AMD inhibition in other cellular systems. This dependency is most clearly seen in HnRNA, the AMD sensitivity of which is measured by the length of the lag period between AMD removal and the appearance of HnRNA fractions in a sucrose density gradient. Low molecular weight HnRNA reappears first, whereas heavier fractions of HnRNA appear in the spectrum after a lag period, the length of which is in direct relation to the position of the HnRNA fraction in the gradient.

Mitochondrial protein synthesis in a mammalian cell-line with a temperature-sensitive leucyl-tRNA synthetase

The temperature-sensitive Chinese hamster ovary cell mutant tsH1, has been shown previously to contain a temperature-sensitive leucyl-tRNA synthetase. At the non-permissive temperature of 40 degrees C cytosolic protein synthesis is rapidly inhibited. The protein synthesis which continues at 40 degrees C appears to be mitochondrial, since: (a) whole-cell protein synthesis at the permissive temperature of 34 degrees C is not inhibied by tevenel, the sulfamoyl analogue of chloramphenicol and a specific inhibitor of mitochondrial protein synthesis; however, whole-cell protein synthesis at 40 degrees C is inhibited by tevenel, (b) Protein synthesis by isolated mitochondria from tsH1 cells is not significantly inhibited at 40 degrees C. (c) At 40 degrees C [14C]leucine is incorporated predominantly into the mitochondrial fraction of tsH1 cells. (d) The incorporation of [14C]leucine at 40 degrees C into mitochondrial proteins of tsH1 cells is inh-bited by tevenel but not by cycloheximide. These results suggest that the mitochondria of tsH1 cells contain a leucyl-tRNA synthetase which is different from the cytosolic enzyme. The inhibition of cytosolic, but not of mitochondrial protein synthesis in tsH1 cells at 40 degrees C allows the selective labelling of mitochondrial translation products in the absence of inhibitors. The mitochondrial translation products labelled in tsH1 cells at 40 degrees C and at 34 degrees C in the presence of cycloheximide have been compared by sodium dodecylsulphate-polyacrylamide gel electrophoresis. Both conditions of labelling give similar profiles. The mitochondrial translation products are resolved into two components, one with an apparent molecular weight range from 40,000 to 20,000 and a second with an apparent molecular weight range from 20,000 to 10,000.

Inhibitor of protein synthesis co-isolating with polyribosomal RNA

Although low concentrations of total polyribosomal RNA from porcine parotid glands or rat pituitary cells in culture (GH3) isolated by standard dodecylsulphate/phenol, chloroform extraction techniques can direct the incorporation of radiolabeled amino acids into proteins using a cell-free protein-synthesizing system derived from wheat germ embryos, higher concentrations inhibit the translation of endogenous wheat germ mRNA, or added rabbit globin mRNA or polyribouridylic acid. This inhibitory activity is separated from poly(A)-rich RNA by oligo(dT)-cellulose chromatography. The inhibitory activity appears to reside in a heat-stable protein since it is inactivated by incubation with various proteases but not by DNAase I, pancreatic ribonuclease, alkaline hydrolysis, or treatment with formamide. Specificity of the inhibition is suggested since the inhibitory fraction prepared from GH3 cells also inhibits protein synthesis in a cell-free protein-synthesizing system derived from porcine parotid gland, while the inhibitory fraction prepared from porcine parotid gland has no inhibitory activity in this homologous system. Radioiodination and dodecylsulphate/polyacrylamide gel electrophoresis reveal several protein bands, the most prominent with an apparent molecular weight of 78 000.

Effect of reduced temperatures on protein synthesis in mouse L cells

The rate of incorporation of leucine into protein, the rate of polypeptide elongation and termination, and the relative quantity and size of polysomes were analyzed in mouse L cells grown in suspension culture at various temperatures between 0 degrees C and 36 degrees C. Between 10 degrees C and 36 degrees C protein synthesis exhibited two different apparent activation energies (39 kcal/mole, 10-25 degrees C; 14 kcal/mole, 25-36 degrees C), whereas elongation and termination had only one (16 kcal/mole). Below 36 degrees C, the polysome level and size decreased, reaching a minimum of 30% of the control 36 degrees C values at 10 degrees C; below 10 degrees C the level increased again back to control values at 0 degrees C. The polysome decline was time dependent, requiring about 5 hr to reach the equilibrium value. This decline is completely reversible within 60 min, even in the presence of 4 mug/ml of actinomycin D, and even after 15 hr of incubation at the lower temperature. The results suggest that polypeptide initiation is rate limiting, particularly below 25 degrees C; whereas above this temperature, elongation or perhaps some other process may be limiting. These results are quite different from those obtained for E. coli and rabbit reticulocyte protein synthesis.

Effects of actinomycin D on the association of newly formed ribonucleoproteins with the cistrons of ribosomal RNA in Triturus oocytes

The effect of actinomycin D(AMD) on the association of the nascent ribonucleo-protein (RNP) fibrils containing the precursors of ribosomal RNA (pre-rRNA) with their template deoxyribonucleoprotein (rDNP) strands has been studied in lampbrush stage oocytes from Triturus alpestris. Ovary pieces were incubated in vitro either in media containing radioactive ribonucleosides and then, for various times, in solutions containing 25 mug/ml AMD, or were directly exposed to the drug. The ultrastructure of the nucleoli and the nuclear periphery was studied by electron microscopy of thin sections and positively stained spread preparations of isolated nuclear contents, and by light and electron microscope autoradiography. The fate of the labeled pre-rRNA was followed by gel electrophoresis of RNA extracted from manually isolated nuclei. Our results show that the growing fibrils which contain the nascent pre-rRNA progressively detach from the DNP strands, the majority being released between 45 and 180 min after application of the drug. The release pattern seems to be random and does not show preference for regions close to the initiator or terminator sites of the transcribed rDNP units. There is a pronounced tendency to removal of groups of adjacent mascent fibrils. The effect of the drug is very heterogeneous. Even after 3 h of treatment with AMD the nucleoli exhibit several individual transcriptional units which appear almost completely covered with lateral fibrils. Autoradiography revealed that most of this released RNP remains within the confinements of the nucleoli which show some foci of aggregation and condensation of fibrillar components but no clear "segregation" phenomenon. In the gel-electrophoretic analysis, a significant but moderate decrease of labeled pre-rRNA was noted only in the first stable pre-rRNA component, whereas pre-rRNA classes of lower molecular weight are very stable under these conditions. The results are discussed in relation to the stability of rDNA transcription complexes and as a basis for an explanation of the ultrastructural changes which are generally observed in nucleoli of AMD-treated cells. It is postulated that inhibition of transcription results in a slow but progressive release of the arrested incomplete RNP fibrils from the template.

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.