Characteristics of the polyadenylic acid segment associated with messenger ribonucleic acid in mouse sarcoma 180 ascites cells

The Dynamic Poly(A) Tail Acts as a Signal Hub in mRNA Metabolism

In eukaryotes, mRNA metabolism requires a sophisticated signaling system. Recent studies have suggested that polyadenylate tail may play a vital role in such a system. The poly(A) tail used to be regarded as a common modification at the 3' end of mRNA, but it is now known to be more than just that. It appears to act as a platform or hub that can be understood in two ways. On the one hand, polyadenylation and deadenylation machinery constantly regulates its dynamic activity; on the other hand, it exhibits the ability to recruit RNA-binding proteins and then interact with diverse factors to send various signals to regulate mRNA metabolism. In this paper, we outline the main complexes that regulate the dynamic activities of poly(A) tails, explain how these complexes participate polyadenylation/deadenylation process and summarize the diverse signals this hub emit. We are trying to make a point that the poly(A) tail can metaphorically act as a "flagman" who is supervised by polyadenylation and deadenylation and sends out signals to regulate the orderly functioning of mRNA metabolism.

Poly a blocks in the nuclear ribonucleoprotein complexes, containing pre-mRNA

Poly A was found in nuclear particles containing pre-mRNA. It was shown that during the isolation of 30S particles from rat liver or Ehrlich ascites carcinoma nuclei, all poly A is detached from the particles containing pre-mRNA and is found in the form of RNP with a sedimentation coefficient of about 14S. When RNP polyparticles are isolated in the presence of RNase inhibitor poly A is distributed among the particles of higher molecular weights.Since the sedimentation properties and buoyant density of the poly A-containing particles are different from the 30S particles it was suggested that the poly A fragments are bound not with informofers, but with another kind of protein.

Joys and surprises of a career studying eukaryotic gene expression

In this Reflections, I review a few early and very lucky events that gave me a running start for the rest of a long and wonderfully enjoyable career. For the main part, a discussion is provided of what I recall as the main illuminating results that my many dozens of students and postdoctoral fellows (approximately 140 in all) provided to our biochemical/molecular biological world.

Ending the message: poly(A) signals then and now

Polyadenylation [poly(A)] signals (PAS) are a defining feature of eukaryotic protein-coding genes. The central sequence motif AAUAAA was identified in the mid-1970s and subsequently shown to require flanking, auxiliary elements for both 3'-end cleavage and polyadenylation of premessenger RNA (pre-mRNA) as well as to promote downstream transcriptional termination. More recent genomic analysis has established the generality of the PAS for eukaryotic mRNA. Evidence for the mechanism of mRNA 3'-end formation is outlined, as is the way this RNA processing reaction communicates with RNA polymerase II to terminate transcription. The widespread phenomenon of alternative poly(A) site usage and how this interrelates with pre-mRNA splicing is then reviewed. This shows that gene expression can be drastically affected by how the message is ended. A central theme of this review is that while genomic analysis provides generality for the importance of PAS selection, detailed mechanistic understanding still requires the direct analysis of specific genes by genetic and biochemical approaches.

Rapid, diffusional shuttling of poly(A) RNA between nuclear speckles and the nucleoplasm

Speckles are nuclear bodies that contain pre-mRNA splicing factors and polyadenylated RNA. Because nuclear poly(A) RNA consists of both mRNA transcripts and nucleus-restricted RNAs, we tested whether poly(A) RNA in speckles is dynamic or rather an immobile, perhaps structural, component. Fluorescein-labeled oligo(dT) was introduced into HeLa cells stably expressing a red fluorescent protein chimera of the splicing factor SC35 and allowed to hybridize. Fluorescence correlation spectroscopy (FCS) showed that the mobility of the tagged poly(A) RNA was virtually identical in both speckles and at random nucleoplasmic sites. This same result was observed in photoactivation-tracking studies in which caged fluorescein-labeled oligo(dT) was used as hybridization probe, and the rate of movement away from either a speckle or nucleoplasmic site was monitored using digital imaging microscopy after photoactivation. Furthermore, the tagged poly(A) RNA was observed to rapidly distribute throughout the entire nucleoplasm and other speckles, regardless of whether the tracking observations were initiated in a speckle or the nucleoplasm. Finally, in both FCS and photoactivation-tracking studies, a temperature reduction from 37 to 22 degrees C had no discernible effect on the behavior of poly(A) RNA in either speckles or the nucleoplasm, strongly suggesting that its movement in and out of speckles does not require metabolic energy.

Poly(A)+ RNAs roam the cell nucleus and pass through speckle domains in transcriptionally active and inactive cells

Many of the protein factors that play a role in nuclear export of mRNAs have been identified, but still little is known about how mRNAs are transported through the cell nucleus and which nuclear compartments are involved in mRNA transport. Using fluorescent 2'O-methyl oligoribonucleotide probes, we investigated the mobility of poly(A)⁺ RNA in the nucleoplasm and in nuclear speckles of U2OS cells. Quantitative analysis of diffusion using photobleaching techniques revealed that the majority of poly(A)⁺ RNA move throughout the nucleus, including in and out of speckles (also called SC-35 domains), which are enriched for splicing factors. Interestingly, in the presence of the transcription inhibitor 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole, the association of poly(A)⁺ RNA with speckles remained dynamic. Our results show that RNA movement is energy dependent and that the proportion of nuclear poly(A)⁺ RNA that resides in speckles is a dynamic population that transiently interacts with speckles independent of the transcriptional status of the cell. Rather than the poly(A)⁺ RNA within speckles serving a stable structural role, our findings support the suggestion of a more active role of these regions in nuclear RNA metabolism and/or transport.

Toxicity and metabolism of 3'-deoxyadenosine N1-oxide in mice and Ehrlich ascites tumor cells

The toxic effect of 3'-deoxyadenosine N1-oxide (3'-dANO) on mice, on their different organs, and on Ehrlich ascites tumor cells was studied. In both healthy and tumour-bearing animals, the lethal dose for 10% of the mice receiving i.p. injections (LD10) of 3'-dANO was estimated to be about 300 mg/kg x 4 days in one mouse strain (Theiller). In another mouse strain (NMRI), we obtained a markedly higher LD10 value (675 mg/kg x 5 days). At nonlethal doses (250 mg/kg x 4 days), we observed reversible neurological symptoms on days 4-12 after treatment, but no macroscopical or microscopical changes was detected in the brain, heart, thymus, lung, lymph node, spleen, liver, kidney, bone marrow, or gastrointestinal tract. At doses of 450 mg/kg x 4 days, severe neurological symptoms were observed, and atony of the gastrointestinal canal and damage to the kidney and liver were registered. Even at doses that were lethal to the mice, no histopathological change was observed in the bone marrow or in the gastrointestinal canal. Pharmacokinetics studies showed that after the i.p. injection of 3'-dANO, the maximal plasma concentration was reached after 10 min, after which it declined showing a half-life of about 40 min. A transient accumulation of 3'-deoxyadenosine triphosphate (3'-dATP) was observed within 24 h in the liver and kidney, with the maximal concentration being reached after about 2-3 h. 3'-dANO was excreted partly as the unchanged substance and partly as the metabolite 3'-deoxyinosine within 24 h. Flow-cytometric DNA analysis of Ehrlich tumor cells treated either in vitro or in vivo with 3'-dANO revealed no therapy-induced change in the cell-cycle perturbations, which indicates that cells were randomly killed during all phases of the cycle.

Products of in vitro cleavage and polyadenylation of simian virus 40 late pre-mRNAs

Formation of mRNA 3' termini involves cleavage of an mRNA precursor and polyadenylation of the newly formed end. Cleavage of simian virus 40 late pre-mRNA in a crude nuclear extract generated two RNAs, 5' and 3' half-molecules. These RNAs were unmodified and linear. The 5' half-molecule contained sequences upstream but not downstream of the poly(A) site and ended in a 3'-terminal hydroxyl. The 3' half-molecules comprised a family of RNAs, each of which contains only sequences downstream of the poly(A) site, and ends in a 5'-terminal phosphate. These RNAs differed only in the locations of their 5' terminus. The 3' terminus of the 5' half-molecule was the adenosine 10 nucleotides downstream of AAUAAA, at the +1 position. The 5' terminus of the longest 3' half-molecule was at +2. Thus, these two RNAs contain every nucleoside and phosphate of the precursor. The existence of these half-molecules demonstrates that endonucleolytic cleavage occurs near the poly(A) site. 5' half-molecules generated in the presence of EDTA (which blocks polyadenylation, but not cleavage) ended at the adenosine at position +1 of the precursor. When incubated in the extract under suitable conditions, they became polyadenylated. 5' half-molecules formed in 3'-dATP-containing reactions contained a single 3'-deoxyadenosine (cordycepin) residue added onto the +1 adenosine and were poor polyadenylation substrates. We infer that the +1 adenosine of the precursor becomes the first A of the poly(A) tract and provides a 3' hydroxyl group to which poly(A) is added posttranscriptionally.

Studies on the mechanism of cytotoxicity of 3'-deoxyadenosine N1-oxide in different strains of Ehrlich ascites tumor cells

The correlation between the metabolic processing of 3'-deoxyadenosine N1-oxide (3'-dANO) in vitro and its effect on tumor growth in vivo has been investigated in seven different strains of Ehrlich ascites tumor cells. The metabolism of 3'-dANO is initiated by reduction to 3'-deoxyadenosine (3'-dA). This process is the rate-limiting process. The 3'-dA does not accumulate, but is converted to 3'-deoxyadenosine triphosphate (3'-dATP) or 3'-deoxyinosine (3'-dI). The ratio between 3'-dATP and 3'-dI inosine corresponds to the ratio between the activities of adenosine kinase and adenosine deaminase in the cell. Two of the cell lines were markedly inhibited by 3'-dANO in vivo. In these cells the accumulation of 3'-dATP was 1.4-2.2 nmol/h per mg cells, which accounts for the major part of the metabolized 3'-dANO. Five of the cell lines were not inhibited by 3'-dANO and the formation of 3'-dATP was 5-10 times less in these than in the sensitive strains. The low level of 3'-dATP is caused primarily by a low ratio between the activities of adenosine kinase and adenosine deaminase, which is 15 time less than in the sensitive cell lines. The rate of reduction of 3'-dANO seems to be of minor importance. These results indicate a correlation between the inhibition of tumor growth by 3'-dANO and the ability of the cell to accumulate 3'-dATP from 3'-dANO and show that this conversion is determined solely by the rate of reduction of 3'-dANO (3'-dANO reductase activity) and the ratio between the activities of adenosine kinase and adenosine deaminase in the cell. Consequently, the estimation of these enzyme activities in cell lysate of a given tumor can be used to predict whether the tumor is susceptible to inhibition by 3'-dANO.

Products of in vitro cleavage and polyadenylation of simian virus 40 late pre-mRNAs

Formation of mRNA 3' termini involves cleavage of an mRNA precursor and polyadenylation of the newly formed end. Cleavage of simian virus 40 late pre-mRNA in a crude nuclear extract generated two RNAs, 5' and 3' half-molecules. These RNAs were unmodified and linear. The 5' half-molecule contained sequences upstream but not downstream of the poly(A) site and ended in a 3'-terminal hydroxyl. The 3' half-molecules comprised a family of RNAs, each of which contains only sequences downstream of the poly(A) site, and ends in a 5'-terminal phosphate. These RNAs differed only in the locations of their 5' terminus. The 3' terminus of the 5' half-molecule was the adenosine 10 nucleotides downstream of AAUAAA, at the +1 position. The 5' terminus of the longest 3' half-molecule was at +2. Thus, these two RNAs contain every nucleoside and phosphate of the precursor. The existence of these half-molecules demonstrates that endonucleolytic cleavage occurs near the poly(A) site. 5' half-molecules generated in the presence of EDTA (which blocks polyadenylation, but not cleavage) ended at the adenosine at position +1 of the precursor. When incubated in the extract under suitable conditions, they became polyadenylated. 5' half-molecules formed in 3'-dATP-containing reactions contained a single 3'-deoxyadenosine (cordycepin) residue added onto the +1 adenosine and were poor polyadenylation substrates. We infer that the +1 adenosine of the precursor becomes the first A of the poly(A) tract and provides a 3' hydroxyl group to which poly(A) is added posttranscriptionally.

The AAUAAA sequence is required both for cleavage and for polyadenylation of simian virus 40 pre-mRNA in vitro

The sequence AAUAAA is found near the polyadenylation site of eucaryotic mRNAs. This sequence is required for accurate and efficient cleavage and polyadenylation of pre-mRNAs in vivo. In this study we show that synthetic simian virus 40 late pre-mRNAs are cleaved and polyadenylated in vitro in a HeLa cell nuclear extract, and that cleavage in vitro is abolished by each of four different single-base changes in AAUAAA. In this same extract, precleaved RNAs (RNAs with 3' termini at the polyadenylation site) are efficiently polyadenylated. This in vitro polyadenylation reaction also requires the AAUAAA sequence.

The AAUAAA sequence is required both for cleavage and for polyadenylation of simian virus 40 pre-mRNA in vitro

The sequence AAUAAA is found near the polyadenylation site of eucaryotic mRNAs. This sequence is required for accurate and efficient cleavage and polyadenylation of pre-mRNAs in vivo. In this study we show that synthetic simian virus 40 late pre-mRNAs are cleaved and polyadenylated in vitro in a HeLa cell nuclear extract, and that cleavage in vitro is abolished by each of four different single-base changes in AAUAAA. In this same extract, precleaved RNAs (RNAs with 3' termini at the polyadenylation site) are efficiently polyadenylated. This in vitro polyadenylation reaction also requires the AAUAAA sequence.

Isolation and characterisation of keratin mRNA from the scale epidermis of the embryonic chick

The keratin polypeptides of the epidermis from the leg scale region of 17-day-old embryonic chicks were extracted as S-carboxymethylated derivatives and characterised by electrophoresis on SDS and pH 9.5 urea gels including a combination of both in two dimensions. Proteins were isolated that gave X-ray diffraction patterns typical of alpha- and beta- (avian feather) keratins. An mRNA fraction was isolated from 17-day-old scale tissue by guanidinium chloride extraction and sucrose gradient fractionation. The mRNA was translated in the wheat germ system to give a major product indistinguishable from the molecular weight class (Mr 14 500) of scale beta-keratin polypeptides. A cDNA library was constructed in pBR322 from a 15 S mRNA subfraction and two recombinant clones were selected by their strong hybridisation to cDNA prepared from the 15 S mRNA. The sequencing of these has yielded details of the relatedness of two scale keratin genes including their 3' untranslated regions. Almost half of the protein sequences of the two homologous scale keratins has been deduced and a notable feature of the scale keratin structure appears to be the presence of at least two sequence domains consisting of 13 amino acid repeats.

Regulation of messenger RNA stability in mouse erythroleukemia cells

The decay rates of several messenger RNA species were determined in mouse erythroleukemia cells. The t1/2 values for the actin and tubulin mRNAs were 16 to 26 hours and about seven hours, respectively. The globin mRNA, and two mRNA species subject to translation repression, the P40 and P21 mRNAs, were about as stable as the ribosomal RNA. A stable tubulin mRNA component also appeared to be present in the cells. Exposure of the cells to dimethylsulfoxide for 48 hours led to considerable increases in the rates of decay of all but the globin mRNA. The induction of erythroid differentiation caused by the drug appears to lead to activation of a mRNA-degradation process that affects individual species to different degrees. The newly synthesized actin and tubulin mRNAs lost their poly(A) rather rapidly. This was accompanied by accumulation of poly(A)-deficient mRNA chains, particularly in the case of actin mRNA. The steady-state distribution of mRNA components, determined by Northern blot analysis, also showed that the actin mRNA and one tubulin mRNA species have a high proportion of poly(A)-deficient molecules. The globin, P40 and P21 mRNAs showed little tendency to lose their poly(A) sequence. The steady-state globin and P40 mRNAs also had a low proportion of chains depleted of poly(A). For all five species, the proportions of poly(A)-deficient chains in newly synthesized mRNA were about the same in uninduced and induced cells, in spite of the large decreases in mRNA stability in the induced cells. The lack of correlation between tendency to lose poly(A) and rate of mRNA decay, and the large accumulation of poly(A)-deficient molecules in the cases of the actin and tubulin mRNAs suggest that the stability of mRNA is not determined solely by the presence of poly(A) on the RNA chains. The behavior of the untranslated species in induced and uninduced cells also fails to support the notion of a relationship between translation and mRNA decay.

Evidence for post-transcriptional regulation by insulin of 3-hydroxy-3-methylglutaryl coenzyme A reductase and sterol synthesis in human mononuclear leucocytes

Incubation of freshly isolated human mononuclear leucocytes in lipid-depleted serum for 4 h resulted in a two-fold increase in 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity. Insulin, when added to the incubation medium at concentrations of 10 and 100 nmol/l at zero time, caused additional increases in the enzyme activity of 30% and 37%, respectively. The hormone action was not immediate because no effect was observed when insulin was added at 4 h and activity examined thereafter. Under these conditions sterol synthesis from 14C-acetate and tritiated water was strictly proportional to the activity of HMG-CoA reductase. Cycloheximide (20 micrograms/ml), a translational inhibitor of protein synthesis, prevented the insulin-mediated increase in the enzyme activity and the incorporation of 14C-acetate into sterols. Cordycepin (50 micrograms/ml) inhibited messenger RNA synthesis by greater than 50%, but had no inhibitory effect on the induction of HMG-CoA reductase and sterol synthesis. Low density lipoprotein (80 micrograms protein/ml) and complete serum blocked the induction of the enzyme and sterol synthesis from 14C-acetate caused by lipid-depleted serum. The insulin-effect, however, remained unchanged. The results suggest that insulin may regulate the de novo synthesis of HMG-CoA reductase and accordingly sterol synthesis at a post-transcriptional level.

Effect of convulsions of the synthesis of heterogeneous nuclear RNA associated with polyadenylate and oligoadenylate sequences from El mouse brain as a convulsive strain

Animals from the El (susceptible to seizures) and ddy (nonsusceptible) mouse strains were subjected to vestibular stimulation by tossing. After convulsions in the El mice, both the stimulated El mice and ddY mice were intracranially injected with [14C]- and [3H]adenosine, respectively. In the control experiment, nonstimulated El and ddY mice received radioactive adenosines in the same manner. The rate of incorporation of adenosine into brain nuclear RNA, expressed as a percentage of the 3H; 14C ratio, was reduced to an average of 68% at 15 min after convulsions, then increased and reached a control value at 5 h. This reduction in nuclear RNA synthesis was not due to alteration of the adenosine triphosphate pool. Gel electrophoresis of RNA revealed no obvious differences in the labeling distribution between El and ddY mice, but the synthesis of RNA species larger than 35S in heterogeneous nuclear RNA (HnRNA) was impaired in convulsed El mice. Nuclear resistant segments of HnRNA with both T1 RNase and RNase A, were chromatographed with poly(U)-Sepharose followed by urea-polyacrylamide gel electrophoresis. The ologo(A) and poly(A) segments consisted of 29, 19, and 11, and 203, 135, and 69 nucleotides, respectively. The convulsions of El mice reduced the incorporation of radioactive adenosines into oligo(A) and poly(A) segments, suggesting that they inhibited transcription as well as polyadenylation within HnRNA.

Messenger RNA species partially in a repressed state in mouse sarcoma ascites cells

Four major mRNA species of mouse sarcoma ascites cells, coding for polypeptides designated P65, P40, P36, and P21, occur predominantly as untranslated messenger ribonucleoprotein particles. Cloned cDNA probes were used to study their distribution in cytoplasmic extracts of these cells. A considerable portion of the mRNA molecules sedimented as small particles, whereas the rest was present in polyribosomes. In contrast, the actin mRNA was present almost exclusively in polyribosomes. Incubation of the ascites cells in culture medium, particularly after a starvation treatment, caused an enhancement in polypeptide chain initiation relative to elongation in these cells, as evidenced by a shift of ribosomes into the polyribosome fraction and by an increase in polyribosome size. Exposure of the cells to a low concentration of cycloheximide, an inhibitor of the elongation step, had a similar effect. The actin mRNA and the active P65, P40, P36, and P21 mRNA molecules were shifted to larger polyribosomes in the treated cells, but no shift of molecules from small particles to polyribosomes was observed. The incubation in culture also led to considerable increases in the proportion of P65 and P40 mRNA molecules in the untranslated state. The results indicate that the untranslated state cannot be attributed to poor initiation efficiency. It is suggested that a portion of the mRNA molecules is maintained in a repressed state and that mRNA repression may represent an important translation control process.

Periodicity in the length of 3'-poly(A) tails from native globin mRNA of rabbit

Globin mRNA from rabbit reticulocytes was labelled at the 3'-end with [5'-32P]pCp by T4 RNA ligase. The 3'-poly(A) tail was released by digestion of mRNA with T1 ribonuclease and its size distribution determined by gel electrophoresis and autoradiography. The length of the 3'-poly(A) tails varied from about 15-150 residues, but the size distribution exhibited peaks in the abundance of poly(A) species at intervals of approx. 25 residues. This periodicity appears to reflect the manner in which proteins bind to the 3'-poly(A) tail. The function of such regular interactions may be to control mRNA breakdown in the cytoplasm.

Inhibitory effects of 3'deoxycytidine 5'-triphosphate and 3'-deoxyuridine 5'-triphosphate on DNA-dependent RNA polymerases I and II purified from Dictyostelium discoideum cells

3'-Deoxycytidine 5'-triphosphate and 3'-deoxyuridine 5'-triphosphate were synthesized starting from cordycepin in good yield. The inhibitory effects of these nucleotides were examined in comparison with that of cordycepin 5'-triphosphate (3'-dATP) using purified DNA-dependent RNA polymerases I and II from Dictyostelium discoideum cells. Both nucleotide analogues strongly and competitively inhibited the incorporations of CTP and UTP into RNA by the RNA polymerases. The Km and Ki values for CTP and 3'-dCTP were 6.3 micro M and 3.0 micro M, respectively, and those for UTP and 3'-dUTP were 6.3 micro M and 2.0 micro M, respectively. These two analogues will be useful in studies at the molecular level on the relationship of template and substrate in RNA synthesis with chromatin, isolated nuclei or permeable cells, because they do not have any effect on poly (rA) synthesis.

Proteins associated with poly(A) and other regions of mRNA and hnRNA molecules as investigated by crosslinking

The proteins associated with poly(A) and other regions of mRNA and hnRNA molecules in mouse L cells were investigated with the aid of ultraviolet light-induced crosslinking of proteins to RNA. The poly(A)s of polyribosomal and free cytoplasmic mRNAs are associated with a protein, p78A. In contrast, the poly(A) of hnRNA is associated with a smaller protein, p60A, that differs from p78A in its partial peptide map. p78A occurs free in the cytoplasm, but p60A does not. There is a second 78 kd protein, p78X, associated with mRNA sequences other than poly(A). p78X differs from p78A in its partial peptide map. The total proteins crosslinked to polyribosomal and free cytoplasmic mRNAs are similar. However, the total proteins crosslinked to hnRNA are quite different from those crosslinked to mRNA. We suggest that newly synthesized mRNA molecules emerging from the nucleus into the cytoplasm shed the proteins with which they were associated in the nucleus and become associated with a new set of proteins derived from the cytosol. Furthermore, the cytoplasmic mRNA-associated proteins continue to exchange with free proteins.

Stability of non-polyadenylated viral mRNAs injected into frog oocytes

The naturally non-polyadenylated mRNAs of reovirus were shown to have a half-life in excess of 3 days when injected into Xenopus laevis oocytes. Hybridization analysis and gel electrophoresis showed that all 10 mRNA species had a similar high stability, despite being translated at widely differing rates. We have confirmed previous findings indicating a role for the 5'-terminal cap structure in determining reovirus mRNA stability [Furuichi et al. (1977) Nature (Lond.) 266, 235-239]. The significance of these results in relation to a general role for poly(A) in messenger RNA function is discussed.

Fractionation and characterization of polyadenylated RNA from broad bean meristematic root cells

Several populations of polyadenylated RNA from Vicia faba méristematic root cells were fractionated by stepwise thermal elution from poly(U)-Sepharose following sequential phenol extraction. Analysis of these fractions showed that the size of the poly(A) segment could influence this fractionation, but in some cases other characteristics of the molecule are involved. Evidence was obtained that 45-60% of the nucleotides of plant polyadenylated RNA are in base paired regions, as was previously demonstrated for mammalian mRNA.

Long-lasting effects of electroconvulsive shock on the pattern of poly(A)-RNA synthesis in rabbit cerebral cortex

The pattern of poly(A)-associated [poly(A)+] RNA synthesis was studied in rabbit cerebral cortex in the period following a single electroconvulsive shock (ECS). Labeled uridine was injected into the brain 2 and 4 hr after ECS and the animals sacrificed 1 hr later. Total and poly(A)+ RNA were then prepared from cortical nuclei and microsomes and analyzed. The amounts of newly synthesized total and poly(A)+ RNA in nuclei and microsomes appeared to be close to the control. However, the pattern of newly synthesized poly(A)+ nuclear RNA appeared to be still displaced toward the high molecular weights as it was in the early post-ECS period. The result indicates a long-lasting disturbance of brain poly(A)+-RNA metabolism by ECS.

The Role of the poly(A) sequence in mammalian messenger RNA

The poly(A) sequence is added to 3' termini of nuclear RNA segments destined to become part of the mRNA, and may play an essential role in the selection of these segments. It appears to be required for at least some of the splicing events involved in mRNA processing. In the cytoplasm, the poly(A) segment is the target of a degradation process which causes its gradual shortening, and leads to a heterogeneous steady-state poly(A)-size distribution. Complete loss of the poly(A) is probably followed by inactivation of the mRNA, since chains depleted of poly(A) do not accumulate in the cells. A role for this sequence in the promotion of mRNA stability is suggested by the behavior of globin mRNA depleted of poly(A) after injection into frog oocytes. The poly(A) shortening process may be part of the mRNA inactivation mechanism, as indicated by the greater sensitivity to degradation of the poly(A) of some short-lived mRNAs. However, the stochastic mRNA decay implies that new and old mRNA chains, with long and short poly(A) segments, respectively are equally susceptible to inactivation. The poly(A)-lacking histone mRNAs are stable only in cells engaged in DNA replication. Present knowledge favors a role for poly(A) in the control of mRNA stability. Loss of this sequence could be controlled through modulation of poly(A)-protein interactions or through masking of a sequence directly adjacent to the poly(A). In the nucleus, the poly(A) sequence could also serve as stabilizing agent, but, in addition, it might interact with the splicing machinery.

Post-transcriptional modification of the poly(A) length of galactose-1-phosphate uridyl transferase mRNA in Saccharomyces cerevisiae

Thermal elution poly(U)-Sepharose chromatography was utilized to fractionate yeast mRNA based on poly(A) size. Analysis of the in vitro translation products of the fractionated RNAs in a wheat-embryo cell-free protein synthesis system shows a heterogeneous but equal distribution of these abundant translatable mRNAs in the different poly(A) size classes. By comparing the translational activity of inducible galactose-1-phosphate uridyl transferase mRNA, which can be monitored as a function of age, to contitutive mRNAs, we demonstrate that initially galactose-1-phosphate uridyl transferase mRNA has a uniformly large poly(A) tail which becomes heterogeneous and shorter with age in the cytoplasm. These observations are consistent with the previously observed cytoplasmic poly(A) catabolism in yeast and with cytoplasmic post-transcriptional modification of the poly(A) length of galactose-1-phosphate uridyl transferase mRNA.

Addition of poly(A) to nuclear RNA occurs soon after RNA synthesis

A kinetic analysis of the appearance of [3H]uridine label in RNA sequences that neighbor poly(A), as well as the incorporation of [3H]adenosine label into both the RNA chain and the poly(A) of poly(A)-containing molecules, shows that poly(A) is added within a minute or so after RNA chain synthesis in Chinese hamster ovary cells and HeLa cells. Previous conclusions by several groups (5-7) that poly(A) might be added as long as 20-30 min after RNA synthesis appear to be in error, and the present conclusion seems much more in line with several different types of recent studies with specific mRNAs that suggest prompt poly(A) addition (13-16).

The effect of diabetes and insulin on the polyadenylic acid-containing RNA of rat skeletal muscle

Poly(A)-containing RNA in various fractions of RNA from rat skeletal muscle has been detected and quantitated by hybridization to [3H]poly(U). Comparison has been made between the RNA in skeletal muscle from normal adult rats, from rats 2 days after induction of diabetes with streptozotocin, and from diabetic rats killed 60 min after injection of insulin. The poly(A)-containing RNA constituted a similar proportion of the total RNA in skeletal muscle from each of the three types of rats. In diabetes there was a decrease in the proportion of skeletal muscle ribosomes sedimenting as polyribosomes, but this was reversed after rats had been injected with insulin. However these changes were not associated with any alterations in the relative amounts of poly(A)-containing RNA in ribosomes isolated from rat skeletal muscle. Diabetes did not significantly alter the size distribution of the poly(A)-containing RNA or its poly(A) segment. If it is assumed that poly(A)-containing RNA is mRNA, these results imply that insulin stimulates protein synthesis in the skeletal muscle of diabetic rats by affecting the translation of pre-existing mRNA. Sucrose density gradient analysis showed that there was less poly(A)-containing RNA in the polyribosome region from diabetic rats than there was in that from normal rats. The balance of the poly(A)-containing RNA in the ribosomes from diabetic rats (presumed to be the untranslated mRNA) was found associated with rapidly sedimenting aggregated ribosomes, but its native form has not yet been determined.

Polyadenylate-deficient analogues of poly(A)-containing mRNA sequences in cultured AKR mouse embryo cells

Five to six percent (by mass) of AKR-2B mouse embryo cell polysomal RNA consists of messenger RNA sequences which may exist in polyadenylated form. In the steady state, however, only 30--40% of these molecules are retained by extensive passage over oligo(dT)-cellulose, the remainder being present in the form of poly(A)-deficient analogues. Within experimental limits, these poly(A)-deficient analogues contain representatives of all poly(A)-containing mRNA sequences in these cells. An analysis of the kinetics of hybridization of cDNA probes enriched for either abundant or rare poly(A)-containing mRNA sequences suggests that the frequency distributions of poly(A)-containing and poly(A)-deficient analogues are dissimilar, and that a relationship exists between the intracellular frequency of a given mRNA sequence and the number of poly(A)-deficient analogues of that sequence. High frequency sequences appear to be enriched in the poly(A)-containing fraction, while low frequency sequences are predominately associated with the poly(A)-deficient fraction, thus, poly(A) may play a role in the regulation of mRNA frequency in the cytoplasm.

Synchronization of MEL cell commitment with cordycepin

The response of differentiating MEL cells to the nucleotide analogue cordycepin reveals a previously unrecognized aspect of the molecular events which cause commitment of these cells to terminal erythroid differentiation. Cordycepin rapidly inhibits commitment of DMSO-treated MEL cells in a dose range which does not cause cytotoxicity. Reversal of cordycepin treatment in the presence of inducer leads to a rapid and synchronous commitment of a significant proportion of cells in the culture. These results suggest that MEL cells can be blocked just prior to the point of commitment by cordycepin treatment.

Regulation of sterol synthesis in human lymphocytes: evidence for post-transcriptional control by low density lipoprotein

The effect of cordycepin (3'-deoxyadenosine), an inhibitor of messenger RNA synthesis, on the induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase mediated by lipid-depleted serum was studied in isolated human lymphocytes. 50 micrograms/ml cordycepin, although inhibiting messenger RNA synthesis by more than 50%, had no inhibitory effect on the two and four-fold induction of hydroxymethylglutaryl-CoA reductase when cells were incubated in a medium containing lipid-depleted serum for 8 and 16 h, respectively. This result suggests that newly synthesises messenger RNA is not required for the effect of lipid-depleted serum on the induction of hydroxymethylglutaryl-CoA reductase in human lymphocytes.

Uptake of immune RNA by normal mouse spleen cells

Normal mouse spleen cells take up in vitro radioactively labeled immune RNA. RNA taken up is present in nuclei, polysomes, membranes and cytoplasm. About 20-40% of immune RNA is nonspecifically associated with cell surface. 45% of RNA taken up is degraded and reutilized inside the cells within 2 hours.