Polyribosomal RNA Research Articles

Characterization of translation products of the polyadenylated RNA of free and membrane-bound polyribosomes of rat forebrain.

Poly(A)+ RNA (polyadenylated RNA) isolated from membrane-bound and free polyribosomes was translated in reticulocyte lysates, and the products were analysed by two-dimensional gel electrophoresis. Several translation products were specific to membrane-bound polyribosomal mRNA, including polypeptides of 47kDa, 35kDa and 21 kDa, whereas others (e.g. of 37 kDa, 17 kDa and 14 kDa) were specific to free polyribosomal mRNA. Although many products were common to both mRNA species, cross-contamination could be ruled out on the basis of the presence of these and other specific products. The common products included a 68 kDa microtubule-associated protein, tubulin, actin, the brain form of creatine kinase, neuron-specific enolase and protein 14-3-3 and calmodulin, all of which were identified on the basis of two-dimensional gel and peptide analyses. The 35 kDa protein product of membrane-specific mRNA was co-translationally processed in vitro by microsomal membranes, resulting in its cleavage to 33 kDa (and partial glycosylation). The 33 kDa processed protein (but not the 35 kDa precursor) was integrated into both dog pancreas and rat brain microsomal membranes. The occurrence of the enzymes and calmodulin as products of membrane-bound polyribosomal mRNA is discussed in the light of their presence on rat brain synaptic plasma membranes [Lim, Hall, Leung, Mahadevan & Whatley (1983) J. Neurochem. 41, 1177-1182] and their existence in a specific component of axonal flow. It is suggested that some of these translation products of the rough endoplasmic reticulum may represent proteins destined for the plasma membrane. However, the identity and location of the 35 kDa membrane-specific product (or its processed form) still remain unestablished.

Differences in sequence content of nuclear and cytoplasmic polyribosomal RNA from adenovirus-infected cells.

RNA molecules from nuclear and cytoplasmic polyribosomes of adenovirus-infected HeLa cells were compared by hybridization to analyse the sequence content. Nuclear polyribosomes were released by exposure of intact detergent-washed nuclei to poly(U) and purified. Cytoplasmic polyribosomes were also purified from the same cells. To show that nuclear polyribosomes contain ribosomes linked by mRNA, polyribosomes were labelled with methionine and uridine in the presence of actinomycin D in adenovirus-infected cells. Purified nuclear polyribosomes were treated with EDTA under conditions which dissociate polyribosomes into ribosomes and subunits with a simultaneous release of mRNA, and sedimented. The treatment dissociated these polyribosomes, releasing the mRNA from them. Radiolabelled total RNA from each polyribosome population was fractionated in sucrose gradients into several pools or hybridized to intact adenovirus DNA to select virus-specific RNA. Sucrose-gradient-fractionated pool-3 RNA (about 28S) and virus-specific RNA were then hybridized to fragments of adenovirus DNA cleaved by restriction endonucleases SmaI, HindIII and EcoRI by the Southern-blot technique and by filter hybridization. The results showed that nuclear RNA contained sequences, from about 0 to 18 map units, which were essentially absent from cytoplasmic RNA. Furthermore, the amount of virus-specific RNA for a particular sequence was also different in the two populations.

Changes in Ribosome Organization and Messenger RNA Abundance in Ripening Tomato Fruits

The involvement of RNA and protein synthesis in fruit ripening was investigated. Mature-green tomato fruits were found to contain about 30% of their ribosomal RNA in polyribosomes. At the 'breaker' or early ripening stage, about 50% of rRNA was in polyribosomes and this distribution of rRNA was maintained until fruits were fully ripe. The continued presence of polyribosomes is consistent with active protein synthesis persisting through and beyond the climacteric period when the wall-hydrolysing enzyme polygalacturonase accumulates and fruits soften, synthesize lycopene and undergo other ripening related changes. Poly(A)-containing RNA purified from polyribosomes extracted from individual fruits was used to prime the synthesis of [35S]methionine-labelled polypeptides by a wheat germ in vitro translation system. The pattern of polypeptides synthesized in response to RNA from mature-green fruits differed from that given by RNA from ripening fruits. The majority of changes were found to occur within approximately 48 h of the increase of ethylene synthesis and were apparent in all fruits with any pink or red colour. Similar results were obtained by translating total cellular RNA and total polyribosomal RNA indicating that the major RNA species shown in this study to change in abundance during ripening are polyadenylated, and hence most probably cytoplasmic, and do not accumulate as 'stored messages' outside of the polyribosomes. The differences between green and ripening fruits in polyribosome profiles were demonstrated in two cultivars of tomato. Differences in mRNA populations between green and ripe fruits were found in three cultivars.

Isolation and sequence analysis of cDNAs for the major potato tuber protein, patatin.

A cDNA library from membrane-bound poly(A)+ mRNA of developing potato tubers was constructed and two classes of essentially full-length cDNA clones for a precursor to the major tuber glycoprotein, patatin were isolated. Sequence analysis shows that the two classes are approximately 99% homologous and correspond to the two major species of patatin identified previously by NH2-terminal amino acid sequence analysis. Sequence analysis also predicts that patatin is synthesized with a 23 amino acid signal sequence. Northern blot analysis shows that patatin mRNAs are 1550 +/- 50 nucleotides in length and are normally not present in polyribosomal or total RNA from stems or leaves.

Characterization of Viral-complementary RNA Associated with Polyribosomes from Tobacco Infected with Sonchus Yellow Net Virus

SUMMARY Polyribosomal RNA from tobacco infected with sonchus yellow net virus was resolved by electrophoresis on agarose gels, transferred to nitrocellulose and hybridized to radioactive viral RNA. Polyadenylated RNA contained four major viral complementary species, molecular weights 2.2 × 106, 0.83 × 106, 0.62 × 106 and 0.46 × 106, plus a minor species, molecular weight approx. 3 × 106. No such species could be detected in the non-polyadenylated RNA fraction. The polyadenylated RNA from membrane-bound polyribosomes contained only the 0.83 × 106 and 0.46 × 106 molecular weight RNAs. The sizes of the RNAs were consistent with the expected sizes of messages for the virus proteins, calculated from the molecular weight of these proteins.

Effect of interferon on the penetration of murine leukemia virus and the binding of its genome RNA to polyribosomes at the early stage of infection.

The effect of interferon (IFN) on the adsorption, penetration and subsequent binding of the incoming genome RNA of Moloney murine leukemia virus (MLV) to polyribosomes, was studied in NIH/3T3 mouse fibroblasts. Virus adsorption was assayed by determining reverse transcriptase activity in the inoculating virus stock and in the cell membrane fraction before and after 45 minutes of infection. Both measurements suggested that IFN had no effect on virus adsorption. Virus penetration was determined by measuring the amount of viral RNA in the cell cytoplasm at 45 minutes after infection. This amount was remarkably lower in IFN-treated than in untreated cells. This reduction was not due to inhibition of a possible induction of endogenous viral genetic information by the penetrating virions, but was proved to be a direct effect of IFN on virus penetration, which was related to the IFN-induced antiviral state. The effect of IFN on binding of parental genome RNA to polyribosomes was then investigated by analysing Crt hybridization kinetics of polyribosomal viral RNA at different time intervals after infection. While in untreated control cells maximal binding occurred at 3 hours postinfection, this maximal binding was observed in IFN-treated cells at 5 hours postinfection. The distribution of viral RNA molecules between sub-cytoplasmic fractions at 3 hours after infection was, in IFN-treated cells, significantly different from that observed in the untreated cells.

Polyribosome binding of rabbit globin messenger RNA and messenger ribonucleoprotein labelled with bacteriophage-T4 RNA ligase and 5′-[32P] phosphocytidine 3′-phosphate

Rabbit polyribosomal globin messenger RNA (mRNA) and messenger ribonucleoprotein (mRNP) were labelled at the 3' poly(A) tail to high specific activity with T4 RNA ligase and [5'-(32)P]pCp without consequent loss of functional activity. Labelled message was translated in both micrococcal nuclease treated and untreated rabbit reticulocyte lysates, as shown by the formation of labelled polyribosomes. The utilisation of labelled messenger was abolished by T2 toxin or sodium fluoride which are known to inhibit protein synthesis.Images

Restoration Effects of Glucose Refeeding on Reduced Synthesis of Albumin and Total Protein and on Disaggregated Polyribosomes in Liver of Starved Rats: Evidence of a Post-Transcriptional Control Mechanism

Previous studies demonstrated that fasting is accompanied with a reduction of liver protein and albumin synthesis. A protein-deficient diet also leads to a marked change in liver RNA and protein metabolism. Although the reduction of protein synthesis and the disaggregated polyribosomes during fasting can be corrected by a single feeding of protein or a complete amino acid mixture, no or little changes of amino acid concentrations were found in portal blood and liver cytosol of fasted animals as compared to those of the fed group. To determine the effect of glucose on the reduced rate of protein and albumin synthesis of fasted rats, free and membrane-bound polyribosomes were isolated quantitatively from liver of starved rats (42-66 h) at different intervals after a single feeding of glucose and after giving glucose ad libitum for 24 h. (1) The yield of polyribosomal RNA decreased dramatically after a 42- to 66-hour starvation. A glucose refeeding did not change the RNA content. However, the restoration of polyribosome size could be observed rapidly. (2) At various levels of RNA, there was a decreased protein synthesis in fasted animals. However, the synthesis was enhanced after glucose refeeding. The albumin synthesis was also proportionately increased (10-12% of total protein synthesis of membrane-bound polyribosomes). (3) Glucose refeeding had no influence on the content of albumin mRNA sequence and liver RNA. These findings suggest that the effect of glucose on the restoration of protein and albumin synthesis is a sole post-transcriptional event.

Protein synthesis by attached pulmonary macrophages effect of phagocytosis

We studied the effect of phagocytosis of polystyrene latex beads on protein synthesis by pulmonary macrophages. To do this we determine the specific radioactivity of extracellular and intracellular free phenylalanine and of phenylalanine released from tRNA and used this information in calculating the rates of protein synthesis. Phagocytosis resulted in an increased rate of protein synthesis irrespective of which precursor specific radioactivity was used in the calculation. The rate of protein synthesis was increased per μg polyribosomal RNA; but there was no increase in the amount of polyribosomal RNA in phagocytizing macrophages. The increase in the rate of protein synthesis (1.4-fold) was almost identical to the increase (1.3-fold) in the rate of ribosome transit in phagocytizing compared to nonphagocytizing macrophages. The decreased ribosome transit time during phagocytosis occurred without a fall in the average molecular weight of macrophage proteins. We conclude that phagocytosis increases the rate of protein synthesis in attached pulmonary macrophages and that this increased rate of synthesis can be accounted for almost completely by an increased rate of polypeptide chain elongation and/or termination.

Hybridization selection and translation of T-DNA encoded mRNAs from octopine tumors

To characterize the functions of T-DNA derived transcripts in plant tumor cells, we isolated these RNAs by a highly sensitive hybridization selection procedure and investigated whether they are translatable into proteins in wheat germ extracts. Results with two independent tobacco cell lines transformed with octopine plasmids pTiA6 and pTiB6S3 show that the cells contained at least three translatable T-DNA derived mRNAs. Each represented 0.0001% or less of the total mRNA activity in polyribosomal RNA. All were detected in polyadenylated as well as in nonpolyadenylated RNA fractions, and translation was inhibited by the cap analogue pm7G. Two of the proteins were encoded close to the right and the left end of the T-DNA (proteins Mr 39,000 and 14,000, respectively); the third protein was derived from the middle of the T-DNA. The results indicate that genetic manipulation of plants by Agrobacteria involves transfer of several genes which are expressed into proteins in eucaryotic cells.

Evidence that Tomato Spotted Wilt Virus RNA is a Positive Strand

SUMMARY RNA of tomato spotted wilt virus (TSWV) was analysed by gel electrophoresis on methylmercury-agarose gels. Three segments were detected with mol. wt. of 2.7 × 106, 1.7 × 106 and 1.1 × 106. The behaviour on oligo(dT)-cellulose columns indicated that TSWV RNA does not contain poly(A) sequences. TSWV RNA directed the synthesis of several proteins in a wheatgerm cell-free system and a messenger-dependent rabbit reticulocyte lysate. Analysis of the translation products by gel electrophoresis revealed that two major polypeptides were synthesized in both systems. One could be identified by immunoprecipitation as the nucleocapsid protein; the identity of the other, having a mol. wt. of 60000, is unknown. The above results demonstrate that TSWV contains a positive-stranded RNA and this conclusion was confirmed by other experiments. TSWV RNA labelled with 125I did not hybridize with polyribosomal RNA from infected leaves and transcriptase activity could not be detected in purified preparations of TSWV or in nucleocapsid extracts from infected leaves. This study supports the view that TSWV is a unique membrane-bounded plant virus whose genome consists of three RNA segments which act as mRNA.

Cell-Free Synthesis of Wheat Prolamins

Total RNA, polyribosomes and polyribosomal RNA have been isolated from immature wheat grain and used to prime the wheat-germ cell-free protein synthesis system to produce storage protein. The synthesis of wheat prolamins, a major fraction of storage proteins, was mainly associated with membrane-bound polyribosomes. The ratio of bound to free polyribosomes increased from one at 8 days to approximately three at 32 days post-anthesis. SDS polyacrylamide gel electrophoresis and fluorography of the prolamins showed that the same components were being synthesized and accumulated from 12 days post-anthesis onward. Evidence is presented that prolamins within the molecular weight range 35 000-45 000 may be translated with a signal sequence attached. These signal sequences are rapidly removed during translation and further post-translational modifications may occur over longer periods.

Metabolism of viral RNA in murine leukemia virus-infected cells; evidence for differential stability of viral message and virion precursor RNA.

Molecular hybridization techniques were used to examine the stability of viral message and virion precursor RNA in murine leukemia virus-infected cells treated with actinomycin D. Under the conditions used, viral RNA synthesis was inhibited, but viral protein synthesis continued, and the cells produced noninfectious particles (actinomycin D virions) lacking genomic RNA (J. G. Levin and M. J. Rosenak, Proc. Natl. Acad. Sci. U.S.A. 73:1154-1158, 1976). Analysis of total RNA in virions revealed that the amount of hybridizable viral RNA decreased steadily after the addition of actinomycin D and by 8 h was 10% of the control value. Studies on fractionated viral RNA showed that this low level of hybridization is due to residual 70S RNA in the virion population. The results indicated that viral RNA which is destined to be encapsidated into virions has a half-life of approximately 3 to 4 h. In contrast, other intracellular virus-specific RNA molecules appeared to be quite stable and persisted for a long period of time, with a half-life of at least 12 h. These observations support the idea that two independent functional pools of 35S viral RNA exist within the infected cell: one serving as message and the other as precursor to virion RNA. The existence of two viral RNA pools was further documented by the finding that 12 h after the addition of actinomycin D, when virion precursor RNA was depleted, 35S and 21S viral nRNA species could be identified in polyribosomal RNA as well as in total polyadenylated cell RNA. Surprisingly, 35S and mRNA declined more rapidly than did 21S mRNA, which appeared to be increased in amount.

Acute effects of ethanol intake on albumin and total protein synthesis in free and membrane-bound polyribosomes of rat liver

Controversial results have been reported in the last few years concerning the effects of ethanol on hepatic protein synthesis. In most of the studies no distinction has been made between the synthetic capabilities of the poly-ribosomes and the secretory product of labelled protein by the hepatocytes. In order to assess the influence of a single feeding of ethanol on the synthesis of albumin and total protein by the polyribosomes of rat liver, free and membrane-bound polyribosomes were isolated quantitatively from rats given 4–8 g ethanol per kg body weight 3–5 h before killing. The following results were obtained: (1) No difference was found in yield and size of free and membrane-bound polyribosomes isolated from control and ethanol-treated rats. The abilities to synthesize albumin and total protein were also equal for polyribosomes from both groups. (2) Addition of 1% ethanol to the incubation mixture of protein synthesis lowered albumin and total protein synthesis by 20%. No effect was observed with 0.5% ethanol. (3) Cell sap prepared from ethanol-treated rats contains a factor or factors which stimulate protein synthesis (10–15%). (4) The albumin mRNA sequence content was not changed in free and membrane-bound polyribosomal RNA fractions of ethanol-treated rats as compared to the control animals.

Ribonucleoprotein organization of eukaryotic RNA: XV. Different nucleoprotein structures of globin messenger RNA sequences in nuclear and polyribosomal ribonucleoprotein particles

Heterogeneous nuclear RNA and polyribosomal messenger RNA are both complexed with specific sets of proteins in the cell, forming ribonucleoprotein complexes known as hnRNP and mRNP, respectively. In the present investigation, the nucleoprotein structures of globin mRNA sequences in hnRNP and mRNP were probed by digestion with nuclease, under conditions in which RNA-protein rearrangements were shown not to occur. Mild digestion with pancreatic RNAase of a Friend erythroleukemia cell RNP fraction containing both hnRNP and mRNP resulted in a preferential depletion of globin mRNA-homologous sequences, as measured by hybridization of the surviving RNA with globin complementary DNA. Hypersensitivity to nuclease typifies 65% of the globin mRNA-homologous sequences, with the other 35% remaining relatively nuclease-resistant. Removal of polyribosomal mRNP by release with EDTA, followed by re-isolation of hnRNP on a sucrose gradient eliminated the nuclease-hypersensitive class of globin mRNA sequences in favor of the relatively nuclease-resistant class. These results suggest that mRNA sequences are more nuclease-sensitive in polyribosomal mRNP than they are in nuclear hnRNP particles. The implication is that mRNA sequences undergo a significant change in RNP structure at some point during their movement from nucleus to cytoplasm.

Characterization of a major late herpes simplex virus type 1 mRNA.

A major, late 6-kilobase (6-kb) mRNa mapping in the large unique region of herpes simplex virus type 1 (HSV-1) was characterized by using two recombinant DNA clones, one containing EcoRI fragment G (0.190 to 0.30 map units) in lambda. WES.B (L. Enquist, M. Madden, P. Schiop-Stansly, and G. Vandl Woude, Science 203:541-544, 1979) and one containing HindIII fragment J (0.181 to 0.259 map units) in pBR322. This 6-kb mRNA had its 3' end to the left of 0.231 on the prototypical arrangement of the HSV-1 genome and was transcribed from right to left. It was bounded on both sides by regions containing a large number of distinct mRNA species, and its 3' end was partially colinear with a 1.5-kb mRNA which encoded a 35,000-dalton polypeptide. The 6-kb mRNA encoded a 155,000-dalton polypeptide which was shown to be the only one of this size detectable by hybrid-arrested translation encoded by late polyadenylated polyribosomal RNA. The S1 nuclease mapping experiments indicated that there were no introns in the coding sequence for this mRNA and that its 3' end mapped approximately 800 nucleotides to the left of the BglII site at 0.231, whereas its 5' end extended very close to the BamHI site at 0.266.

Detailed characterization of the mRNA mapping in the HindIII fragment K region of the herpes simplex virus type 1 genome.

We have isolated as recombinant DNA clones, in the plasmid pBR322, regions of the herpesvirus type 1 genome spanning the region between 0.53 and 0.6 on the prototypical arrangement. This 11,000-base-pair region corresponds to 10% of the large unique region and encodes five major and several minor mRNA species abundant at different times after infection, which range in length from 7 to 1 kilobase. In this report, we have used RNA transfer blots and S1 nuclease digestion of hybrids between viral DNA and polyribosomal RNA to precisely localize (+/- 0.1 kilobase) these mRNA's. Comparison of neutral and alkaline gels of S1 nuclease-digested hybrids indicates no internal introns in the coding sequences of these mRNA's, although noncontiguous leader sequences near (ca. 0.1 kilobase) the 5' ends of any or all mRNA's could not be excluded. The 5' ends of several late mRNA's that are encoded opposite DNA strands map very close to one another, and the 3' ends of a major late and a major early mRNA, which are partially colinear, terminate in the same region. In vitro translation of the viral mRNA's isolated by hybridization with DNA bound to cellulose and fractionation of mRNA species on denaturing agarose gels allowed us to assign specific polypeptide products to each of the mRNA's characterized. Among other results, it was demonstrated unequivocally that two major late mRNA's, which partially overlap, encode the same polypeptide.

Precursor Forms of Pea Vicilin Subunits

Polyribosomal RNA isolated from pea cotyledons at various developmental stages programmed the cell-free synthesis of polypeptides which were recognized by antibodies specific for pea storage proteins. There were quantitative and qualitative changes in the template activity during seed maturation. Most of the polysomal RNA was associated with the membrane fraction, and all of the template for storage protein occurred in this fraction. Using RNA from a stage of seed maturation at which the synthesis of the high-molecular weight vicilin polypeptides predominate, it was found that the major translation products, although antigenically recognizable as storage protein, did not coincide with the authentic vicillin polypeptides on denaturing polyacrylamide gels. The addition during translation of microsomal membranes from dog pancreas or pea cotyledons resulted in the appearance of new polypeptides which did coincide with some of the authentic vicilin polypeptides (in the apparent molecular weight regions of 75,000 and 50,000) and were antigenically recognizable as storage protein. Other translation products related to storage protein were not visibly altered in their electrophoretic mobility by the addition of membranes. Microsomal membranes treated with Triton X-100 were not effective in modifying the cell-free products. The modified vicilin polypeptides and at least two other translation products were protected from proteolytic degradation, suggesting that they were sequestered within microsomal vesicles. Thus, these storage protein components may be synthesized by a mechanism analogous to that described for membrane and secretory proteins (Blobel G, B Dobberstein 1975 J Cell Biol 67: 835-851).

Ethanol and cycloheximide alter protein and RNA synthesis of Cox astrocytoma cells in culture.

AbstractChronic ethanol ingestion or cycloheximide treatment results in alterations in the properties and synthesis of protein and RNA of polyribosomes in the whole brain. To analyze the effects on a homogeneous neural cell population, Cox astrocytoma (glioma) cells were grown in tissue culture media with 100 mM ethanol or 0.017 mg/ml of cycloheximide. When ethanol had been present for ten days, the cell densities remained unchanged but had markedly reduced RNA and protein contents. Furthermore, the ethanol treatment reduced the whole‐cell pulse‐labeling of RNA with (5–3H) orotic acid and protein with (14C) leucine in the postmitochondrial supernatant. These results suggest that chronic ethanol treatment reduced the whole cell synthesis of RNA and protein or increased their degradation. Analysis of the polyribosomes on sucrose density gradients showed that dense polyribosomal chains were decreased after the ethanol treatment, supporting the concept that the polyribosomes were degraded with an alteration in the metabolism of mRNA. The cell‐free incorporation of (14C) leucine into hot TCA precipitable protein by the purified polyribosomes in the presence of ATP, GTP, a heterologous source of soluble factors, and endogenous mRNA was also reduced following the ethanol treatment, further indicating that the previous chronic exposure to ethanol had inhibited the translation of mRNA. When the control cells were grown in the presence of cycloheximide for one hour prior to harvesting, the cell densities remained unchanged, but again, as with the ethanol treatment, the polyribosomal protein and RNA yields decreased. In contrast to ethanol, however, cycloheximide treatment caused increases both in the whole‐cell incorporation of labeled RNA and protein precursors into the supernatant fraction and in the cell‐free incorporation of (14C) leucine into protein. These results suggest that, like the ethanol effects, cycloheximide reduces the total polyribosomes, but unlike the ethanol effects, the remaining polyribosomes have stable mRNA and rapidly incorporate radioactive amino acids, even more than untreated controls. The one‐hour cycloheximide treatment also caused an increase in the ratio of dense polyribosomes to monosomes plus 40s and 60s ribosomal subunits of control cells. In addition, it increased the incorporation of the labeled precursor into protein in the polyribosomal region of the sucrose gradients of both control and ethanol treated cells, suggesting that cycloheximide inhibited the termination step of protein synthesis. When cycloheximide was present for 24 hours prior to harvesting, the ethanol treated cells, in contrast to the controls, still had increased cell‐free incorporation of amino acid into protein, indicating that the stimulatory effects of cycloheximide are prolonged to 24 hours when ethanol is present. Thus, while the ethanol treatment in general inhibits polyribosomal biogenesis in the cells, it alters the complex of stimulating and then inhibiting effects of cycloheximide by preserving cycloheximide's stimulating effects on the amino acid incorporation activities of the polyribosomes.

Polyribosome formation and poly(A)-containing RNA in embryos of the sand dollar,Dendraster excentricus.

The pattern of appearance of ribosomes, newly synthesized mRNA, and poly(A)-containing mRNA in polyribosomes has been examined in sand dollar embryos. From early blastula until shortly before hatching small polyribosomes engaged in histone synthesis predominate. At the time of hatching, when the rate of cell increase is maximal, the proportion of poly(A)-containing RNA in polyribosomes is low. After hatching a new class of large polyribosomes appears and the amount of poly(A)-containing polyribosomal RNA increases. Cordycepin, an inhibitor of RNA adenylylation, prevents the appearance of the large polyribosomes after hatching as well as the increase in poly(A)-containing polyribosomal RNA.