Translational efficiency of cytoplasmic nonpolysomal messenger ribonucleic acid from sea urchin embryos

Informational content of the echinoderm egg

The sea urchin egg contains a store of mRNA synthesized during oogenesis but translated only after fertilization, which accounts for a large, rapid increase in the rate of synthesis of largely the same set of proteins synthesized by eggs. Starfish oocytes contain a population of stored maternal mRNA that becomes actively translated upon GVBD and codes for a set of proteins distinct from that synthesized by oocytes. The sequence complexity of RNA in echinoderm eggs is about 3.5 x 10(8) nucleotides, enough to code for about 12,000 different mRNAs averaging 3 kb in length. About 2-4% of the egg RNA functions as mRNA during early embryonic development; most of the sequences are rare, represented in a few thousand copies per egg, but some are considerably more abundant. Many of the stored RNA sequences accumulate during the period of vitellogenesis, which lasts a few weeks. The mechanisms of storage and translational activation of maternal mRNA are not well understood. Histone mRNAs are sequested in the egg pronucleus until first cleavage, but other mRNAs are widely distributed in the cytoplasm. The population of maternal RNA includes many very large molecules having interspersed repetitive sequence transcripts colinear with single-copy sequences. The structural features of much of the cytoplasmic maternal RNA is thus reminiscent of incompletely processed nuclear precursors of mRNA. The functional role of these strange molecules is not understood, but many interesting possibilities have been considered. For instance, they may be segregated into different cell lineages during cleavage and/or they may become translationally activated by selective processing during development. Maternal mRNA appears to be underloaded with ribosomes when translated, possibly because the coding sequences are short relative to the size of the mRNA. Most abundant and many rare mRNA sequences persist during embryonic development. The rare sequence molecules are replaced by newly synthesized RNA, but some abundant maternal transcripts appear to persist throughout embryonic development. Most of the proteins present in the egg do not change significantly in mass during development, but a few decline or accumulate substantially. Together, these observations indicate that much of the information for embryogenesis is stored in the egg, although substantial changes in gene expression occur during development.

Rapid changes in tubulin RNA synthesis and stability induced by deflagellation in Chlamydomonas

Detachment of the flagella of Chlamydomonas induces a rapid accumulation of mRNAs for tubulin and other flagellar proteins. Measurement of the rate of alpha and beta tubulin RNA synthesis during flagellar regeneration shows that deflagellation elicits a rapid, 4-7-fold burst in tubulin RNA synthesis. The synthesis rate peaks within 10-15 min, then declines back to the predeflagellation rate. Redeflagellation of cells at times before the first flagellar regeneration is completed (and when cells have already accumulated elevated levels of tubulin RNA) induces another burst in tubulin RNA synthesis which is identical to the first in magnitude and duration. This finding indicates that the induction signal may act to simply reprogram the tubulin genes for a transient burst of maximal synthesis. Evidence is presented that the stability of the tubulin RNAs changes during regeneration. Stability changes include both an apparent stabilization during regeneration and accelerated decay following regeneration.

Interspersed poly(A) RNAs of amphibian oocytes are not translatable

The poly(A) RNA of the Xenopus oocytes has been shown to include both single copy and interspersed transcripts. Interspersed maternal poly(A) RNAs contain repetitive sequence elements distributed within regions transcribed from single copy sequences. When renatured these RNAs form partially double-stranded RNA networks, and as shown earlier this can be utilized for preparative separation of interspersed maternal transcripts from maternal transcripts that remain single-stranded after renaturation (Anderson et al., 1982). The translational activity of these RNA fractions was tested in vitro, in wheat germ and reticulocyte systems. While the single-stranded fractions supported protein synthesis, the interspersed oocyte RNAs displayed little translational activity. Translational activity was measured in vivo by injection into the Xenopus oocyte. Oocytes previously injected with globin mRNA were injected with increasing amounts of single-stranded, double-stranded, or denatured double-stranded RNA fractions, and the amount of globin synthesis was determined. It was found that single-stranded RNA competes with globin mRNA for the limited translational apparatus of the oocyte, as manifested by a quantitative reduction of globin synthesis. However, globin synthesis was not affected when double-stranded RNA, either in renatured or denatured form, was injected. We conclude that the interspersed RNAs are not translated within the oocyte. The amount of single and double-stranded RNAs loaded onto polysomes in the injected oocytes was also determined. Sixty seven per cent of radio-iodinated single-stranded RNA pelleted with polysomes in injected oocytes, whereas less than 20% of similarly labeled double-stranded RNA pelleted with polysomes. This value is similar to that obtained when partially hydrolyzed RNA is injected, suggesting again that essentially none of the interspersed RNA is translated in vivo. The significance of these findings in relation to translational regulation during oogenesis and early development is discussed.

Regulation of sea urchin glycoprotein mRNAs during embryonic development

Gastrulation in sea urchin embryos is accompanied by a striking increase in the synthesis of N-linked glycoproteins, and inhibitors of this process block gastrulation. In this report, the messages coding for N-glycosylatable proteins in the developing embryo of the sea urchin, Strongylocentrotus purpuratus, were examined. Total mRNA and mRNA isolated from membranes of the embryos at various stages of development were used to program a cell-free translation/glycosylation system prepared from rabbit reticulocyte lysate supplemented with dog pancreas microsomes. The glycosylated translation products were separated from the nonglycosylated products by concanavalin A-agarose and analyzed by gel electrophoresis. The results indicate that although the RNA derived from the membranes of gastrula-stage embryos contains messages coding for numerous glycoproteins, only trace amounts of glycoprotein messages are associated with membranes at earlier stages of development. mRNAs coding for four glycoproteins of M(r)s 70,000, 65,000, 51,000, and 30,000 were examined further in total RNA preparations from the developing embryo. The data indicate that the messages coding for the glycoproteins of M(r)s 65,000 and 51,000 are present also in the unfertilized egg and in the pregastrulation embryo. Because these two messages are not found associated with the membranes until gastrula stage, it is likely that the synthesis of these glycoproteins during gastrulation is regulated at the translational level. The messages coding for glycoproteins of M(r)s 70,000 and 30,000, on the other hand, are not detectable in the unfertilized egg and may be synthesized de novo by the embryos. Thus, the expression of these two glycoproteins during gastrulation is regulated at least in part on the transcriptional level. On the basis of these findings, it appears that different modes of regulation are used for different glycoproteins that are synthesized during gastrulation.

Inhibition of cell-free protein synthesis by low-molecular-weight RNAs from free cytoplasmic ribonucleoprotein particles

Free cytoplasmic messenger ribonucleoprotein (mRNP) particles from rat liver were treated with EDTA and separated into two populations of RNP particles with sedimentation maxima of 20 S and 35 S respectively. The 20-S and 35-S RNP particles, treated with 0.5 M KCl, have protein-to-RNA ratios of 0.31:1 and 5.7:1 respectively. Whereas 20-S and 35-S RNP particles exhibit a similar protein complement of seven major polypeptides, the low-molecular-weight RNA components of the two particle populations are different. A characteristic set of distinct low-molecular-weight RNAs is found for 20-S and 35-S RNP particles. When the individual low-molecular-weight RNAs of 20-S and 35-S RNP particles isolated from preparative polyacrylamide gels were assayed for their capability to inhibit protein synthesis in vitro, several potent translational inhibitory RNAs were detected. In particular, the low-molecular-weight RNAs of 147, 203 and 263 nucleotides in length associated with the 35-S RNP particles turned out to be strong inhibitors of protein synthesis.

Nonrandom distribution of histone mRNAs into polysomes and nonpolysomal ribonucleoprotein particles in sea urchin embryos

In the early sea urchin embryo, newly synthesized cytoplasmic histone mRNA is found both on polysomes and free of ribosomes as nonpolysomal messenger ribonucleoprotein particles (free RNPs). The distribution of newly synthesized histone mRNAs between translating and nontranslating compartments is nonrandom and dependent on the developmental stage. Gel electrophoresis and autoradiography of polysomal and free RNP RNA from embryos at various stages show that (i) the fraction of total newly synthesized histone mRNA that is in polysomes is greater than the fraction of total newly synthesized poly(A)+mRNA that is in polysomes, at all stages examined, and (ii) among the five histone mRNAs. H1 mRNA and H4 mRNA are relatively more enriched in the free RNPs than are the mRNAs for H2A, H2B, and H3. These data suggest that histone mRNA, as a class, is more efficiently utilized as a template than the average mRNA and, of the cytoplasmic histone mRNAs, the mRNAs for histones H2A, H2B, and possibly H3 are selected more frequently for translation than those for H1 and H4. Cell-free translations of polysomal and free RNP RNAs yield different ratios of in vitro histone products, consistent with the RNA distribution data. To test the possibility that the in vivo distribution of the histone mRNAs is the consequence of different intrinsic initiation capabilities of the individual mRNAs, ribosome-binding assays were carried out and unequal binding abilities of the histone mRNAs in the reticulocyte lysate were shown. A translational level component in the regulation of histone synthesis in the sea urchin embryo is indicated.

Effect of secondary structure of messenger ribonucleic acid on the formation of initiation complexes with prokaryotic and eukaryotic ribosomes

The effect of modification of the secondary structure of phage f2 RNA and brome mosaic virus (BMV) RNA 3 on the formation of initiation complexes in Escherichia coli and wheat germ protein-synthesizing systems was studied. Modification of the RNAs was achieved by using O-methylhydroxylamine, which specifically reacts with cytosines; this leaves the initiation codons unchanged and, under denaturing conditions, leads to irreversible unfolding of the RNA. E. coli ribosomes interact with newly exposed AUG/GUG codons in the modified templates forming polysomes, whereas they form monosomes with native f2 RNA or BMV RNA 3. With wheat germ ribosomes, disomes are formed in the presence of BMV RNA 3, either native or modified. With f2 RNA, eukaryotic ribosomes form monosomes, independent of the secondary structure of the template. The results indicate that, in contrast to prokaryotic ribosomes, binding of eukaryotic ribosomes to f2 RNA or BMV RNA 3 is not affected by modification of the secondary structure of these messengers.

Stored messenger ribonucleoprotein particles in differentiated sclerotia of Physarum polycephalum

Starvation induces vegetative microplasmodia of Physarum polycephalum to differentiate into translationally-dormant sclerotia. The existence and the biochemical nature of stored mRNA in sclerotia is examined in this report. The sclerotia contain about 50% of the poly (A)-containing RNA [poly(A)+RNA] complement of microplasmodia as determined by [3H]-poly(U) hybridization. The sclerotial poly(A)+RNA sequences are associated with proteins in a ribonucleoprotein complex [poly(A)+mRNP] which sediments more slowly than the polysomes. Sclerotial poly(A)+RNP sediments more rapidly than poly(A)+RNP derived from the polysomes of microplasmodia despite the occurrence of poly(A)+RNA molecules of a similar size in both particles suggesting the existence of differences in protein composition. Isolation of poly(A)+RNP by oligo (dT)-cellulose chromatography and the analysis of its associated proteins by polyacrylamide gel electrophoresis show that sclerotial poly(A)+RNP contains at least 14 major polypeptides, 11 of which are different in electrophoretic mobility from the polypeptides found in polysomal poly(A)+RNP. Three of the sclerotial poly(A)+RNP polypeptides are associated with the poly(A) sequence (18, 46, and 52 x 10(3) mol. wt. components), while the remaining eight are presumably bound to non-poly(A) portions of the poly(A)+RNA. Although distinct from polysomal poly(A)+RNP, the sclerotial poly(A)+RNP is similar in sedimentation behavior and protein composition (with two exceptions) to the microplasmodial free cytoplasmic poly(A)+RNP. The results suggest that dormant sclerotia store mRNA sequences in association with a distinct set of proteins and that these proteins are similar to those associated with the free cytoplasmic poly(A)+RNP of vegetative plasmodia.

Translational control in sea urchin eggs and embryos: initiation is rate limiting in blastula stage embryos

To determine whether initiation is rate-limiting in protein synthesis during the embryogenesis of sea urchins, polyribosome profiles of unfertilized eggs and cleavage, blastula and prism stage embryos were examined after incubation of the eggs and embryos in the presence and absence of low amounts of emetine, an inhibitor of polypeptide elongation. The ribosomes were radioactively labeled with [³H]uridine by injection of the adults during oogenesis so that we could monitor emetine-dependent shifts of monoribosomes to polyribosomes. Although initiation is not rate limiting in unfertilized eggs or 2- to 16-cell embryos of Strongylocentrotus purpuratus, it is rate limiting in blastula and prism embryos. We suggest that initiation becomes rate limiting to allow the selective translation of certain classes of mRNA during later development.

Cell-free translation of free and membrane-bound polysomes and polyadenylated mRNA from rabbit brain following administration of d-lysergic acid diethylamide in vivo

Free and membrane-bound polysomes and polyadenylated mRNA isolated from rabbit brain were translated in an mRNA-dependent rabbit reticulocyte lysate system. Electrophoretic analysis of the cell-free translation products demonstrated that although most of the nascent proteins were common to both free and membrane-bound brain polysomes, qualitative and quantitative differences were observed. Compared with the results obtained with purified polyadenylated mRNA, the addition of intact polysomes to the cell-free translation assay was more efficient and produced higher molecular weight products. Analysis of the translation products of free and membrane-bound polysomes revealed the appearance of 74K protein following neither LSD administration or hyperthermia induced by elevated temperature treatment. The presence of this 74K protein was verified by analysis of the translation products by two-dimensional gel electrophoresis.

Poly(adenylic acid)-containing and -deficient messenger RNA of mouse liver

RNA was isolated and fractionated into poly(A)-containing and -deficient classes by oligo(dT) chromatography. Approximately 99% of the poly(A) material bound to the oligo(dT); that which did not bind contained substantially shorter poly(A) chains. All RNA fractions retained an ability to initiate cell-free translation, with the poly(A)-deficient fraction containing half the total translational activity, i.e., mRNA. Two-dimensional polyacrylamide gel analysis of the cell-free translation products revealed three classes of mRNA: 1, mRNA preferentially containing poly(A), including the abundant liver mRNA species; 2, poly(A)-deficient mRNA, including many mid- and low-abundant mRNAs exhibiting less than 10% contamination in the poly(A)-containing fraction fraction; and 3, bimorphic species of mRNA proportioned between both the poly(A)-containing and -deficient fractions. Poly(A)-containing and bimorphic mRNA classes were further characterized by cDNA hybridizations. The capacity of various RNA fractions to prime cDNA synthesis was determined. Compared to total RNA, the poly(A)-containing RNA retained 70% of the priming capacity, while 20% was found in the poly(A)-deficient fraction. Poly(A)-containing, poly(A)-deficient, and total RNA fractions were hybridized to cDNAs synthesized from (+)poly(A)RNA. Poly(A)-containing RNA hybridized with an average R0t 1/2 approximately 20 times faster than total RNA. Poly(A)-deficient RNA hybridized with an average R0t 1/2 approximately 3-4 times slower than total RNA. These R0t 1/2 shifts indicated that in excess of three-quarters of the total hybridizable RNA was recovered in the poly(A)-containing fraction and that less than one-quarter was recovered in the poly(A)-deficient RNA fraction. Abundancy classes were less distinct in heterologous hybridizations. In all cases the extent of hybridization was similar, indicating that while the amount of various mRNA species varied among the RNA fractions, most hybridizing species of RNA were present in each RNA fraction. cDNA to the abundant class of mRNAs was purified and hybridized to both (+)- and (-)poly(A)RNA. Messenger RNA corresponding to the more abundant species was enriched in the poly(A)-containing fraction at least 2-fold over the less abundant species of mRNA, with less than 10% of the abundant mRNAs appearing inthe poly(A)-deficient fraction.

Cytoplasmic polyadenylate processing events accompany the transfer of mRNA from the free mRNP particles to the polysomes in Physarum

The relationship between the mRNA in the polysomes and the free cytoplasmic messenger ribonucleoprotein of Physarum polycephalum was studied by microinjection techniques. Labeled free cytoplasmic ribonucleoprotein, prepared from donor plasmodia, was microinjected into unlabeled host plasmodia, and its fat was followed in the host ribonucleoprotein particles. Approximately one-half of the poly(A)-containing RNA [poly(A)+RNA] that originated from the microinjected particles was incorporated into the host polysomes by normal translational processes within 1 hr. Very short poly(A) sequences (approximately 15 nucleotide residues) were found in these poly(A)+RNA molecules. These short poly(A) sequences were sensitive to digestion with micrococcal nuclease, suggesting that they were not associated with protein. Because the poly(A)+RNA molecules of the microinjected free cytoplasmic mRNP had originally contained poly(A) sequences 50-65 nucleotides long and were associated with protein extensive poly(A) degradation and poly(A).protein complex dissociation must have occurred during their incorporation into the polysomes or during their translation. These results demonstrate a precursor-product relationship between free cytoplasmic mRNP and polysomal mRNA and suggest that the incorporation process in Physarum is accompanied by structural modifications in the poly(A) region of mRNA. They also imply that the polysome is a site for disruption of the poly(A).protein complex and poly(A) degradation.

Polysome-dependent in vitro translation system capable of peptide chain reinitiation

A sensitive in vitro translation system has been developed which makes use of cellular polysomes as the source of mRNA and ribosomes. The soluble factors are derived from the preincubated S-30 fraction by centrifugation through a discontinuous sucrose gradient. Of the four fractions tested, fraction 1 (topmost fraction in the gradient) and fraction 2 (fraction sedimenting in 0.5 M sucrose) were stimulatory. These two fractions together yield the highest activity, corresponding to about 125 times the background incorporation. The polysome-directed system exhibits optimal activity in the range 1.8-2 mM Mg2+ and 125-175 mM KCl. The polysome-directed in vitro products exhibit a complexity comparable to the in vivo products resolved on the two-dimensional polyacrylamide gels of O'Farrell [O'Farrell, P. (1975) J. Biol. Chem. 250, 4007-4021]. The system is capable of active chain reinitiation as indicated by partial inhibition by 7-methylguanosine 5'-monophosphate and pactomycin and N-terminal end analysis of in vitro products. This system can also translate polysomes from diverse tissues such as mouse liver, rat liver, and rat brain. The levels and also the authenticity of translation of rat liver albumin and mouse liver carbamoyl phosphate synthetase I were tested by immunoprecipitation with monospecific antibodies. The results show that the major as well as the minor translation products are synthesized in this system at levels comparable to the physiological levels.