Messenger ribonucleoprotein particles in developing sea urchin embryos

Structure of germ line immunoglobulin alpha heavy-chain RNA and its location on polysomes

We describe the structure of the major germ line RNA transcribed from unrearranged immunoglobulin alpha heavy-chain genes in immunoglobulin M-expressing cells of the I.29 mu B-cell lymphoma, a cell line capable of switching to immunoglobulin A expression upon lipopolysaccharide treatment. This germ line alpha RNA has a small open reading frame that does not include the C alpha domain, and this RNA appears to be present on polysomes in I.29 mu cells.

Structure of germ line immunoglobulin alpha heavy-chain RNA and its location on polysomes

We describe the structure of the major germ line RNA transcribed from unrearranged immunoglobulin alpha heavy-chain genes in immunoglobulin M-expressing cells of the I.29 mu B-cell lymphoma, a cell line capable of switching to immunoglobulin A expression upon lipopolysaccharide treatment. This germ line alpha RNA has a small open reading frame that does not include the C alpha domain, and this RNA appears to be present on polysomes in I.29 mu cells.

The accumulation and translation of a spicule matrix protein mRNA during sea urchin embryo development

In this report we further characterize the expression of the gene that encodes the 50-kDa spicule matrix protein (SM50) during development of the sea urchin Strongylocentrotus purpuratus. Quantitative measurements of SM50 mRNA levels using the single-stranded probe excess titration technique indicate that SM50 transcript levels attain a maximum level of 8000 to 10,000 transcripts per embryo by the gastrula stage, representing 120 to 200 SM50 mRNAs per primary mesenchyme cell. Experiments analyzing run-on transcription in nuclei isolated at different stages of development indicate that the sharp increase in SM50 mRNA levels occurring at the time of primary mesenchyme ingression is concomitant with an increase in transcription of the SM50 gene. We have also analyzed the RNA sequences present on polyribosomes at different stages of development. These studies indicate that SM50 mRNA is present on polyribosomes as soon as it begins to accumulate (which is well in advance of overt spicule formation) and SM50 mRNA remains on polyribosomes through subsequent development. From estimates of the rate of SM50 protein synthesis based on these data, we calculated that the maximum amount of SM50 accumulated during development through the 4-day pluteus stage is approximately 7.4 pg/embryo. This approximation is concordant with the amount of SM50 actually found in the sea urchin embryo.

Three translationally regulated mRNAs are stored in the cytoplasm of clam oocytes

In situ hybridization was used to examine the spatial distributions of three translationally controlled maternal RNAs in oocytes and two-cell embryos of the clam Spisula. 3H-labeled single-stranded RNA probes were generated from SP6 recombinant clones containing DNA inserts encoding portions of histone H3 (the DNA sequence which is presented here), cyclin A, and the small subunit of ribonucleotide reductase. Hybridization of these probes to oocytes, in which the mRNAs are translationally inactive, shows that these mRNAs are stored in the cytoplasm. There is no evidence for sequestration of any of the RNAs within the nucleus or any other discrete structure. Instead they appear to be evenly distributed throughout the cytoplasm.

Three types of muscle-specific gene expression in fusion-blocked rat skeletal muscle cells: translational control in EGTA-treated cells

When rat skeletal muscle cells were treated with EGTA, an inhibitor of cell fusion, a battery of muscle-specific mRNAs was synthesized but not translated despite the synthesis of many other proteins. Most of the muscle-specific mRNAs were associated with polysomes in fused myotubes, whereas they were found in postpolysomal fractions in EGTA-treated cells. Therefore, in addition to the well-documented transcriptional and posttranscriptional control of muscle-specific genes, translational control of this specific group of genes, presumably involving a Ca2+-dependent process, is also observed in these fusion-blocked cells. These findings and results obtained with other fusion inhibitors demonstrate that three types of muscle-specific gene expression take place in the fusion-blocked cells depending on the inhibitors used: one, neither muscle-specific mRNAs nor proteins are synthesized; two, the mRNAs are synthesized but not translated; and three, both the mRNAs and the proteins are synthesized.

Widespread changes in the translation and adenylation of maternal messenger RNAs following fertilization of Spisula oocytes

We have reported previously that sequence-specific adenylations and deadenylations accompany changes in the translation of maternal mRNA following fertilization of Spisula oocytes (E.T. Rosenthal, T.R. Tansey, and J.V. Ruderman, 1983, J. Mol. Biol. 166, 309-327). The data presented here confirm and extend those observations. We have identified four classes of maternal mRNA with respect to translation: Class 1-not translated in oocytes and translated at very high efficiency immediately after fertilization, Class 2-not translated in oocytes and partially utilized for translation following fertilization, Class 3-translated in oocytes and not translated in embryos, and Class 4-not translated either before or after fertilization. There is an excellent, although not perfect, correlation between the translation of an mRNA and its polyadenylation status. The poly(A) tails of all the mRNAs which are translated in oocytes and untranslated in embryos are shortened at fertilization, and the poly(A) tails of those mRNAs which are untranslated in oocytes and translated in embryos are lengthened at fertilization. These adenylations and deadenylations occur simultaneously during the first 20 min following fertilization.

Expression of alpha- and beta-tubulin genes during development of sea urchin embryos

Mature unfertilized eggs of the sea urchin Lytechinus pictus contain multiple alpha-tubulin mRNAs, which range in size from 1.75 to 4.8 kb, and two beta-tubulin mRNAs, 1.8 and 2.25 kb. These mRNAs were found at similar levels throughout the early cleavage stages. RNA gel blot hybridizations showed that prominent quantitative and qualitative changes in tubulin mRNAs occurred between the early blastula and hatched blastula stages. The overall amounts of alpha- and beta-tubulin mRNAs increased two- to fivefold between blastula and pluteus. These increases were due mainly to a rise in a 1.75-kb alpha RNA and a new 2.0-kb beta RNA. Other, minor changes also occurred during subsequent development. All size classes of alpha- and beta-tubulin RNAs in early and late embryos contained poly(A)+ translatable sequences. As reported earlier, some of each of the alpha RNAs, but neither of the beta RNAs, are translated in the egg and a small portion of each of the stored alpha and beta RNAs is recruited onto polysomes within 30 min of fertilization. In the work described here, subsequent development up to the morula stage was accompanied by a gradual recruitment of tubulin mRNAs into polysomes. By the early blastula stage, most of the maternal tubulin sequences were associated with polysomes. In contrast to the gradual recruitment of maternal sequences throughout cleavage, the tubulin mRNAs which appeared at the blastula stage showed no delay in entering polysomes. The exact fraction of each mRNA that was translationally active at later stages varied somewhat among the individual mRNAs. From the differential hybridization patterns of egg, embryo, and testis RNAs to various tubulin cDNA and genomic DNA probes, it is concluded that at least one gene producing maternal alpha mRNA is different from a second one which is expressed only in testis. Each of the three embryonic beta RNAs is encoded by a different beta gene; at least two of these different beta genes are also expressed in testis.

Multiple polymorphic alpha- and beta-tubulin mRNAs are present in sea urchin eggs

Multiple alpha- and beta-tubulin RNAs were found in the mature unfertilized eggs of the sea urchin Lytechinus pictus. The alpha-tubulin RNAs were polymorphic in number, size, and relative amounts in the eggs of different females. Five to seven different size classes [1.75-4.2 kilobases (kb)] were detected on RNA gel blots. All egg preparations contained variable amounts of 1.8- and 2.25-kb beta-tubulin RNAs, and a few of them contained an additional 2.9-kb beta-tubulin RNA. The total amount of alpha-tubulin RNA did not always parallel that of beta-tubulin RNA. A portion of all of the various alpha- and beta-tubulin RNAs were polyadenylylated. RNase H digestions ruled out the possibility that some of these RNAs represented a single transcript bearing different lengths of 3' poly(A). One class of alpha-tubulin RNAs (2.4-2.65 kb) was reduced to 2 kb by RNase H, suggesting the presence of internal oligo(A) regions. All of the egg beta-tubulin RNAs sedimented as free ribonucleoprotein particles. Only a small portion of the 1.75- to 3.6-kb alpha-tubulin RNAs, but most of the 4.2-kb alpha-tubulin RNA, were found on polysomes before fertilization. In the 30-min embryo, small amounts of each of the various alpha- and beta-tubulin RNAs were recruited onto polysomes. Thus, each of the multiple polymorphic alpha- and beta-tubulin RNAs in the egg represent translationally competent mRNA.

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.

Poly(A)-containing messenger ribonucleoprotein complexes from sea urchin eggs and embryos: polypeptides associated with native and UV-crosslinked mRNPs

Fertilization of sea urchin eggs results in the rapid recruitment of stored messages into polyribosomes. Whether translational control in sea urchin eggs is mediated by macromolecules associated with the stored messages remains unknown, since preparations of messenger ribonucleoprotein complexes (mRNPs) were active in protein synthesis in a rabbit reticulocyte lysate. To facilitate the study of mRNPs, chromatography on oligo(dT)-cellulose was used to purify poly(A)-containing mRNPs from eggs and embryos of the sea urchin Strongylocentrotus purpuratus. Nonpolyribosomal mRNPs purified from eggs had a similar sedimentation in sucrose to unpurified mRNPs, a peak buoyant density in metrizamide of 1.22 g/cm3, and peak buoyant densities in Cs2SO4 in 1.42 g/cm3 after fixation with glutaraldehyde and 1.46 g/cm3 without fixation. Nonpolyribosomal mRNPs from eggs and zygotes contained 5-10 major proteins on sodium dodecylsulfate (SDS) polyacrylamide gels, and numerous minor bands. UV-irradiation of living eggs of the sea urchin Arbacia punctulata produced cross-linked mRNPs which contained a similar pattern of polypeptides to noncross-linked mRNPs. The polypeptides associated with embryonic polyribosomal mRNPs were also qualitatively similar to those present in nonpolyribosomal mRNPs, although stoichiometric differences may exist.

76-kDa poly(A)-protein is involved in the formation of 48 S initiation complexes

In erythropoietic mouse cells induced by Friend leukemia virus, approximately 50% of non-polyribosomal globin mRNA is found in 48 S initiation complexes ready to be translated. EDTA releases 15 S globin mRNPs, homologous to polyribosomal globin mRNPs. The 76-kDa poly(A)-protein is one of its main protein components. The other 50% of non-polyribosomal message can be separated as 20 S 'free' mRNPs. Its protein composition is different, especially the 76-kDa protein is lacking. The role of this protein is discussed.

Sequence-specific adenylations and deadenylations accompany changes in the translation of maternal messenger RNA after fertilization of Spisula oocytes

A dramatic change in the pattern of protein synthesis occurs within ten minutes after fertilization of Spisula oocytes. This change is regulated entirely at the translational level. We have used DNA clones complementary to five translationally regulated messenger RNAs to follow shifts in mRNA utilization at fertilization and to characterize alterations in mRNA structure that accompany switches in translational activity in vivo. Four of the mRNAs studied are translationally inactive in the oocyte. After fertilization two of these mRNAs are completely recruited onto polysomes, and two are partially recruited. All four of these mRNAs have very short poly(A) tracts in the oocyte; after fertilization the poly(A) tails lengthen considerably. In contrast, a fifth mRNA, that encoding alpha-tubulin mRNA, is translated very efficiently in the oocyte and is rapidly lost from polysomes after fertilization. Essentially all alpha-tubulin mRNA in the oocyte is poly(A)+ and a large portion of this mRNA undergoes complete deadenylation after fertilization. These results reveal a striking relationship between changes in adenylation and translational activity in vivo. This correlation is not perfect, however. Evidence for and against a direct role for polyadenylation in regulating these translational changes is discussed. Changes in poly(A) tails are the only alterations in mRNA sizes that we have been able to detect. This indicates that, at least for the mRNAs studied here, translational activation is not due to extensive processing of larger translationally incompetent precursors. We have also isolated several complementary DNA clones to RNAs encoded by the mitochondrial genome. Surprisingly, the poly(A) tracts of at least two of the mitochondrial RNAs also lengthen in response to fertilization.

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.

Utilization of maternal and embryonic histone RNA in early sea urchin development

Histone RNA in early sea urchin embryos is derived from maternal stores and from new transcription. We show that the sedimentation of maternal free RNPs, containing histone RNA, is somewhat more rapid than the sedimentation of the newly made histone RNPs. Yet, prior to the 2- to 4-cell stage, both the maternally derived and the newly synthesized histone RNA are localized to the same extent in the non-polysomal-free RNPs, and the timing of their recruitment into embryonic polysomes appears to be the same. The levels of hybridization of histone probe to RNAs in cleaving embryos increases severalfold in intensity, and the increase occurs primarily in the polysomes. These data suggest that new transcription may provide an important contribution to the total histone RNA mass by as early as the 32- to 64-cell stage of development.

Molecular biology of the sea urchin embryo

Research on the early development of the sea urchin offers new insights into the process of embryogenesis. Maternal messenger RNA stored in the unfertilized egg supports most of the protein synthesis in the early embryo, but the structure of maternal transcripts suggests that additional functions are also possible. The overall developmental patterns of transcription and protein synthesis are known, and current measurements describe the expression of specific genes, including the histone genes, the ribosomal genes, and the actin genes. Possible mechanisms of developmental commitment are explored for regions of the early embryo that give rise to specified cell lineages, such as the micromere-mesenchyme cell lineage.