Sequence organization of the poly(A) RNA synthesized and accumulated in lampbrush chromosome stage Xenopus laevis oocytes

Gene expression during oogenesis and oocyte development in mammals

Mouse oocytes progress through early meiotic prophase during fetal life and reach the diplotene stage by birth. During prepubertal and reproductive life, oocytes are continuously selected to grow from the pool of small primordial oocytes. Growing oocytes reach full size in 2 weeks, and full-grown oocytes are present in rapidly enlarging follicles for about 5 days before meiotic maturation and ovulation. RNA synthesis during early meiotic prophase, as estimated from [3H]uridine incorporation followed by autoradiography and from electron microscopic analysis of nuclear components, proceeds at a moderate rate throughout except for a brief period in early pachytene when synthesis is low or absent. RNA synthesis continues in primordial oocytes at a moderate rate. Incorporation studies, electron microscopic analyses, and particularly measurements of ongoing RNA polymerase activity (completion of initiated chains as analysed in tissue sections) indicate a distinctly increased rate of synthesis during oocyte growth over that of primordial oocytes, followed by a decline in full-grown oocytes. During growth, this rate increases severalfold. The absolute rate of synthesis of heterogeneous nuclear RNA (using rRNA as a standard) during mid-growth is very rapid, but nevertheless still much lower than that in typical lampbrush chromosomes. Most of the hnRNA turns over with a half-life of about 20 min, as is typical in somatic cells. Newly synthesized mRNA-like RNA enters the cytoplasm at about one-half the rate of rRNA, and about one-third of the ribosomes and one-fourth of the mRNA appear in polysomes. In full-grown oocytes, the rate of synthesis falls distinctly, but a significant level of synthesis continues until it essentially ceases at breakdown of the germinal vesicle. During meiotic prophase, chromosomes are most compact at pachytene and unfold lateral projections as RNA synthesis increases in late pachytene-early diplotene. In primordial oocytes, the diplotene state of chromosomes is obvious in most mammals, but in rodents the chromosomes are more evenly dispersed and are said to be in a dictyate state, although they are still presumably in a diplotene configuration. The chromosome core, which is present in leptotene through early diplotene stages, apparently disappears in the dictyate stage.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Expression of two Xenopus laevis ribosomal protein genes in injected frog oocytes. A specific splicing block interferes with the L1 RNA maturation

The expression of two Xenopus laevis ribosomal protein genes (L1 and L14) has been analysed by microinjection of the cloned genomic sequences into frog oocyte nuclei. While the injection of the L14 gene causes the accumulation of the corresponding protein in large excess with respect to that synthesized endogenously, the L1 gene does not. Analysis of the RNA shows that both genes are actively transcribed. The seven-intron-containing L14 transcript is completely processed to a mature form, while two out of nine intron sequences persist in the L1 transcript. This precursor RNA is confined to the nucleus; its accumulation is due to a specific block of splicing operating at the level of two defined introns and not to saturation of the processing apparatus of the oocyte. The different behaviour of the two genes may reflect different mechanisms of regulation which, in the case of the L1 gene, could operate at the level of splicing.

Lineage segregation and developmental autonomy in expression of functional muscle acetylcholinesterase mRNA in the ascidian embryo

Acetylcholinesterase is a histospecific marker of cell differentiation occurring only in the muscle and mesenchyme tissues of the ascidian embryo. The distribution of functional mRNA coding for this enzyme has been investigated and it is shown here that only cells of muscle and mesenchyme lineages possess such a template. Blastomeres of four cell lineage quadrants were separated microsurgically from eight-cell-stage embryos of Ciona intestinalis and raised in isolation until muscle development was well advanced. Measurement of enzyme activity in the resulting partial embryos revealed that acetylcholinesterase was limited to descendants of one blastomere pair, the B4.1 blastomeres containing muscle and mesenchyme lineages. To study the tissue distribution of acetylcholinesterase mRNA, RNA from partial embryos was translated in Xenopus laevis oocytes. When oocytes were injected with an appropriate template, they synthesized a biologically active acetylcholinesterase that could be selectively immunopurified with an antiserum to the ascidian enzyme. Under the conditions used the quantity of acetylcholinesterase mRNA was directly related to the enzyme activity in immunoprecipitates. Acetylcholinesterase mRNA was found only in B4.1 lineage partial embryos where it occurred in approximately the same amount as in whole embryos of the same age. Since there is a limited period from gastrulation until the middle tail-formation stage when functional acetylcholinesterase mRNA accumulates, the results of our mRNA distribution experiments strongly suggest that the gene for ascidian acetylcholinesterase is active only in muscle and mesenchyme tissues. The histospecific occurrence of this enzyme apparently does not involve selective, cell-specific control of translation.

Transcription of a long, interspersed, highly repeated DNA element in Xenopus laevis

We have analyzed the transcription of 1723, a long, repeated DNA element that is interspersed in the genome of Xenopus laevis (B. K. Kay and I. B. Dawid (1983) J. Mol. Biol. 170, 583-596). We have detected RNA homologous to 1723 in total cellular RNA from ovaries, embryos, liver, and cultured kidney cells. Transcripts from both strands of the element are present at similar concentrations in these different RNA preparations. In oocytes, approximately 100 pairs of lampbrush chromosome loops are active in the transcription of 1723 elements. The abundance of 1723 RNA increases during embryogenesis, with the highest level reached at the tadpole stage. From cellular fractionation studies, we conclude that 1723 transcripts are largely limited to the nucleus.

Small nuclear U-ribonucleoproteins in Xenopus laevis development. Uncoupled accumulation of the protein and RNA components

The accumulation of protein and RNA components of small nuclear U-ribonucleoprotein particles is non-co-ordinate during oogenesis and early embryogenesis in Xenopus laevis. Northern blot hybridization of a cloned Xenopus U2-RNA gene to oocyte and embryo RNAs demonstrates that the amount of small nuclear U2-RNA per oocyte reaches a plateau early in oogenesis (at the start of yolk deposition); further accumulation is not observed in oogenesis, nor in embryogenesis until the late blastula stage. In contrast, we show by immunoblot analysis that the proteins that bind to small nuclear U-RNAs continue to be accumulated after vitellogenesis begins, reaching maximum amounts only at the end of oocyte development. No further accumulation of these proteins is seen during embryogenesis. The consequences of this non-co-ordinate synthesis of small nuclear RNA and small nuclear RNA-binding proteins are as follows: a 10- to 20-fold excess of the protein components of the small ribonucleoprotein particles over small nuclear RNA exists in large oocytes; the bulk of the protein is cytoplasmic, while the RNA is nuclear. Thus the excess protein in the cytoplasm is uncomplexed with RNA. The imbalance between protein and RNA is not corrected until the late blastula or early gastrula stages of embryogenesis, when a tenfold increase in the amount of small nuclear U2-RNA is detected. Thus the protein, but not the RNA, components of small nuclear U-ribonucleoprotein particles are stockpiled in oocytes for later use in embryonic development. During the course of these studies, we also found that there are tissue-specific differences in the Sm-antigenic proteins of X. laevis.

Histone RNA in amphibian oocytes visualized by in situ hybridization to methacrylate-embedded tissue sections

We present an in situ hybridization method for detecting cellular RNAs in tissue sections using methacrylate as the embedding medium. The technique offers the advantage of superior morphological preservation compared with previously published procedures. Since sections can be cut 1 micron or less in thickness, full advantage is taken of the short path length of 3H electrons. Applying this procedure to developing amphibian oocytes, we investigated the accumulation and localization of RNA complementary to the histone genes and their adjacent spacers. Histone RNA begins to accumulate in the cytoplasm of late pachytene-early diplotene oocytes, rapidly reaching a maximum concentration during Dumont stage 1. After this stage the concentration of histone RNA declines. RNA transcribed from histone coding regions is located almost exclusively in the cytoplasm of oocytes. Transcripts of the spacer regions, which are known to be synthesized on oocyte lampbrush chromosomes, do not accumulate in the oocytes. [3H]RNA complementary to U2 small nuclear RNA, used in these experiments as a control, hybridized predominantly to the nucleus of the oocytes.

Isolation and characterization of calmodulin genes from Xenopus laevis

Two cDNAs derived from Xenopus laevis calmodulin mRNA have been cloned. Both cDNAs contain the complete protein-coding region and various lengths of untranslated segments. The two cDNAs encode an identical protein but differ from each other by 5% nucleotide substitutions. The 5' and 3' untranslated regions, to the extent available, are highly homologous between the two cDNAs. The predicted sequence of X. laevis calmodulin is identical to that of vertebrate calmodulins from mammals and chickens and shows one substitution compared with electric eel calmodulin. Genomic DNA sequences homologous to each of the two cDNA clones have been isolated and were shown to account for the major calmodulin-coding DNA sequences in X. laevis. These data suggest that X. laevis carries two active, nonallelic calmodulin genes. Although no complete analysis has been carried out, it appears that the X. laevis calmodulin genes are interrupted by at least four introns. The relative concentrations of calmodulin mRNA have been estimated in different embryonic stages and adult tissues and found to vary by up to a factor of 10. The highest levels of calmodulin mRNA were found in ovaries, testes, and brains. In these three tissues, the two calmodulin genes appear to be expressed at approximately equal levels.

Reversible inhibition of translation by Xenopus oocyte-specific proteins

A characteristic of growing oocytes of all animal species is the synthesis and accumulation of messenger RNA which is destined to be used primarily by the early embryo. The mechanism(s) which regulates the translation of this maternal mRNA remains unknown. However, the inability of the oocyte to translate all of its putative mRNA has been attributed to at least three limitations: (1) The rate of translation is limited by the availability of components of the translational apparatus other than mRNA, (2) the structural organization of the mRNA prevents translation, and (3) proteins associated with the mRNA prevent translation. Several investigators have suggested that proteins associated with maternal mRNA suppress translation in sea urchin eggs, although others claim that such results may be due to experimental artefacts. Oocyte-specific proteins have been identified in association with non-translating poly(A)+ mRNAs from Xenopus laevis oocytes, and we report here that when these proteins are reconstituted with mRNAs in vitro the translation of the mRNAs in vitro is reversibly repressed. The implication is that these proteins are involved in the regulation of translation of stored maternal mRNAs.

Isolation and characterization of calmodulin genes from Xenopus laevis

Two cDNAs derived from Xenopus laevis calmodulin mRNA have been cloned. Both cDNAs contain the complete protein-coding region and various lengths of untranslated segments. The two cDNAs encode an identical protein but differ from each other by 5% nucleotide substitutions. The 5' and 3' untranslated regions, to the extent available, are highly homologous between the two cDNAs. The predicted sequence of X. laevis calmodulin is identical to that of vertebrate calmodulins from mammals and chickens and shows one substitution compared with electric eel calmodulin. Genomic DNA sequences homologous to each of the two cDNA clones have been isolated and were shown to account for the major calmodulin-coding DNA sequences in X. laevis. These data suggest that X. laevis carries two active, nonallelic calmodulin genes. Although no complete analysis has been carried out, it appears that the X. laevis calmodulin genes are interrupted by at least four introns. The relative concentrations of calmodulin mRNA have been estimated in different embryonic stages and adult tissues and found to vary by up to a factor of 10. The highest levels of calmodulin mRNA were found in ovaries, testes, and brains. In these three tissues, the two calmodulin genes appear to be expressed at approximately equal levels.

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.

Two-dimensional gel analysis of the fate of oocyte nuclear proteins in the development of Xenopus laevis

The amphibian oocyte nucleus is thought to provide a maternal store of protein required in embryogenesis. The fate of germinal vesicle proteins has been studied by comparing polypeptide patterns of oocytes, embryos, and several adult organs of Xenopus laevis on two-dimensional gels. A combination of silver staining and fluorography of radiolabeled protein on gels was used to analyze maternal and newly synthesized polypeptides in embryogenesis. Comparison of protein patterns was facilitated and corroborated by application of monoclonal antibodies against several germinal vesicle proteins. These were characterized by immunoblotting from two-dimensional gels, and polypeptides of identical structure were recognized in oocyte nuclei, embryos, and tadpoles. The following conclusions were drawn: (1) Almost all prevalent germinal vesicle proteins can be continuously traced in embryos up to swimming tadpole stages, although their patterns of new synthesis are greatly different, some are not radiolabeled in the embryo but solely provided by the maternal store. (2) Many of the polypeptides occurring in oocyte nuclei are also found in one or several organs of the adult. (3) Tissue specificities of germinal vesicle proteins, previously detected by immunocytochemistry with monoclonal antibodies, could be confirmed by independent biochemical methods. (4) As has been previously shown by immunohistological methods, oocyte nuclear antigens are shed into the cytoplasm of the maturing egg, and are reaccumulated in the nuclei of the embryonic cells, each at a characteristic developmental stage. These shifts between intracellular compartments are not accompanied by a change of the covalent structure of the antigen.

Satellite DNA from Xenopus laevis: comparative analysis of 745 and 1037 base pair Hind III tandem repeats

Highly repetitive Hind III restriction fragments of 0.72-0.76 KBP from total Xenopus laevis genomic DNA are organized in a tandem like arrangement. Cloning of these fragments in pBR 322 with subsequent restriction site mapping and nucleotide sequence analysis of some selected clones showed two different types of sequences. 25-30% of material represent the oocyte specific 5 S DNA repeat units, 70-75% are similar to the recently described repeat elements of satellite 1 DNA. Hybridization of a genomic DNA library to such a 745 BP monomeric repeat unit and investigation of some clones with positive autoradiographic signals revealed structural heterogeneities of repeat elements, in that the 745 BP sequence cross-hybridized with 1037 BP Hind III repeat units. Nucleotide sequence analysis demonstrated that the two types of sequences show a homology of 84.3% and that the 1037 BP sequence additionally contains duplicated elements of the 745 BP sequence as well as apparently unrelated DNA sequences.

Differential gene expression in the gastrula of Xenopus laevis

A modified cloning method designed to produce differential complementary DNA libraries permits the isolation of sequences that are present in the RNA population of any developmental stage or tissue, but are not present or are much less abundant in another stage or tissue. Selective complementary DNA cloning is especially useful when the differentially expressed RNA's are of low to moderate abundance in the cells in which they occur. A class of cytoplasmic polyadenylated RNA's differentially expressed in gastrula embryos of Xenopus laevis (DG RNA's) has been isolated. These DG RNA's occur very rarely or not at all in unfertilized eggs and blastulae, accumulate as the result of transcription before and during gastrulation, and, with some exceptions, decline in abundance as development proceeds. Many of these RNA molecules appear to be translated at the gastrula stage. Thus, DG RNA's may encode proteins that are important in the process of gastrulation.

Nuclear exclusion of transcription factor IIIA and the 42s particle transfer RNA-binding protein in Xenopus oocytes: a possible mechanism for gene control?

The intracellular location of 7S and 42S RNP particles in Xenopus oocytes has been determined by immunohistochemistry. Using antibodies directed against the 48-mol-wt protein component of the 42S particle and against transcription factor IIIA, the protein moiety of the 7S particle, we show that these ribonucleoprotein particles are detectable only in the oocyte cytoplasm, being excluded from the nucleus. The mechanism of this nuclear exclusion, and its possible significance in the regulation of 5S RNA gene expression, are discussed.

Development of translationally active mRNA for larval muscle acetylcholinesterase during ascidian embryogenesis

Relative quantities of translationally active acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) mRNA present at various developmental stages were compared in embryos of the ascidian Ciona intestinalis. Purified RNA was tested for its translational capacity by microinjection into Xenopus laevis oocytes; the acetylcholinesterase produced was immunoprecipitated with antibody to Ciona acetylcholinesterase and enzyme activity was assayed radiometrically. With this protocol, enzyme synthesis was found to be directly related to the amount of RNA injected and to the oocyte incubation time. A functional template for acetylcholinesterase was first detected at 6 hr of development (late gastrula) and is probably present as early as 5 hr. The level of this template activity increased until the middle tail formation stage (11-12 hr after fertilization) and then remained constant until 16 hr of development (the final stage examined), 2 hr before hatching. These findings, and the results of previous actinomycin D inhibition experiments, indicate that mRNA for ascidian larval muscle acetylcholinesterase is first synthesized during gastrulation.

Half-lives and relative amounts of stored and polysomal ribosomes and poly(A) + RNA in mouse oocytes

Growing mouse oocytes were labeled in vitro with [3H]uridine and chased for 2 or for 7 days to estimate the relative amounts of RNA appearing in different fractions and to follow their turnover. Oocytes were lysed and thoroughly dispersed in the presence of 1% DOC, and centrifuged on sucrose gradients to separate polysomes from smaller components not engaged in translation. After the short chase, one-third of the labeled ribosomes appeared in EDTA-sensitive polysomes. The proportion of ribosomes in both fractions remained stable during the long chase, demonstrating no net flow from one fraction to the other. When gradient fractions were analyzed by poly(U) Sepharose chromatography, it was found that about 20% of the labeled poly(A)+ RNA appeared in polysomes after the short chase. The half-lives of stored and translated mRNA were followed relative to stable rRNA during the long chase. Stored mRNA was completely stable, but translated mRNA turned over with a t1/2 of about 6 days. Other methods for separating stored from translated components were not successful, including sedimentation of putative large complexes (fibrillar lattices) containing stored components, or chromatography of lysates on oligo(dT)-cellulose. Results presented here combined with our previous results demonstrate that, during meiotic maturation, the percent of labeled stable RNA which is polyadenylated declines from 19 to 10%, suggesting deadenylation or degradation of half of the accumulated maternal mRNA.

Small nuclear RNA transcription and ribonucleoprotein assembly in early Xenopus development

The Xenopus egg and embryo, throughout the transcriptionally inactive early cleavage period, were found to contain a store of approximately 8 X 10(8) molecules of the small nuclear RNA (snRNA) U1, sufficient for 4,000-8,000 nuclei. In addition, when transcription is activated at the twelfth cleavage (4,000 cell-stage), the snRNAs U1, U2, U4, U5, and U6 are major RNA polymerase II products. From the twelfth cleavage to gastrulation, U1 RNA increases sevenfold in 4 h, paralleling a similar increase in nuclear number. This level of snRNA transcription is much greater than that typical of somatic cells, implying a higher rate of U1 transcription or a greater number of U1 genes active in the embryo. The Xenopus egg also contains snRNP proteins, since it has the capacity to package exogenously added snRNA into immunoprecipitable snRNP particles, which resemble endogenous particles in both sedimentation coefficient and T1 RNase digestibility. SnRNP proteins may recognize conserved secondary structure of U1 snRNA since efficient packaging of both mouse and Drosophila U1 RNAs, differing 30% in sequence, occurs. The Xenopus egg and embryo can be used to pose a number of interesting questions about the transcription, assembly, and function of snRNA.

Interspersed sequence organization and developmental representation of cloned poly(A) RNAs from sea urchin eggs

A random primed complementary DNA (cDNA) clone library constructed from total maternal poly(A) RNA of sea urchin eggs was screened with two cloned genomic repetitive sequence probes. Sets of cDNA clones reacting with each of these repetitive sequences were recovered. Most of the cloned transcripts included both single copy and repeat sequence elements. Except for the shared repeat sequence element, both the repetitive and single copy regions of the members of each set of clones failed to crossreact. Single copy probes linked to the repeats on the cloned maternal RNAs are represented in an asymmetric manner. It follows that many different genomic members of a given dispersed repeat sequence family are represented in the maternal RNA. RNA gel blots carried out with several repeat probes display about 10 to 20 prominent maternal poly(A) RNAs containing transcripts of each repetitive sequence family. The interspersed maternal transcripts are 3000 to 15,000 bases in length. Maternal transcripts reacting with single copy probes derived from the cloned cDNAs persist during embryonic development, and in some cases appear to be augmented by similar, newly synthesized embryo transcripts. Two examples were found in which additional transcripts of different length appear at specific developmental stages. The transcribed single copy regions are highly polymorphic in the genomes of different individual sea urchins, and comparisons of closely related sea urchin species showed that both the prevalence and length of specific maternal transcripts change rapidly during evolution. Nucleotide sequences of two homologous repeat elements occurring on different cloned transcripts displayed translation stop codons in every possible reading frame. These repeat sequences display structural features suggesting that there has been evolutionary transposition into transcription units active during oogenesis. The repeat elements and their flanking single copy regions reside either in very long 3' or 5'-terminal sequences, or in unprocessed intervening sequences in the maternal poly(A) RNA. These findings lead us to the proposal that the majority of the cytoplasmic poly(A) RNA in echinoderm eggs and early embryos is similar in form to RNAs that occur in the nucleus rather than to the messenger RNA of later cells.

[Do repetitive DNA sequences have a biological function?]

By DNA reassociation kinetics it is known that the eucaryotic genome consists of non-repetitive DNA, middle-repetitive DNA and highly repetitive DNA. Whereas the majority of protein-coding genes is located on non-repetitive DNA, repetitive DNA forms a constitutive part of eucaryotic DNA and its amount in most cases equals or even substantially exceeds that of non-repetitive DNA. During the past years a large body of data on repetitive DNA has accumulated and these have prompted speculations ranging from specific roles in the regulation of gene expression to that of a selfish entity with inconsequential functions. The following article summarizes recent findings on structural, transcriptional and evolutionary aspects and, although by no means being proven, some possible biological functions are discussed.

The activation of RNA synthesis by somatic nuclei injected into amphibian oocytes

Previous work has shown that nuclei of cultured cells or erythrocytes are transcriptionally activated when injected into the germinal vesicle of Xenopus oocytes. We now find that the total amount of stable RNA synthesized by an oocyte with injected nuclei is about twice that of an uninjected oocyte (approximately 15 ng/day). At least half of the RNA transcribed by the injected nuclei is low-molecular-weight RNA of discrete sizes, and is transcribed by RNA polymerase III. Part of this is attributable to a new class of gene (referred to as OAX) which makes a transcript about 180 nucleotides long, and which is activated by at least 50-fold in somatic nuclei injected into oocytes. OAX RNA is not a degradation or processing variant of ribosomal or 5 S RNA, and is not coded by mitochondrial DNA. It enters oocyte cytoplasm, but turns over with a half-life of less than 3 hr, as judged by the culture of enucleate oocyte cytoplasms for several days during which 4 and 5 S RNAs are stable. OAX RNA synthesis is not detected by labelling cultured cells or their nuclei. The results emphasize the selectivity with which components of an oocyte germinal vesicle activate genes in injected somatic nuclei.

Nucleocytoplasmic distribution of snRNPs and stockpiled snRNA-binding proteins during oogenesis and early development in Xenopus laevis

The distribution of small nuclear ribonucleoprotein particles containing U snRNAs (U snRNPs) during oogenesis and early development in Xenopus was analyzed with a lupus antibody (anti-Sm) that reacts with snRNA-binding proteins. Fully grown oocytes and embryos prior to gastrulation were found to be relatively depleted of U snRNPs in their nuclei and to contain an excess of snRNA-binding proteins stored in the cytoplasm. During late blastula-early gastrula, or after microinjection of U snRNAs into the cytoplasm of a mature oocyte, the proteins migrate into the nucleus. Dot hybridization analysis showed that small previtellogenic oocytes already contain a maximal amount of U1 (and U2) snRNAs, which then decreases to about 20% of that value in fully mature oocytes, even though the cell's volume has increased enormously. Thus fully grown oocytes and eggs accumulate snRNA-binding proteins for use during early development, but this is not coupled with the accumulation of U snRNA.

Mobilization of maternal mRNA in amphibian eggs with special reference to the possible role of membraneous supramolecular structures

Current knowledge of the mechanism of mobilization of maternal mRNA is summarized herein and a working hypothesis has been constructed to explain the mechanism on the assumption that the mRNA enters the cytoplasm in association with the cytoplasmic membraneous structures and is then stored in the structures until liberation and relocation at the step of oocyte maturation. An extensive turnover of poly(A) sequences as well as the occurrence of repetitive sequences in the maternal mRNA may be relevant to mRNA activation.

Post-transcriptional processing of simian virus 40 late transcripts in injected frog oocytes

The capacity of fully grown Xenopus oocytes to process messenger RNA precursors has been assessed using transcripts synthesized from simian virus 40 (SV40) DNA microinjected into the oocyte nucleus. In oocytes, stable transcripts of the SV40 virion protein genes have undergone at least four post-transcriptional maturation steps: cleavage at 3' splice sites, formation of a mature 3' terminus, addition of poly(A), and selective intracellular partitioning, such that only those RNAs with a mature 3' terminus and poly(A) are located in the cytoplasm. Apparently unspliced transcripts with mature 3' termini are transported into the oocyte cytoplasm. A prominent transcript of roughly the full length of SV40 DNA, bearing a 5' terminus in the same region as late mRNA and confined to the nucleus, is found in oocytes injected with SV40 DNA. The possibility that this transcript may serve as a precursor to late mRNA is discussed.

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.