Ribosomal RNA synthesis in pre- and post-gastrula-stage sea urchin embryos

Psoralen crosslinking of active and inactive sea urchin histone and rRNA genes

Chromatin structure is highly correlated with the transcriptional activity of specific genes. For example, it has been found that the regularity of nucleosome spacing is compromised when genes are transcribed. The rRNA genes from fungi, plants, and animals give distinctly bimodal distributions of psoralen crosslinking, which has led to the suggestion that these genes might be largely devoid of nucleosomes when transcriptionally active. We investigated the chromatin structure of the multicopy rRNA and histone genes during sea urchin early embryogenesis. The rRNA genes, which are weakly expressed, give a unimodal distribution of weak psoralen crosslinking, in contrast to the situation in all other organisms studied. The early histone genes were more accessible to psoralen crosslinking when active than inactive. The pattern of crosslinking suggests that these polII genes have a homogeneous structure and are still highly protected by nucleosomes when in the active conformation, unlike the situation in polI genes.

Structural analysis of ribosomal RNA genes from the Hawaiian sea urchin species, Tripneustes gratilla

Ribosomal gene sequences (rDNA) were isolated from the genomic DNA of the Hawaiian sea urchin Tripneustes gratilla by cloning in plasmid and phage vectors. The rRNA coding regions in four clones were localized by hybridizing Southern transfers of endonuclease-digested DNA with 32P-labeled 26S rRNA and 32P-labeled 18S rRNA. Three of the rDNA clones were isolated from a library of DNA from a single sea urchin and represent the two major types of rDNA repeats present in that individual's genome. Both repeat types appear identical within their rRNA coding regions but are dissimilar in an area of the nontranscribed spacer (NTS) adjacent to the 3' end of the 26S coding region. The cloned rDNA repeats were shorter than their genomic counterparts due to deletions occurring within internally repetitious NTS domains, probably as a result of unequal recombination during phage propagation.

The role of cap methylation in the translational activation of stored maternal histone mRNA in sea urchin embryos

Cap methylation was examined in the early sea urchin embryo. Nucleotide analyses of 3H-methyl methionine-labeled RNA in two-cell embryos and in unfertilized eggs show that fertilization activates the cap methylation of about 10(7) RNA molecules. Greater than 37% of methyl-labeled RNAs following fertilization hybridize with so-called early histone genes H1, H4, and H2B, which encode a subpopulation of the maternal mRNA molecules. Activation of RNA cap methylation is inhibited by aphidicolin, but not by actinomycin D, suggesting that this process is temporally coordinated with DNA replication, but independent of RNA transcription. These results indicate that the translational activation of maternal early histone mRNA during fertilization is a consequence of cap methylation of mRNAs incompletely formed during oogenesis.

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.

Regulation of cytoplasmic mRNA prevalence in sea urchin embryos. Rates of appearance and turnover for specific sequences

Complementary DNA clones representing cytoplasmic poly(A) RNAs of sea urchin embryos were hybridized with metabolically labeled cytoplasmic RNA preparations and the rates of appearance and of decay for each transcript species were determined at the blastula-gastrula stage of development. The prevalence of the transcripts chosen for this study ranged, on average, from about one molecule per cell to a few hundred molecules per cell. The embryos were labeled continuously for 18 hours with [3H]guanosine, beginning at 24 hours post-fertilization. The amount of cytoplasmic [3H]poly(A) RNA that hybridized to each cloned sequence was determined and the specific activity of the [3H]GTP pool was measured in the same embryos. Rate constants for the entry of each transcript species into the cytoplasm, and for its decay were extracted from these data. The embryo transcript species identified by the cloned probes displayed a range of stabilities. Half-lives of only a few hours were measured both for a very rare sequence and for a moderately prevalent sequence. Other newly synthesized transcripts, including sequences that first appear during embryonic development, as well as sequences also represented in maternal RNA, are far more stable. We conclude that cytoplasmic RNA turnover rate is a major variable in the determination of the cytoplasmic level of expression of embryo genes. The entry rates of the transcripts into the cytoplasm also varied, from a few molecules per embryo per minute to several hundred, depending on the sequence. By comparing the mass of transcripts of a given sequence in the embryo to the mass of transcripts of that sequence accumulating as a result of new synthesis, the point at which embryo transcription accounts for the major fraction of the cytoplasmic molecules could be estimated. This calculation showed that for some sequences maternal transcripts persist well beyond gastrulation, while other embryo poly(A) RNA species are largely the product of transcription in the embryo nuclei from the blastula stage onwards. There is no single stage at which all maternal transcripts are suddenly replaced by newly synthesized embryo transcripts. Primary transcription rates were measured for two sequences by determining accumulation of label in these RNA species soon after addition of [3H]guanosine to the cultures. Comparing these rates to the cytoplasmic entry rates, we did not detect a significantly greater nuclear transcription of the sequence homologous to the cloned probe.

Modification of ribosomal proteins in sea urchin eggs following fertilization

Analysis of the ribosomal proteins in sea urchin eggs by two-dimensional polyacrylamide gel electrophoresis revealed postfertilization changes in the proteins of both the small and the large subunits. Five egg-ribosomal proteins (S7, S16, S19, L19, L31) appeared to undergo rapid changes to the corresponding embryo-specific proteins. These changes were completed within 30 min after fertilization, and identical electrophoretic patterns were observed among the different developmental stages of embryos. One of the five proteins, S7, showed an increase in the phosphorylated form. The remainder showed qualitative shifts to the corresponding embryo-specific proteins; however, peptide map analyses revealed the existence of common structural units between the corresponding proteins. These modifications were observed in the three species of sea urchin studied (Pseudocentrotus depressus, Hemicentrotus pulcherrimus and Anthocidaris crassispina), except in the case of one protein (L31). Purification of ribosomes by different procedures based on high-salt treatment gave the same results with respect to the egg-specific and embryo-specific proteins.

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.

The region of transcriptional initiation in Lytechinus variegatus rRNA genes

A sea urchin ribosomal DNA 1.9 kilobase BamHI fragment adjacent to the 5' end of the 18 S gene has been mapped with the restriction enzymes, XhoI, EcoRi, SmaI and HinfI. A 270 basepair fragment which most likely contains the 5' end of the presumed primary transcript of rRNA was identified by hybridization of [32P]DNA fragments to total nuclear RNA separated on methylmercury hydroxide gels and bound to diazobenzyloxymethyl paper. Under these denaturing conditions the size of Lytechinus variegatus precursor rRNA was determined to be 7.2 kilobases (33 S).

Ultrastructure and 3H-thymidine incorporation by chromosome vesicles in sea urchin embryos

The ultrastructural features of chromosome vesicle formation in early sea urchin embryos and chromosome vesicle uptake of tritiated thymidine is described. Envelopes which resemble typical nuclear envelopes form around the condensed anaphase chromosomes. In late anaphase or early telophase, the chromosomes swell and decondense and it is at this time when tritiated thymidine is incorporated. This study shows that DNA synthesis in the rapidly dividing cells or early sea urchin embryos occurs in chromosome vesicles which form during anaphase.