Polyadenylation of maternal and newly-synthesized RNA during starfish oocyte maturation

Clearance of Maternal RNAs: Not a Mummy's Embryo Anymore

Until the zygotic genome is activated, early development relies on the products deposited by the mother. Once the zygotic genome starts to be transcribed, most maternal products are not needed anymore by the developing embryo. This emancipation from the maternal genome occurs during the Zygotic Genome Activation (ZGA). Although the process by which the maternal content is replaced with zygotic products differs from species to species, there is a common theme to all of them: maternal transcripts are actively degraded. Here, a review of how the degradation of maternal RNAs is regulated during early development and discussions on some computational tools that may be of use in this research area are outlined.

A nuclear factor required for specific translation of cyclin B may control the timing of first meiotic cleavage in starfish oocytes

In most animals, the rate of cyclin B synthesis increases after nuclear envelope breakdown during the first meiotic cell cycle. We have found that cyclin B-cdc2 kinase activity drops earlier in emetine-treated than in control starfish oocytes, although the protein synthesis inhibitor does not activate the cyclin degradation pathway prematurely. Moreover, protein synthesis is required to prevent meiotic cleavage to occur prematurely, sometimes before chromosomes have segregated on the metaphase plate. In normal conditions, increased synthesis of cyclin B after germinal vesicle breakdown (GVBD) balances cyclin degradation and increases the time required for cyclin B-cdc2 kinase to drop below the level that inhibits cleavage. Taken together, these results point to cyclin B as a possible candidate that could explain the need for increased protein synthesis during meiosis I. Although direct experimental evidence was not provided in the present work, cyclin B synthesis after GVBD may be important for correct segregation of homologous chromosomes at the end of first meiotic metaphase, as shown by a variety of cytological disorders that accompany premature cleavage. Although the overall stimulation of protein synthesis because of cdc2 kinase activation is still observed in oocytes from which the germinal vesicle has been removed before hormonal stimulation, the main increase of cyclin B synthesis normally observed after germinal vesicle breakdown is suppressed. The nuclear factor required for specific translation of cyclin B after GVBD is not cyclin B mRNA, as shown by using a highly sensitive reverse transcription followed by polymerase chain reaction procedure that failed to detect any cyclin B mRNA in isolated germinal vesicles.

Changes in polyadenylation of lactate dehydrogenase-X mRNA during spermatogenesis in mice

The expression of the mRNA for mouse testicular lactate dehydrogenase (LDH-X) was examined by RNA:cDNA hybridization in situ in the testis and by Northern analyses of meiotic and postmeiotic spermatogenic cell populations. Silver grains accumulated in cells inside the second layer from the periphery of the seminiferous tubule, confirming previous findings that LDH-X mRNA first appears in the spermatocyte and continues to accumulate until the late spermatid stage. Northern analyses showed that meiotic and postmeiotic cells contained 1.2 and 1.3 kb classes of hybridizing mRNA, respectively. RNase H digestion of oligo (dT)-hybridized RNA and poly(U)-Sepharose column chromatography with differential elution by formamide revealed that the difference in size of the two classes of mRNAs was due to the poly(A) tail length of the LDH-X mRNA. When the distribution of the LDH-X mRNA was examined across polysome gradients, both mRNAs were partially associated with polysomes. These results suggest that the changes in the polyadenylation of LDH-X mRNA were associated with the meiotic division during spermatogenesis in the mouse. They raise the possibility that the stable accumulation of the LDH-X mRNAs in the postmeiotic cells is enhanced by poly(A) tails of increased length.

Changes in RNA titers and polyadenylation during oogenesis and oocyte maturation in Xenopus laevis

The titers of over 90 sequences isolated by cDNA cloning of oocyte poly(A)+RNA were examined during oogenesis in Xenopus laevis. The relative titers of most sequences in unfertilized eggs are established in pre-lamp brush oocytes and persist throughout oogenesis. We have identified several sequences whose titers decrease significantly during the growth phase of oogenesis as well as a few sequences whose titers increase slightly during this period. Among 21 sequences analyzed by RNA gel blots, all remained unchanged in titer during oocyte maturation. A significant fraction of early oocyte RNA does not bind to oligo(dT)cellulose, but by the end of oogenesis transcripts for many RNA species examined are detected exclusively in the poly(A)+RNA fraction. During oocyte maturation a slight size shift or a broadening of the hybridizing band can be seen for many sequences, indicative of poly(A) elongation or degradation.

Biosynthesis of 1-methyladenine by isolated segments of starfish ovaries

Acid-soluble 1-methyladenine (1-MeA) and an insoluble fraction containing 1-MeA were formed when radioactive L-methionine or adenine (A) were incubated with starfish ovary segments. Attempts to prepare free ribonucleic acid (RNA) from ovaries failed since it was strongly bonded to protein as ribonucleoprotein (RN-P) which was therefore used in studies involving RNA. Incubation of ovary segments with [8-14C]adenine-8 (A-8-14C) yielded both soluble 1-Me-A-8-14C and RN-P-8-14C, and similar incubation with L-[methyl-14C]methionine yielded soluble 1-MeA-14CH3 and RN-P-14CH3. Hydrolysis of RN-P-8-14C with 1 N HCl at 100 degrees yielded 91% of the initial radioactivity in the purine fraction, and of this 90% was in A, 9% in 1-MeA, and 1% in 4(5)-amino-5(4)-imidazole carboxamidine (AIMCAD). With RN-P-14CH3 the corresponding figures were 20, 45, 27, and 28%. Degradation of 1-MeA-8-14C or RN-P-8-14C with 6 N HCl at 110 degrees yielded radioactive AIMCAD which, on heating at pH 12, gave radioactive 4(5)-amino-5(4)-imidazole carboxamide (AICA). When 1-MeA-14CH3 or RN-P-14CH3 were similarly degraded, radioactive AIMCAD was formed, but the AICA possessed little or no radioactivity due to the loss of the radioactive methyl group. Addition of radial nerve factor (GSS) increased the yield of 1-MeA up to 19 times when radioactive L-methionine was substrate, but was ineffective with radioactive A. S-[8-14C]Adenosylmethionine was only about 3% as effective as L-[methyl-14C]methionine in supporting formation of 1-MeA, and its slight activity was not enhanced by GSS. Immature ovary segments were much more active than those from mature ovaries in synthesizing 1-MeA. The results support the contention that formation of free 1-MeA in starfish ovarian tissues involves methylation of adenine residues in a polynucleotide followed by liberation of 1-MeA by enzymatic hydrolysis.

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.

Metabolism of poly(A) sequences during 3T3 cell growth regulation

The metabolism and the intracellular distribution of RNA-associated poly(A) sequences from contact-inhibited and serum-induced proliferating 3T3 cells were examined by hybridization with [3H]poly(U). Evidence is presented for the growth-specific changes in the 'steady-state' populations of poly(A) sequences as determined by this technique. The results indicate that reinitiation of growth in 3T3 cells is accompanied by characteristic changes in total poly(A) content and the size of the 'steady-state' poly(A) sequence. The serum-induced state is characterized by: (1) a higher content of RNA-associated poly(A) sequences; (2) the redistribution of poly(A)-containing RNA species from sub-ribosomal to polysomal fraction; and (3) an overall shortening of the RNA-associated poly(A) sequences. The size distribution of newly synthesized poly(A) in both resting and serum-induced cells are similar even though there is a marked reduction in the size of steady-state poly(A) sequences of the RNA from growing cells. The suggestion that there is an enhanced poly(A) processing in serum-induced cells is further substantiated by an increase in poly(A) hydrolyzing activity in growing cells. These findings of the growth-specific cellular pattern of poly(A) sequences in RNA suggest a possible physiological role for the modulation of poly(A) size in the regulation of mRNA expression with respect to cell growth.

Translational regulation of polysome formation during dormancy of Physarum polycephalum

The translational activity of actively growing microplasmodia and dormant microsclerotia of Physarum polycephalum was investigated by analyzing the distribution of ribosomes in polysomes. Microplasmodial post-mitochondrial fractions contained substantial amounts of polysomes and ribosomal subunits but very few native monosomes. During the starvation period which preceded microsclerotium formation, polysome levels remained constant, whereas the subunit titer began to increase. During encystment ribosomal subunits continued to accumulate as the level of polysomes gradually decreased. Dormant microsclerotia contained a large surplus of stored ribosomal subunits but no detectable polysomes. However, incubation of microsclerotia with concentrations of cycloheximide sufficient to slow polypeptide elongation without affecting initiation caused the gradual reappearance of polysomes at the expense of the subunits. Under these conditions the percentage of subunits driven into polysomes reached values similar to those of actively growing microplasmodia. Microsclerotia returned to nutrient medium contained very low levels of polysomes during the lag period which preceded germination. These were formed with preexisting, stored messenger ribonucleic acid. During the germination period, polysome levels were markedly increased. This elevation was dependent on new ribonucleic acid transcription. It is concluded that dormant microsclerotia contain functional messenger ribonucleic acid and ribosomes which are subject to translational repression at the level of initiation.

Postfertilization poly(A) . protein complex formation on sea urchin maternal messenger RNA

A two-fold increase in polyadenylate [poly(A)] content occurs between fertilization and the two-cell stage in sea urchin zygotes. In this report the role of this cytoplasmic polyadenylation process in the provision of binding sites for poly(A)-associated proteins during early development of Lytechinus pictus is evaluated. Protein-associated poly(A) sequences, from ribonuclease-treated, post-mitochondrial supernatants of various developmental stages, were collected by nitrocellulose filtration and quantified by 3H-poly(U) complex formation. The proportion of protein-associated poly(A) rose from about 27% to about 60% of the total poly(A), on a nucleotide basis, during the period between fertilization and the eight-cell stage. However, the actual increase in number of poly(A) sequences associated with protein was more extensive, about 2.5-fold, since protein-associated poly(A) sequences average about 45 nucleotides longer than free poly(A). The protein-associated poly(A) of eggs and zygotes is found in two types of protease-sensitive complexes which sediment at 8--12 S and 15--20 S. The 8--12 S complex appears to be selectively increased in amount following fertilization. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the poly(A) protein complex fraction indicates the presence of 87,000 and 130,000 molecular weight polypeptides in both eggs and zygotes. It is concluded that quantitative, but not qualitative, alterations in the proportion of protein-associated poly(A) accompanies post-fertilization cytoplasmic polyadenylation in sea urchin zygotes. The attachment of specific proteins to the 3' terminus of maternal RNA's may be involved in their subsequent activities during early embryogenesis.