Poly (A)-containing polyribosomal RNA in sea urchin embryos: changes in proportion during development

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.

Developmental changes in the molecular weight of heterogeneous nuclear RNA

The size distribution of heterogeneous nuclear RNA (hnRNA) in the sea urchin embryo changes markedly during early development. Measurement of cleavage (4.5 h) to late gastrula (40 h) hnRNA by sedimentation in aqueous and denaturing solvent indicates that in the stages tested cleavage (4.5 h) hnRNA is smallest and rotating blastula (13 h) hnRNA is largest. The molecular weight of cleavage hnRNA is calculated to be about one-third that of rotating blastula hnRNA. Sedimentation of early embryo hnRNA in denaturing solvent to disaggregate complexes demonstrated mean S values lower than those obtained in aqueous solvent.

Polyadenylate-deficient analogues of poly(A)-containing mRNA sequences in cultured AKR mouse embryo cells

Five to six percent (by mass) of AKR-2B mouse embryo cell polysomal RNA consists of messenger RNA sequences which may exist in polyadenylated form. In the steady state, however, only 30--40% of these molecules are retained by extensive passage over oligo(dT)-cellulose, the remainder being present in the form of poly(A)-deficient analogues. Within experimental limits, these poly(A)-deficient analogues contain representatives of all poly(A)-containing mRNA sequences in these cells. An analysis of the kinetics of hybridization of cDNA probes enriched for either abundant or rare poly(A)-containing mRNA sequences suggests that the frequency distributions of poly(A)-containing and poly(A)-deficient analogues are dissimilar, and that a relationship exists between the intracellular frequency of a given mRNA sequence and the number of poly(A)-deficient analogues of that sequence. High frequency sequences appear to be enriched in the poly(A)-containing fraction, while low frequency sequences are predominately associated with the poly(A)-deficient fraction, thus, poly(A) may play a role in the regulation of mRNA frequency in the cytoplasm.

Purification of cDNA complementary to sea urchin histone mRNA

Complementary DNA (cDNA) was transcribed from a polyadenylated sea urchin histone mRNA preparation isolated by density gradient centrifugation. By hybridization, this cDNA was shown to be extensively contaminated (85% of hybridizable cDNA) with DNA complementary to RNA derived from the large ribosomal subunit. Purification of a mRNA specific cDNA fraction was achieved by hybridization of purified rRNA to cDNA followed by fractionation on hydroxyapatite. After further purification to remove nonhybridizable cDNA our purified cDNA showed only 8% hybirdization to rRNA.

Purification and properties of soybean leghemoglobin messenger RNA

Poly(A)-containing leghemoglobin mRNA from soybean root nodules has been purified 84-fold, as judged by its ability to direct the in vitro synthesis of leghemoglobin in a wheat germ system. It has a poly(A) content of 8.6% and a molecular weight, estimated by formamide gel electrophoresis, of 260 000. mRNA with a molecular weight of around 143 000 would be sufficient to code for leghemoglobin. Thus, with respect to both its poly(A) content and its unexpectedly high molecular weight, leghemoglobin mRNA is similar to mRNAs isolated from animal tissues.

Methylated constituents of poly(A)- and poly(A)+ polyribosomal RNA of sea urchin embryos

We have examined the poly(A)- and poly(A)+ polyribosomal (Ps) RNAs of developing sea urchin embryos to see whether they contain the capped structures previously observed in the poly(A)+ cytoplasmic RNA (mRNA) of a variety of eucaryotic organisms. We have found that both classes of Ps RNA contain 7-methylguanosine linked to the RNA by a 5'-5' triphosphate bridge. Capped structures derived from either class of Ps RNA appear to be of the type m7G5'ppp5'NmpNp, with only a single sugar-methylated nucleoside per cap, and we have estimated that both classes of Ps RNA are capped to the same extent. The 5' terminal 2'-0-methylated nucleosides have been identified and quantitated. The relative amounts are Am = 78%, Gm = 14%, Cm = 5%, Um = 3% for poly(A)- Ps RNA, and Am = 78%, Gm = 15%, Cm = 5%, Um = 2% for poly (A)+ Ps RNA. In addition, both classes of Ps RNA contain 2-4 internal residues of N6-methyladenylic acid per molecule.

Cordycepin and early effects of estradiol on the immature rat uterus

Estradiol induces the synthesis of a specific protein fraction (IP) in the uterus of the immature rat. The injection of cordycepin (3' deoxyadenosine), an inhibitor of poly A synthesis, inhibits the synthesis of IP. This fact suggests that one of the earliest effects of estrogen is the production of Hn-RNA poly-A relative to IP. Moreover, using electron microscopy, the stimulation by estradiol of the nucleolus of the immature rat uterine epithelium has been shown. Cordycepin does not affect this stimulation to any appreciable extent. Biochemical studies (incorporation of radioactive stracers into NRA, affinity chromatography on poly U-Sepharose) carried out in parallel with and under conditions comparable to those used in electron microscopy show that cordycepin does not greatly affect the increase in ribosomal RNA observed under the effect of estradiol. The blocking of IP by cordycepin and the lack of inhibition at the nucleolus level under the same conditions, show that the two early effects of the action of estrogen on the immature rat uterus are not directly correlated.

Translation of nonpolyadenylylated messenger RNA of sea urchin embryos

A large proportion of newly synthesized polyribosomal RNA of sea urchin blastulae is not polyadenylylated. The size distributions of the polyadenylylated and nonpolyadenylylated RNA are indistinguishable (mean size of 26 S). Upon translation of sea urchin polyribosomal RNA containing poly(A) and that without poly(A) in a wheat embryo cell-free protein-synthesizing system, where polypeptide synthesis is dependent on added messenger, both classes of RNA support peptide synthesis to the same extent. A preliminary analysis of the proteins synthesized in response to added mRNA (polyadenylylated and nonpolyadenylylated) indicates that these classes of mRNA molecules may code for different populations of proteins.