Some somatic sequences are absent or exceedingly rare in Xenopus oocyte RNA

A variety of Xenopus laevis cDNA clones derived from somatic cell RNAs were hybridized to oocyte pA⁺ RNA separated on Northern gels. We were unable to detect oocyte pA⁺ sequences complementary to three undefined tadpole cDNA clones. With one of these clones, a complex pattern of bands appears during embryogenesis. With the other two clones, a single band appears. Two additional tadpole clones hybridize to both oocyte and tadpole RNA, but yield a more complex RNA pattern from embryos than from oocytes. One of these additional tadpole clones has complementarity to actin DNA, suggesting that the additional RNA band which appears during embryogenesis is α-actin mRNA (E. A. Sturgess, J. E. M. Ballantine, H. R. Woodland, P. R. Mohun, C. D. Lane, and G. J. Dimitriadis, 1980, J. Embryol. Exp. Morphol.58, 303–320). We have also failed to detect hybridization to oocyte pA⁺ RNA with one vitellogenin and three adult globin cDNA clones. Reconstruction experiments with purified globin mRNA from anemic adult blood cells set the lower level of sensitivity for globin mRNA at one part in 10⁶. The data suggest that some Xenopus mRNA sequences are absent or very rare in the oocyte pA⁺ RNA population.     

mRNA usage during Drosophila melanogaster embryonic development. Analysis of nine cloned DNA segments

A set of nine phage lambda clones containing inserts from Drosophila melanogaster which are complementary to cDNA made from oocyte poly(A)+ RNA were selected from a larger group. These cloned elements code for a range of middle abundant RNA sequences which show no appreciable change in abundance during Drosophila embryogenesis. Seven of the nine clones are complementary to two oocyte RNAs, one to three RNAs and one to four RNAs. This study describes the changes that occur in these RNAs during embryonic development in the polysomal and non-polysomal fraction, and in the poly(A)+ RNA and poly(A)- RNA fraction. In all nine of these clones, greater than 70% of the complementary RNA is found in the polysomal region of a sucrose gradient. This proportion increases somewhat during development. Specific changes have been found during development in the proportion of RNA that is poly(A)+. Depending to the cloned sequence, this proportion may increase, decrease, or remain unchanged. For those clones that show a change, most of this change occurs between 8 and 19 h of development. Our data suggest, furthermore, the presence of a class of non-adenylated RNA being utilized during embryogenesis.     

The expression and genomic organization of randomly selected cloned Drosophila melanogaster genes

A lambda recombinant DNA library containing Drosophila melanogaster nuclear DNA inserts was screened with cDNA made from oocyte and gastrula poly(A)+ RNA. 124 clones were isolated which represented sequences complementary to a distribution of abundancies of their RNAs. The clone set was then used as probes to identify those whose RNA abundancies changed during embryonic development. The vast majority of clones showed little difference during development. Four different clones were identified whose poly(A)+ RNAs were quantitatively regulated; two were oocyte-specific, and two were embryonic-specific. 44 clones were chosen for in situ hybridization to salivary gland polytene chromosomes. The location and distribution of their sites are described. A class of clones, identified by in situ hybridization to the nucleolus, is further described. These clones contain a scrambled array of ribosomal intervening sequences.     

Molecular cloning and characterisation of cDNAs complementary to mRNAs from wounded potato (Solanum tuberosum) tuber tissue

Five cDNA clones complementary to mRNAs representing different abundances and responses to wounding have been isolated from a library of Sau 3A fragments in the bacteriophage M13 mp8. These were characterised by hybrid-release translation and hybridisation to RNA blots. The levels of RNA complementary to two of the clones show a marked increase during the 24h after wounding, one shows a small increase and two show no appreciable changes except that caused by a general increase in the total amount of polyadenylated RNA per microgram of total RNA which increases 2.5-fold during the same period. The would-induced RNAs are not induced in diluted suspension-culture cells, but RNA complementary to each clone is present in varying levels in stems, leaves and roots of intact potato plants.Abbreviations cDNA complementary DNA - poly(A) polyadenosine - poly(A)⁺ RNA polyadenylated RNA - poly(U) polyuridine     

Biochemical studies of mammalian oogenesis: synthesis and stability of various classes of RNA during growth of the mouse oocyte in vitro

The synthesis of various classes of RNA in mouse oocytes at different stages of growth has been examined after incubating follicles in medium containing radiolabeled uridine. After fractionation on poly(U)-Sepharose of radiolabeled oocyte RNA, of which about 83% is associated with the nucleus after a 5-hr labeling period, revealed that about 40–50% of the radiolabeled RNA behaved as poly(A)-containing RNA. This value remained fairly constant during the period of oocyte growth in which oocyte diameter increased from about 35 to about 55 μm. After a 5-hr labeling, the percentage of radiolabeled poly(A)-containing RNA in either the fully grown dictyate oocyte, metaphase II oocyte, or one-cell embryo was about 20%. After a 5-hr labeling, agarose gel electrophoretic analysis of the radiolabeled species of oocyte RNA obtained after fractionation on poly(U)-Sepharose revealed the presence of a putative ribosomal RNA precursor, ribosomal (28 and 18 S) RNA, transfer plus 5 S RNA and heterodisperse poly(A)-containing RNA. A significant fraction of the radiolabeled RNA species was quite large (>40 S). The ratios of the relative proportions of the radiolabeled ribosomal RNAs and transfer plus 5 S RNA remained essentially constant during oocyte growth. The stability of various classes of RNA was examined by incubating follicles with radiolabeled uridine, washing the follicles free of radioactivity and culturing the follicles under conditions which support oocyte growth in vitro (Eppig, 1977). Under these conditions, total oocyte radiolabeled RNA was quite stable as determined by retention of acid-insoluble radioactive material ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 28}\text{days}$). However, under conditions in which oocytes are viable but do not grow, the half-life of total RNA was about 4.5 days. Poly(A)-containing RNA was also very stable; after 8 days in culture, about 50% of the radiolabeled poly(A)-containing RNA present after 5 hr of labeling was still present. Agarose gel electrophoretic analysis of radiolabeled RNA in oocytes after 4 days of culture and after fractionation on poly(U)-Sepharose revealed the presence of ribosomal (28 and 18 S) RNA, transfer plus 5 S RNA, and heterodisperse poly(A)-containing RNA. At this time, these RNAs are located in the oocyte cytoplasm. In addition, the molecular weight distribution of poly(A)-containing RNA was significantly lower than that after 5 hr of labeling. The ratios of the relative proportions of radiolabeled ribosomal RNAs and transfer plus 5 S RNA were quite similar to those after 5 hr of labeling.     

Cloning and characterisation of the abundant cytoplasmic 7S RNA from mouse cells.

A cDNA library has been prepared from mouse embryo small RNAs and screened for the presence of clones complementary to the highly abundant cytoplasmic 7S RNA. One clone (pA6) was selected which hybridized exclusively with 7S RNA on a Northern blot prepared from cytoplasmic RNA run on high resolution polyacrylamide/urea gels. Sequence analysis of this clone has shown that at least 65 nucleotides at the 5' end of 7S RNA are extensively homologous with the highly repeated mouse B1 family. Heterologous hybridisations between the cloned mouse 7S sequence and RNAs prepared from rat, human and chick cells have shown that the non-B1 part of the 7S RNA molecule has been highly conserved during recent eucaryotic evolution. There are multiple copies of 7S RNA genes in the genomes of mouse, human, rat and chick cells, but substantial differences exist in copy number and genomic organisation in these organisms.     

A comparison of Xenopus laevis oocyte and embryo mRNA

Total RNA, extracted from mature oocytes and tadpoles of Xenopus laevis, was used as a template for in vitro protein synthesis. The oocyte RNA is markedly deficient in abundant mRNA species by comparison to tadpole RNA or other somatic RNAs, in agreement with previous experiments using RNA-cDNA hybridization analysis (S. Perlman and M. Rosbash, 1978, Develop. Biol.63, 197–212). Oocyte pA⁺ RNA is also larger than tadpole pA⁺ RNA or other somatic pA⁺ populations. The larger oocyte pA⁺ RNA and smaller oocyte pA⁺ RNA are equally good templates for in vitro protein synthesis, which implies that much, and perhaps all, of the large oocyte pA⁺ RNA is bona fide mRNA. We suggest that the relatively large size of the oocyte pA⁺ RNA population is due, at least in part, to the relative lack of abundant mRNA species in the population. This suggestion follows from the observation of 0. Meyuhas and R. P. Perry (1979, Cell16, 139–148) that L-cell-abundant mRNAs are preferentially small and rare mRNAs preferentially large. Most of the oocyte pA⁺ sequences are also present in tadpoles and are still adenylated at this stage. Oocyte proteins synthesized in vivo do not appear deficient in abundant proteins, suggesting that a translational control mechanism operates to select certain pA⁺ RNAs at higher frequencies than others.     

Program of early development in the mammal: Changes in absolute rates of synthesis of ribosomal proteins during oogenesis and early embryogenesis in the mouse

The absolute rates of synthesis of specific ribosomal proteins have been determined during growth and meiotic maturation of mouse oocytes, as well as during early embryogenesis in the mouse. These measurements were made possible by the development of a high-resolution twodimensional gel electrophoresis procedure capable of resolving basic proteins with isoelectric points between 9.1 and 10.2. Mouse ribosomal proteins were separated on such gels and observed rates of incorporation of [³⁵S]methionine into each of 12 representative ribosomal proteins were converted into absolute rates of synthesis (femtograms or moles synthesized/hour/oocyte or embryo) by using previously determined values for the absolute rates of total protein synthesis in mouse oocytes and embryos (R. M. Schultz, M. J. LaMarca, and P. M. Wassarman, 1978,Proc. Nat. Acad. Sci. USA,75, 4160;R. M. Schultz, G. E. Letourneau, and P. M. Wassarman, 1979,Develop. Biol.,68, 341–359). Ribosomal proteins were synthesized at all stages of oogenesis and early embryogenesis examined and, while equimolar amounts of ribosomal proteins were found in ribosomes, they were always synthesized in nonequimolar amounts during development. Rates of synthesis of individual ribosomal proteins differed from each other by more than an order of magnitude in some cases. Synthesis of ribosomal proteins accounted for 1.5, 1.5, and 1.1% of total protein synthesis during growth of the oocyte, in the fully grown oocyte, and in the unfertilized egg, respectively. During meiotic maturation of mouse oocytes the absolute rate of synthesis of ribosomal proteins decreased about 40%, from 620 to 370 fg/hr/cell, as compared to a 23% decrease in the rate of total protein synthesis during the same period. On the other hand, during early embryogenesis the absolute rates of synthesis of each of the 12 ribosomal proteins examined increased substantially as compared with those of the unfertilized egg, such that at the eight-cell stage of embryogenesis synthesis of ribosomal proteins (4.17 pg/hr/embryo) accounted for about 8.1% of the total protein synthesis in the embryo. Consequently, while the absolute rate of total protein synthesis increased about 1.5-fold during development from an unfertilized mouse egg to an eight-cell compacted embryo, the absolute rate of ribosomal protein synthesis increased more than 11-fold during the same period. These results seem to reflect the differences reported for the patterns of ribosomal RNA synthesis during early development of mammalian, as compared to nonmammalian, animal species. The results are compared with those obtained using oocytes and embryos fromXenopus laevis.     

Identification and genetic localization of mRNAs from ovarian follicle cells of Drosophila melanogaster.

RNA synthesis in ovarian follicles of Drosophila melanogaster was studied by methods which eliminate experimentally induced alterations in gene expression. Gel electrophoresis of follicular RNA, labeled after injection of precursors into females, revealed qualitative and quantitative differences in synthesis during the course of oogenesis. A highly heterogeneous group of poly(A)-containing RNAs is produced during much of the course of follicular development. However, post-vitellogenic stages synthesize a small number of stage-specific poly(A)-containing RNAs. During this period, RNA synthesis is known to take place primarily in the follicle cells, which are engaged in the production of the endochorion and exochorion. Two intense bands of nonmitochondrial poly(A)⁺ RNA are labeled between stage 11 and early stage 13. The synthesis of a more heterogeneous group of very small poly(A)-containing RNAs characterizes the last part of oogenesis, stages 13 and 14. Evidence is presented to show that these RNAs are specifically localized in the follicle cells of the egg chamber. We propose that they represent mRNAs for chorion proteins. In situ hybridization of preparations of late stage poly(A)-containing RNA to salivary gland chromosomes revealed two major sites of complementarity, 7E11 and 12E, as well as several minor sites. Experiments in which RNAs were separated on gels prior to hybridization in situ suggested that both the major stage 12-specific RNA bands contained molecules which were complementary to DNA in the 7E11 region. It is particularly interesting that this site is within a small chromosomal interval known to contain the gene ocelliless. Females homozygous for ocelliless have been shown to produce structurally abnormal chorions (Johnson and King, 1974).     

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.     

The isolation of plasmids containing dna complementary to messenger rna for variant surface glycoproteins of Trypanosoma brucei

We have isolated poly(A)⁺ RNA from four antigenic variants (117, 118, 121, 221) of one clone of Trypanosoma brucei. Translation of these poly(A)⁺ RNAs in a rabbit reticulocyte lysate gave rise to proteins that could be precipitated with antisera against homologous variant surface glycoprotein, the protein responsible for antigenic variation in trypanosomes. From the electrophoretic mobility of these in vitro products in sodium dodecyl sulphate (SDS) gels we infer that variant surface glycoproteins (VSGs) are made as pre-proteins, which require trimming to yield mature VSGs.The total translation products from the four poly(A)⁺ RNAs produced a complex set of bands on SDS gels, which only differed in the region where the variant pre-glycoproteins migrated. The only detectable variation in the messenger RNA populations of these variants is, therefore, in the messenger RNA for variant pre-glycoproteins.We have made duplex DNA copies of these poly(A)⁺ RNAs, linked the complementary DNA to plasmid pBR322 by GC tailing and cloned this recombinant DNA in Escherichia coli. Colony hybridization with complementary DNA made on poly(A)⁺ RNA showed that 7–10% of the colonies contained DNA that hybridized only with the homologous probe. Plasmid DNA was isolated from ten such colonies (two or three of each variant complementary DNA), bound to diazobenzyloxymethyl-cellulose (DBM) paper and used to select complementary messenger RNA from total poly(A)⁺ RNA by hybridization. In eight cases the RNA recovered from the filter gave variant pre-glycoprotein as the predominant product of in vitro translation.Poly(A)⁺ RNA from each of the variants only hybridized to the homologous complementary DNA in filter hybridizations. Each trypanosome variant, therefore, contains no detectable messenger RNAs for the three heterologous variant-specific glycoproteins tested. We conclude from this lack of cross-hybridization that antigenic diversity in trypanosomes, unlike antibody diversity in mammals, does not involve the linkage of a repertoire of genes for the variable N-terminal half to a single gene for the C-terminal half of the VSGs.     

Identification and Characterization of Neuron-Specific and Developmentally Regulated Gene Transcripts in the Chick Embryo Spinal Cord

Clones corresponding to neuron-specific and developmentally regulated messenger RNA species in the chick have been isolated from a complementary DNA library prepared using polyadenylated RNA from 7-day embryonic spinal cord. The library was initially screened by differential complementary DNA hybridization procedures for clones identifying polyadenylated RNAs present in embryonic spinal cord but absent from or at low abundance in liver tissue. A high proportion of selected recombinant plasmids were found to identify different RNA species which, although present in 14-day embryonic spinal cord, could not be detected in a corresponding region of the developing chick CNS that is devoid of neuronal cell bodies, the optic nerve. The neuron-specific assignment of these mRNAs within the developing neuroectoderm was confirmed using bulk-isolated neuronal and glial-enriched cell fractions from 7-day embryonic spinal cord. In addition, several distinctive patterns of developmentally regulated expression of neuron-specific messenger RNA species have been observed in the chick spinal cord. The studies lay a foundation for detailed examination of the regional and temporal distribution and control of neuronal gene expression in the chick spinal cord during embryogenesis.     

Gene expression during induction of somatic embryogenesis in carrot cell suspensions

We report the identification, via their cDNAs, of genes which are temporarily transcribed during the initiation of somatic embryogenesis in carrot (Daucus carota L.) cells cultured in an auxin-free medium. Their expression is roughly associated with the first morphogenetic, or globular, stage. A cDNA library ( gt 10) was established using poly(A)⁺ -rich RNAs from cells deprived of auxin for 8 d. By differential screening a number of clones corresponding to early-induced embryogenic genes were identified. For several a temporary accumulation of the specific mRNA between 6 and 16 d after induction was observed. With regard to the nucleotide sequence and the respective deduced amino-acid sequence, two glycine-rich proteins and a polypeptide with a proline-rich domain were among the products of genes activated at the onset of somatic embryogenesis.Abbreviations b, bp bases, basepairs - 2,4-D 2,4-dichlorophenoxyacetic acid Sequence data reported here will appear in the EMBL Genbank and DDBJ Nucleotide Sequence Databases under the following accession numbers: X 15436 for clone DC 2.15 (proline-rich protein), X 15706 for clone DC 7.1 (glycine-rich protein, DCGRP) and X 14067 for clone DC 9.1 (glycine-rich protein, DCGRP)This research was supported by the Deutsche Forschungsgemeinschaft. We thank Mrs. I. Liebscher for her competent assistance.