The functional stability of sea urchin histone mRNA injected into oocytes of Xenopus laevis

Cholinoceptive properties of human primordial, preantral, and antral oocytes: in situ hybridization and biochemical evidence for expression of cholinesterase genes

In addition to their well-known involvement in neuromuscular junctions and in brain cholinergic synapses, cholinergic mechanisms have been implicated in the growth and maturation of oocytes in various species. Functional acetylcholine receptors were electrophysiologically demonstrated in amphibian and mammalian oocyte membranes, and activity of the acetylcholine-hydrolyzing enzyme, acetylcholinesterase (AChE), was biochemically measured in the exceptionally big oocytes of the frog Xenopus laevis. However, biochemical methods could not reveal whether AChE was produced within the oocytes themselves or in the surrounding follicle cells. Furthermore, this issue is particularly important for understanding growth and fertilization processes in the much smaller human oocytes, in which the sensitivity of AChE biochemical measurements is far too low to be employed. To resolve this question, a molecular biology approach was combined with biochemical measurements on ovarian extracts and sections. To directly determine whether the human cholinesterase (ChE) genes are transcriptionally active in oocytes, and, if so, at what stages in their development, the presence of ChE mRNA was pursued. For this purpose frozen ovarian sections were subjected to in situ hybridization using 35S-labeled human ChE cDNA. Highly pronounced hybridization signals were localized within oocytes in primordial, preantral, and antral follicles, but not in other ovarian cell types, demonstrating that within the human ovary ChE mRNA is selectively synthesized in viable oocytes at different developmental stages.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of histone gene expression during early development of Xenopus laevis

Using literature data for transcriptional and translational rate constants, gene copy numbers, DNA concentrations, and stability constants, we have calculated the expected concentrations of histones and histone mRNA during embryogenesis of Xenopus laevis. The results led us to conclude that: (i) for X. laevis the gene copy number of the histone genes is too low to ensure the synthesis of sufficient histones during very early development, inheritance from the oocyte of either histone protein or histone mRNA (but not necessarily both) is necessary; (ii) from the known storage of histones in the oocyte and the rates of histone synthesis determined by Adamson & Woodland (1977), there would be sufficient histones to structure the newly synthesized DNA up to gastrulation but not thereafter (these empirical rates of histone synthesis may be underestimates); (iii) on the other hand, the amount of H3 mRNA recently observed during early embryogenesis (Koster, 1987, Koster et al., 1988) could direct a higher and sufficient synthesis of H3 protein, also after gastrulation. We present a quantitative model that accounts both for the observed H3 mRNA concentration as a function of time during embryogenesis and for the synthesis of sufficient histones to structure the DNA throughout early embryogenesis. The model suggests that X. laevis exhibits a major (i.e. some 14-fold) reduction in transcription of histone genes approximately 11 hours after fertilization. This reduction could be due to a decrease in the number of transcribed histone genes, a decreased rate constant of transcription with continued transcription of all the histone genes, and/or a reduction in the time during the cell cycle in which histone mRNA synthesis takes place. Alternatively, the histone mRNA stability might decrease approximately 16-fold 11 hours after fertilization.

Differential stimulation of sea urchin early and late H2B histone gene expression by a gastrula nuclear extract after injection into Xenopus laevis oocytes

Sea urchin early histone genes are active in preblastula embryos; late histone genes are maximally expressed during subsequent stages of embryogenesis. We used the Xenopus laevis oocyte to assay for trans-acting factors involved in this differential regulation. Sea urchin nuclear proteins were prepared by extracting gastrula-stage chromatin successively with 0.45, 1, and 2 M NaCl. We injected three fractions into oocytes along with plasmids bearing sea urchin early and late H2b histone genes. While neither the 0 to 0.45 M nor the 1 to 2 M salt fraction affected H2b gene expression, the 0.45 to 1 M salt fraction stimulated early and late H2b mRNA levels significantly. Late H2b gene expression was stimulated preferentially when the early and late genes were coinjected into the same oocytes. This extract did not stimulate the accumulation of transcripts of injected herpesvirus thymidine kinase genes or of the sea urchin Spec 1 gene, suggesting that the stimulatory activity is not a general transcription factor. We localized the DNA sequence required for the stimulatory effect to a region of the late H2b gene located between -43 and +62 relative to the transcription start site. A component of the 0.45 to 1 M salt wash fraction specifically bound to the 105-base-pair late gene DNA sequence and to the corresponding early gene fragment. The abundance of this binding activity decreased on a per genome basis during early development of the sea urchin.

Differential stimulation of sea urchin early and late H2B histone gene expression by a gastrula nuclear extract after injection into Xenopus laevis oocytes

Sea urchin early histone genes are active in preblastula embryos; late histone genes are maximally expressed during subsequent stages of embryogenesis. We used the Xenopus laevis oocyte to assay for trans-acting factors involved in this differential regulation. Sea urchin nuclear proteins were prepared by extracting gastrula-stage chromatin successively with 0.45, 1, and 2 M NaCl. We injected three fractions into oocytes along with plasmids bearing sea urchin early and late H2b histone genes. While neither the 0 to 0.45 M nor the 1 to 2 M salt fraction affected H2b gene expression, the 0.45 to 1 M salt fraction stimulated early and late H2b mRNA levels significantly. Late H2b gene expression was stimulated preferentially when the early and late genes were coinjected into the same oocytes. This extract did not stimulate the accumulation of transcripts of injected herpesvirus thymidine kinase genes or of the sea urchin Spec 1 gene, suggesting that the stimulatory activity is not a general transcription factor. We localized the DNA sequence required for the stimulatory effect to a region of the late H2b gene located between -43 and +62 relative to the transcription start site. A component of the 0.45 to 1 M salt wash fraction specifically bound to the 105-base-pair late gene DNA sequence and to the corresponding early gene fragment. The abundance of this binding activity decreased on a per genome basis during early development of the sea urchin.

Widespread changes in the translation and adenylation of maternal messenger RNAs following fertilization of Spisula oocytes

We have reported previously that sequence-specific adenylations and deadenylations accompany changes in the translation of maternal mRNA following fertilization of Spisula oocytes (E.T. Rosenthal, T.R. Tansey, and J.V. Ruderman, 1983, J. Mol. Biol. 166, 309-327). The data presented here confirm and extend those observations. We have identified four classes of maternal mRNA with respect to translation: Class 1-not translated in oocytes and translated at very high efficiency immediately after fertilization, Class 2-not translated in oocytes and partially utilized for translation following fertilization, Class 3-translated in oocytes and not translated in embryos, and Class 4-not translated either before or after fertilization. There is an excellent, although not perfect, correlation between the translation of an mRNA and its polyadenylation status. The poly(A) tails of all the mRNAs which are translated in oocytes and untranslated in embryos are shortened at fertilization, and the poly(A) tails of those mRNAs which are untranslated in oocytes and translated in embryos are lengthened at fertilization. These adenylations and deadenylations occur simultaneously during the first 20 min following fertilization.

Histone mRNA degradation in vivo: the first detectable step occurs at or near the 3' terminus

The first detectable step in the degradation of human H4 histone mRNA occurs at the 3' terminus in a cell-free mRNA decay system (J. Ross and G. Kobs, J. Mol. Biol. 188:579-593, 1986). Most or all of the remainder of the mRNA is then degraded in a 3'-to-5' direction. The experiments described here were designed to determine whether a similar degradation pathway is followed in whole cells. Two sets of short-lived histone mRNA decay products were detected in logarithmically growing erythroleukemia (K562) cells. These products, designated the -5 and -12 RNAs, were generated by the loss of approximately 4 to 6 and 11 to 13 nucleotides, respectively, from the 3' terminus of histone mRNA. The same decay products were observed after a brief incubation in vitro. They were in low abundance or absent from cells that were not degrading histone mRNA. In contrast, they were readily detectable in cells that degraded the mRNA at an accelerated rate, i.e., in cells cultured with a DNA synthesis inhibitor, either cytosine arabinoside or hydroxyurea. During the initial stages of the decay process, as the 3' terminus of the mRNA was being degraded, the 5'-terminal region remained intact. These results indicate that the first detectable step in human H4 histone mRNA decay occurs at the 3' terminus and that degradation proceeds 3' to 5', both in cells and in cell-free reactions.

Histone mRNA degradation in vivo: the first detectable step occurs at or near the 3' terminus

The first detectable step in the degradation of human H4 histone mRNA occurs at the 3' terminus in a cell-free mRNA decay system (J. Ross and G. Kobs, J. Mol. Biol. 188:579-593, 1986). Most or all of the remainder of the mRNA is then degraded in a 3'-to-5' direction. The experiments described here were designed to determine whether a similar degradation pathway is followed in whole cells. Two sets of short-lived histone mRNA decay products were detected in logarithmically growing erythroleukemia (K562) cells. These products, designated the -5 and -12 RNAs, were generated by the loss of approximately 4 to 6 and 11 to 13 nucleotides, respectively, from the 3' terminus of histone mRNA. The same decay products were observed after a brief incubation in vitro. They were in low abundance or absent from cells that were not degrading histone mRNA. In contrast, they were readily detectable in cells that degraded the mRNA at an accelerated rate, i.e., in cells cultured with a DNA synthesis inhibitor, either cytosine arabinoside or hydroxyurea. During the initial stages of the decay process, as the 3' terminus of the mRNA was being degraded, the 5'-terminal region remained intact. These results indicate that the first detectable step in human H4 histone mRNA decay occurs at the 3' terminus and that degradation proceeds 3' to 5', both in cells and in cell-free reactions.

Patterns of maternal messenger RNA accumulation and adenylation during oogenesis in Urechis caupo

We have investigated the accumulation and adenylation of the maternal mRNA during oogenesis in the oocytes of the marine worm Urechis caupo. The analysis, using in vitro translation and cDNA probes to assay for specific mRNAs, demonstrates that different maternal mRNAs accumulate with different patterns during oogenesis. One class of maternal mRNAs accumulates throughout oogenesis and remains at a steady level in the full-grown oocyte. These mRNAs do not have a poly(A) tail long enough to mediate binding to oligo(dT)-cellulose in oocytes, but are rapidly adenylated immediately following fertilization. The other maternal mRNAs accumulate in growing oocytes as poly(A)+ RNA and undergo some deadenylation in full-grown oocytes and embryos. Some of these mRNAs attain their highest concentration fairly early in oogenesis, while others continue to accumulate during later stages. Many of the mRNAs that accumulate as poly(A)+ RNA in growing oocytes diminish dramatically in concentration in full-grown oocytes.

Differential stability of Drosophila embryonic mRNAs during subsequent larval development

The relative stabilities of specific embryonic mRNAs that persist in Drosophila melanogaster larvae were determined using an approach that combined RNA density labeling with cell-free translation. Unlike the other methods commonly used to measure the decay of individual mRNAs, the density labeling approach does not depend on the use of transcriptional inhibitors or on the measurement of precursor pool specific activities. Using this approach, we have determined that different embryonic mRNA species persist for varying periods during subsequent development, with half-lives ranging from approximately 2 to approximately 30 h. The embryonic histone mRNAs are relatively unstable; they are no longer detectable by 9 h of larval development. By 41 h of larval development, 90% of the nonhistone mRNAs assayed have decayed considerably; computerized scanning densitometry of translation products indicates that these transcripts are not decaying as members of discrete half-life classes. The persisting mRNAs that remain are very long-lived; their in vitro translation products can still be detected after 91 h of larval development. We have tentatively identified the mRNAs that encode actin, tropomyosin, and tubulin as members of this stable mRNA population. Although embryonic mRNAs do fall into these three broad classes of stability, they appear to decay with a continuum of half-lives. Because the range of half-lives is so great, mRNA stability is probably an important factor controlling mRNA abundance during Drosophila development.

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

Poly(A)-containing RNA in early embryogenesis of sea urchins

The content, biosynthesis and template activity of poly(A)+ RNA in the early stages of sea urchin development have been studied. The amount of poly(A)+ RNA reaches a maximum at the middle blastula stage in polyribosomes and at the 8-blastomere stage in the cytoplasm. Poly(A)+ RNA synthesis becomes noticeable at the 64-blastomere stage and the spectrum of newly synthesized molecules is different from that at the middle blastula stage. The products of translation in vitro of poly(A)+ RNA at all the stages studied show insignificant differences and contain a major group of polypeptides of molecular mass 10-20 kDa.

Human beta and delta globin messenger RNAs turn over at different rates

We have tested the hypothesis of Winslow & Ingram (1966) that the steady-state levels of human beta and delta globin proteins are determined, in part, by the stabilities of their respective messenger RNAs. Nucleated bone marrow cells were cultured under different RNA "chase" conditions, and samples were harvested at intervals thereafter. The quantities of beta and delta globin mRNA sequences in total RNA from these cells were measured by solution hybridization with specific 32P-labeled DNA probes. The average half-lives of beta and delta globin mRNAs are 16.5 and 4.5 hours, respectively. Both mRNAs are polyadenylated. The rapid turnover of delta globin mRNA accounts, at least in part, for the low level of delta globin mRNA in non-nucleated peripheral blood reticulocytes. The possibility that the rate of mRNA decay is determined by nucleotide sequence signals located in the 3' untranslated region is discussed.

Translational activity and functional stability of human fibroblast beta 1 and beta 2 interferon mRNAs lacking 3'-terminal RNA sequences

Polyadenylylated mRNA was purified from poly(I).poly(C)- and cycloheximide-superinduced human fibroblast (FS-4) cultures. The mRNA was subjected to electrophoresis through an agarose/CH3HgOH gel, and human fibroblast beta 1 and beta 2 interferon mRNAs were isolated. Each mRNA preparation was phosphorolyzed at 0 degrees C for 20 min by using a molar excess of polynucleotide phosphorylase to produce RNAs lacking poly(A) and then incubated at 37 degrees C for varying lengths of time to allow the phosphorylase to further digest the deadenylylated RNA from the 3' end in a processive and synchronous manner. Removal of the poly(A) (less than or equal to 100 residues) and approximately 100 adjacent residues from human fibroblast beta 1 interferon mRNA (native length, 900 residues, including a 3'-noncoding region of 203 residues) did not alter the translational activity or the functional stability of this mRNA in Xenopus oocytes, whereas deletion of the poly(A) and approximately 200 adjacent residues decreased its translational efficiency. On the other hand, removal of the poly(A) (approximately 200 residues) and approximately 200 adjacent residues from human fibroblast beta 2 interferon mRNA (native length, 1300 residues) did not alter the translational activity or the functional stability of this molecule in oocytes. Thus, neither the poly(A) nor large segments of the 3'-noncoding region (which includes the hexanucleotide A-A-U-A-A-A sequence, at least in the case of beta 1 mRNA) are required for the maintenance of the functional stability of human beta 1 and beta 2 interferon mRNAs in Xenopus oocytes.

Transcriptional fidelity of histone genes injected into Xenopus oocyte nuclei

Previous work has indicated that at least some of the genetic information required for the expression of sea urchin histone genes is recognized following injection of the gene repeat (h22) into Xenopus oocyte nuclei. The ability to elicit the expression of cloned genes and their sequence-manipulated counterparts is proving invaluable in analysing the molecular details of gene expression. Direct injection of such genes into Xenopus oocyte nuclei remains one of the simplest methods for obtaining such expression and a remarkable degree of transcriptional fidelity has been demonstrated using this system with RNA polymerase III genes, and to a lesser extent with rDNAs transcribed by RNA polymerase I. In the case of polymerase II genes there is ample evidence for coupled transcription-translation, but the degree of transcriptional fidelity involved may, as has recently been shown for the ovalbumin gene, be minimal. However, clearly if the oocyte is to be used to investigate transcriptional regulation of such genes, transcriptional fidelity defined as the production of correct RNA termii, rather than the production of 'functional mRNAs' (ref. 15), must pertain. Here we demonstrate such fidelity in the expression of all five Psammechinus miliaris histone genes comprising a repeat unit. However, we find large quantitative variations in the levels of synthesis of the individual correct termini and hence of the mRNAs. In addition to the mRNAs, species with no detectable counterparts in the sea urchin are generated off the coding strand, as are heterogeneous noncoding species.