Transcript levels and translational control of hsp70 synthesis in Xenopus oocytes

Established Protocols for cRNA Expression and Voltage-Clamp Characterization of the P2X7 Receptor in Xenopus laevis Oocytes

P2X7 receptors (P2X7Rs) are fast ATP^4--gated ion channels that, like other members of the P2X receptor family, function as homotrimers. A high-resolution cryo-EM structure of the full-length rat P2X7R is available. Using voltage-clamp experiments in Xenopus laevis oocytes, even the earliest steps of P2X7R activation can be quantitatively recorded in the millisecond range. Site-directed mutagenesis combined with voltage-clamp recordings can reveal residues and domains of the P2X7R involved in ATP^4- binding, gating (i.e., opening and closing of the channel pore) and ion selectivity. We present here proven voltage-clamp protocols that take into account requirements that are important at the levels of cDNA and vector sequences, cRNA synthesis, and Xenopus laevis oocyte isolation for reliable results.

Amphibian oocytes release heat shock protein A during spawning: evidence for a role in fertilization

Heat shock proteins A (HSPAs, previously known as HSP70s) are widely distributed proteins originally linked with heat shock but now associated with several normal cellular functions. We recently found indirect evidence suggesting a role for HSPAs in sperm-oocyte interaction in the amphibian Bufo arenarum. In the present study our aim was to study its expression, subcellular distribution, and role during fertilization. By Western blot analysis using two different antibodies we detected HSPAs present in B. arenarum oocytes in the absence of any stress. We performed two-dimensional electrophoresis and detected two isoforms with isoelectric points of 5.25 and 5.45. We studied its subcellular distribution isolating total membranes, cytosol, and plasma membranes. HSPAs were present in all of these fractions. We confirmed these results by immunofluorescence microscopy and also found that the HSPA signal was present in the vitelline envelope. To further test this, we performed Western blot analysis in isolated vitelline envelopes and in egg water (diffusible material from deposited oocytes). HSPAs were present in these two fractions. Moreover, human recombinant his-tagged HSPA (HSPA1A) was able to specifically bind to sperm in vitro (midpiece) and enhance sperm membrane integrity. In vitro fertilization assays in the presence of anti-HSPA polyclonal antibodies showed diminished fertilization scores at low sperm concentrations (10(5) cells per milliliter). Our results suggest that HSPAs are present in intracellular and extracellular structures of nonstressed B. arenarum oocytes and participates in fertilization by and that their release during spawning plays a role in sperm membrane integrity.

Heat shock induces mini-Cajal bodies in the Xenopus germinal vesicle

Cajal bodies are evolutionarily conserved nuclear organelles that are believed to play a central role in assembly of RNA transcription and processing complexes. Although knowledge of Cajal body composition and behavior has greatly expanded in recent years, little is known about the molecules and mechanisms that lead to the formation of these organelles in the nucleus. The Xenopus oocyte nucleus or germinal vesicle is an excellent model system for the study of Cajal bodies, because it is easy to manipulate and it contains 50-100 Cajal bodies with diameters up to 10 μm. In this study we show that numerous mini-Cajal bodies (less than 2 μm in diameter) form in the germinal vesicle after oocytes recover from heat shock. The mechanism for heat shock induction of mini-Cajal bodies is independent of U7 snRNA and does not require transcription or import of newly translated proteins from the cytoplasm. We suggest that Cajal bodies originate by self-organization of preformed components, preferentially on the surface of B-snurposomes.

Programmed cell death in the neurulating embryo is prevented by the chaperone heat shock cognate 70

Neuronal cell death is a genuine developmental process, with precise regulation and defined roles. In striking contrast, characterization of cell death that occurs at early stages of neural development is very limited. We previously showed that embryonic proinsulin increases the level of the chaperone heat shock cognate 70 (Hsc70) and reduces the incidence of apoptosis in the neurulating chick embryo [de la Rosa, et al. (1998), Proc. Natl. Acad. Sci. USA, 95, 9950]. We now demonstrate that Hsc70 is directly involved in cell survival during neurulation, as specific downregulation of endogenous Hsc70 by antisense oligodeoxynucleotide interference provoked an increase in apoptosis both in vitro and in ovo. In parallel, activation of caspase-3 was increased after hsc70 antisense oligodeoxynucleotide treatment. Dead cells were located mostly in the developing nervous system, distributed in areas where the incidence of cell death was high. These areas coincided both in vivo and under different death-inducing conditions, including antisense interference and growth factor deprivation. Hsc70 immunostaining was strong in at least some areas of high cell death. Apoptotic cells within these areas presented undetectable Hsc70 levels, however, suggesting that this protein acts as an intrinsic protector of neuroepithelial and neural precursor cells.

Heat shock proteins in retinal neurogenesis: identification of the PM1 antigen as the chick Hsc70 and its expression in comparison to that of other chaperones

While the role of heat shock proteins under experimental stress conditions is clearly characterized, their expression in unstressed cells and tissues and their functions in normal cell physiology, besides their chaperone action, remain largely undetermined. We report here the identification in chicken of the antigen recognized by the monoclonal antibody PM1 [Hernández-Sánchez et al. (1994) Eur. J. Neurosci., 6,1801-1810] as the noninducible chaperone heat-shock cognate 70 (Hsc70). Its identity was determined by partial peptide sequencing, immuno-crossreactivity and two-dimensional gel-electrophoresis. In addition, we examined its expression during chick embryo retinal neurogenesis. The early widespread Hsc70 immunostaining corresponding to most, if not all, of the neuroepithelial cells becomes restricted to a subpopulation of these cells in the peripheral retina as development proceeds. On the other hand, retinal ganglion cells, differentiating in the opposite central-to-peripheral gradient, retained Hsc70 immunostaining. Other molecular chaperones, the heat-shock proteins Hsp40, Hsp60 and Hsp90, did not seem to compensate the loss of Hsc70. They also showed decreasing immunostaining patterns as neurogenesis proceeds, although distinctive from that of Hsc70, whereas Hsp70 was not detected in the embryonic retina. This precise cellular and developmental regulation of Hsc70, a generally considered constitutive molecular chaperone, in unstressed embryos, together with the expression of other chaperones, provides new tools and a further insight on neural precursor heterogeneity, and suggests possible specific cellular roles of chaperone function during vertebrate neurogenesis.

HSP90 interacts with and regulates the activity of heat shock factor 1 in Xenopus oocytes

Transcriptional activation of heat shock genes is a reversible and multistep process involving conversion of inactive heat shock factor 1 (HSF1) monomers into heat shock element (HSE)-binding homotrimers, hyperphosphorylation, and further modifications that induce full transcriptional competence. HSF1 is controlled by multiple regulatory mechanisms, including suppression by additional cellular factors, physical interactions with HSP70, and integration into different cellular signaling cascades. However, the signaling mechanisms by which cells respond to stress and control the HSF1 activation-deactivation pathway are not known. Here we demonstrate that HSP90, a cellular chaperone known to regulate several signal transduction molecules and transcription factors, functions in the regulation of HSF1. The existence of HSF1-HSP90 heterocomplexes was shown by coimmunoprecipitation of HSP90 with HSF1 from unshocked and heat-shocked nuclear extracts, recognition of HSF1-HSE complexes in vitro by using HSP90 antibodies (Abs), and recognition of HSF1 in vivo by HSP90 Abs microinjected directly into oocyte nuclei. The functional impact of HSP90-HSF1 interactions was analyzed by using two strategies: direct nuclear injection of HSP90 Abs and treatment of cells with geldanamycin (GA), an agent that specifically blocks the chaperoning activity of HSP90. Both HSP90 Abs and GA delayed the disassembly of HSF1 trimers during recovery from heat shock and specifically inhibited heat-induced transcription from a chloramphenicol acetyltransferase reporter construct under control of the hsp70 promoter. HSP90 Abs activated HSE binding in the absence of heat shock, an effect that could be reversed by subsequent injection of purified HSP90. GA did not activate HSE binding under nonshock conditions but increased the quantity of HSE binding induced by heat shock. On the basis of these findings and the known properties of HSP90, we propose a new regulatory model in which HSP90 participates in modulating HSF1 at different points along the activation-deactivation pathway, influencing the interconversion between monomeric and trimeric conformations as well as transcriptional activation. We also put forth the hypothesis that HSP90 links HSF1 to cellular signaling molecules coordinating the stress response.

Modulation of the chaperone heat shock cognate 70 by embryonic (pro)insulin correlates with prevention of apoptosis

Insights have emerged concerning insulin function during development, from the finding that apoptosis during chicken embryo neurulation is prevented by prepancreatic (pro)insulin. While characterizing the molecules involved in this survival effect of insulin, we found insulin-dependent regulation of the molecular chaperone heat shock cognate 70 kDa (Hsc70), whose cloning in chicken is reported here. This chaperone, generally considered constitutively expressed, showed regulation of its mRNA and protein levels in unstressed embryos during early development. More important, Hsc70 levels were found to depend on endogenous (pro)insulin, as shown by using antisense oligodeoxynucleotides against (pro)insulin mRNA in cultured neurulating embryos. Further, in the cultured embryos, apoptosis affected mainly cells with the lowest level of Hsc70, as shown by simultaneous Hsc70 immunostaining and terminal deoxynucleotidyltransferase-mediated UTP nick end labeling. These results argue in favor of Hsc70 involvement, modulated by embryonic (pro)insulin, in the prevention of apoptosis during early development and suggest a role for a molecular chaperone in normal embryogenesis.

Nuclear history of a pre-mRNA determines the translational activity of cytoplasmic mRNA

The pathways of synthesis and maturation of pre-messenger RNA in the nucleus have a direct effect on the translational efficiency of mRNA in the cytoplasm. The transcription of intron-less mRNA in vivo directs this mRNA towards translational silencing. The presence of an intron at the 5' end of the transcript relieves this silencing, whereas an intron at the 3' end further represses translation. These regulatory events are strongly dependent on the transcription of pre-mRNA in the nucleus. The impact of nuclear history on regulatory events in the cytoplasm provides a novel mechanism for the control of gene expression.

Conditions for a heat shock response during oogenesis and embryogenesis of the amphibian Pleurodeles waltl

The optimal conditions capable of inducing an increase in HSP70 neosynthesis during development of the urodele amphibian Pleurodeles waltl were determined in this study. These conditions depend on temperature, heat shock duration and recovery duration. In oocytes, a heat shock response was repeatedly obtained at 37 degrees C for 15 min followed by 1 h recovery. These results provided evidence for heat shock response at every stage considered. An increase in HSP70 synthesis was noted throughout oogenesis, but it did not lead to an increase in the amount of soluble HSP70, except for stage VI oocytes. Such results suggest that from stage II to stage IV oocytes, an equilibrium occurs between the HSP70 used and the HSP70 neosynthesized. In contrast, in stage VI oocytes, heat shock led to overproduction of HSP70. During early development, the heat shock response was repeatedly obtained only from the gastrula stage with a 37 degrees C shock and a 15 min duration of treatment. Surprisingly, during cleavage stage, the soluble HSP70 total amount increased after heat shock at a time when no HSP70 neosynthesis occurred.

Role of chromatin and Xenopus laevis heat shock transcription factor in regulation of transcription from the X. laevis hsp70 promoter in vivo

Xenopus laevis oocytes activate transcription from the Xenopus hsp70 promoter within a chromatin template in response to heat shock. Expression of exogenous Xenopus heat shock transcription factor 1 (XHSF1) causes the activation of the wild-type hsp70 promoter within chromatin. XHSF1 activates transcription at normal growth temperatures (18 degrees C), but heat shock (34 degrees C) facilitates transcriptional activation. Titration of chromatin in vivo leads to constitutive transcription from the wild-type hsp70 promoter. The Y box elements within the hsp70 promoter facilitate transcription in the presence or absence of chromatin. The presence of the Y box elements prevents the assembly of canonical nucleosomal arrays over the promoter and facilitates transcription. In a mutant hsp70 promoter lacking Y boxes, exogenous XHSF1 activates transcription from a chromatin template much more efficiently under heat shock conditions. Activation of transcription from the mutant promoter by exogenous XHSF1 correlates with the disappearance of a canonical nucleosomal array over the promoter. Chromatin structure on a mutant hsp70 promoter lacking Y boxes can restrict XHSF1 access; however, on both mutant and wild-type promoters, chromatin assembly can also restrict the function of the basal transcriptional machinery. We suggest that chromatin assembly has a physiological role in establishing a transcriptionally repressed state on the Xenopus hsp70 promoter in vivo.

The cDNA encoding Xenopus laevis heat-shock factor 1 (XHSF1): nucleotide and deduced amino-acid sequences, and properties of the encoded protein

We have isolated a cDNA encoding Xenopus laevis (Xl) heat-shock factor 1 (XHSF1). XHSF1, translated from the mRNA synthesized in vitro, will bind specifically to the Xl hsp70 promoter (hsp70). Microinjection of XHSF1 mRNA into Xl oocytes leads to synthesis of XHSF1 which accumulates in the nucleus and selectively activates Xl phsp70p activity at 18 degrees C.

Expression and activity of p40MO15, the catalytic subunit of cdk-activating kinase, during Xenopus oogenesis and embryogenesis

Threonine 161 phosphorylation of p34cdc2 and its equivalent threonine 160 in p33cdk2 by cdk-activating kinase (CAK) is essential for the activation of these cyclin-dependent kinases. We have studied the expression and associated kinase activity of p40MO15, the catalytic subunit of CAK, during Xenopus oogenesis, meiotic maturation, and early development to understand in more detail how cdk kinases are regulated during these events. We find that p40MO15 is a stable protein with a half-life > 16 h that is accumulated during oogenesis. p40MO15 protein and its associated CAK activity are localized predominantly to the germinal vesicle; however, a small but significant proportion is found in the cytoplasm. The amount of p40MO15 detected in stage VI oocytes remains unchanged through meiotic maturation, fertilization, and early embryogenesis. Significantly, p40MO15 was found to be constitutively active during oogenesis, meiotic maturation, and the rapid mitotic cycles of early development. This suggests that regulation of p34cdc2 and p33cdk2 activity during cell cycle progression does not involve changes in the level or activity of p40MO15/CAK.

Constitutive expression of a somatic heat-inducible hsp70 gene during amphibian oogenesis

We isolated and characterized a sequence coding for heat-shock protein 70 (HSP70) of the amphibian Pleurodeles waltl. Results from S1 nuclease protection assays led us to conclude that an hsp70 gene, strictly inducible in somatic cells during heat shock, is constitutively active during oogenesis. By quantitative northern and western blot analysis, we showed that both hsp70 mRNA and HSP70-related protein levels increased in oocytes from stage II to stage VI under physiological conditions. Furthermore, by in situ hybridization to the nascent transcripts of lampbrush chromosome loops, we provided evidence for a clear-cut relationship between this increase in hsp70 mRNA and transcriptional activity during the lampbrush stage of oogenesis. These results strongly suggest that hsp70 genes are actively transcribed throughout oogenesis. HSP70-related proteins localized in the cytoplasm of young oocytes are progressively transferred to the nucleus in the course of oogenesis and preferentially accumulated in the nuclei of some stage VI oocytes.

Up-regulation of sodium pump activity in Xenopus laevis oocytes by expression of heterologous beta 1 subunits of the sodium pump

Recent evidence suggests that the beta subunit of the Na+ pump is essential for the alpha subunit to express catalytic activity and for assembly of the holoenzyme in the plasma membrane. We report here that injection into Xenopus laevis oocytes of cRNAs specific for beta 1 subunit isoforms of the Na+ pump of four species (Torpedo californica, chicken, mouse and rat) causes a time-dependent increase in the number of ouabain-binding sites, both in the plasma membrane and in internal membranes. Expression of the beta 1 subunit of the Na+ pump of mouse and rat in the oocytes could be substantiated by immunoprecipitation using a polyclonal antiserum against the mouse beta 1 subunit. Scatchard analysis in permeabilized cells disclosed that the affinity for ouabain is unchanged after expression of each of the beta 1 subunits. A proportional increase in ouabain-sensitive 86Rb+ uptake indicates that the additionally expressed ouabain-binding sites on the cell surface represent functional Na+ pumps. The findings support the concept of Geering. Theulaz, Verrey, Häuptle & Rossier [(1989) Am. J. Physiol. 257, C851-C858] that beta 1 subunits expressed in oocytes associate with an excess of endogenous alpha subunits of the Na+ pump to form a hybrid enzyme. In addition, all of the beta 1 isoforms investigated in the present study were also capable of combining with the co-expressed alpha 1 subunit of the Torpedo Na+ pump to produce a functional enzyme. Injection of cRNA encoding for the Torpedo alpha 1 subunit alone had no effect on the ouabain-binding capacity of the surface and intracellular membranes of the oocyte.

Identification of two HSP70-related Xenopus oocyte proteins that are capable of recycling across the nuclear envelope

Two 70-kD polypeptides, B3 and B4, are present in equivalent concentrations in the nucleus and cytoplasm of Xenopus oocytes. The objectives of this study were to determine if they (a) are members of the 70-kD family of heat shock proteins, and (b) recycle between the nuclear and cytoplasmic compartments. Evidence based on high-affinity binding to ATP, cross-reactivity of B3/B4-specific antibodies with rat hsc70, and a comparison of cyanogen bromide cleavage peptide maps with hsc70, verified that B3 and B4 are members of the 70-kD family of heat-shock proteins. Nuclear uptake studies were performed by microinjecting 125I-labeled B3/B4, rat hsc70, and BSA into the cytoplasm of oocytes, and examining their subsequent intracellular distributions. By 6 h postinjection, the nuclear concentration of B3/B4 and hsc70 were approximately 24-fold greater than BSA controls. It was also found that B3/B4-coated gold particles as large as 120A in diameter were able to enter the nucleus by passing through the pores. Nuclear efflux was analyzed by microinjecting the iodinated proteins directly into the oocyte nuclei. 2 h after nuclear injection, at least 46% of the B3/B4 and 60% of the hsc70 were found in the cytoplasmic fractions, compared with less than 10% for the BSA controls. Cell fusion experiments, in which labeled, anucleate oocyte vegetal hemispheres were fused, under oil, with nucleate unlabeled animal hemispheres, demonstrated that cytoplasmic B3 and B4 could enter the nucleus after equilibration was reached, arguing against the existence of separate nuclear and cytoplasmic populations. Collectively, these results show that B3, B4, and rat hsc70 are transported across the nuclear envelope and recycle between the nucleus and cytoplasm.

Induction of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Induction of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Effects of cold shock treatment on lampbrush chromosomes of amphibian oocytes

When females of the newt Pleurodeles waltl, normally raised in our laboratory at 20 degrees C, were placed in 8 degrees C water for several days, striking modifications occurred in the structure of oocyte lampbrush chromosomes: numerous normal-type lateral loops were partially reduced in size and number, while hyperdeveloped loops of a new morphological type occurred at constant, reproducible loci. However, induction of these cold loops apparently was not accompanied by preferential RNA synthesis at their levels. Cold loops resulted from the decompaction of specific landmarks, the granular loops. Autoradiography revealed a significant drop in lampbrush chromosome transcriptional activity at 8 degrees C. Both structural and transcriptional modifications were fully reversible when oocytes were returned to normal temperature. These modifications are discussed in relation to processes which could determine the morphological appearance of landmark loops.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Isolation and nucleotide sequence analysis of the rat testis-specific major heat-shock protein (HSP70)-related gene

The isolation of a rat hsp 70-related gene which is specifically and highly expressed in testis is described together with the complete nucleotide sequence of the transcription unit (2947 bp), 5' flanking (about 1 kbp) and 3' flanking (about 0.3 kbp) regions. The sequence analysis and nuclease S1 mapping revealed that the isolated gene (referred to as the hst70 gene) represents a novel, distinct member of the hsp70 multigene family. Its transcription unit lacks introns and a single open reading frame encodes a protein of 69.5 kDa. The predicted amino acid sequence of this protein is highly similar (only four out of 633 amino acids are different) to that encoded by the mouse testis-specific hsp70.2 gene (Zakeri, Z.F., Wolgemuth, D.J. and Hunt, C.R. (1988) Mol. Cell. Biol. 8, 2925-2932). The functional significance of multiple potentially regulatory sequences (e.g. TATA-boxes, heat-shock element and estrogen receptor binding site) present in the 5' flanking region of the rat hst70 gene is discussed.

An oocyte-expressed alpha-tubulin gene in Xenopus laevis; sequences required for the initiation of transcription

We have studied the expression of X alpha T14, a member of the alpha-tubulin multigene family in Xenopus laevis. Small amounts of X alpha T14 RNA are detectable in a range of cell types, but much higher levels are present in ovary and tissue culture cells. In oocytes X alpha T14 transcripts accumulate during early vitellogenesis but their level declines in more advanced stages. Faithful and efficient initiation of transcription occurred on cloned X alpha T14 injected into oocytes even at low template levels. We have examined the amount of transcript produced by various deletion mutants relative to a co-injected control gene. The presence of 200bp of DNA 5' and 53bp of DNA 3' to the initiation site sufficed for high levels of promoter activity, although maximum activity required 560 bp of 5' flanking DNA. The DNA between -200 and -60 was necessary for transcription in oocytes and contains several sequence motifs implicated in transcriptional regulation including three CCAAT boxes and a sequence resembling a heat shock element. An 8 bp deletion that removed the latter element from 5kb of 5'-flanking DNA reduced promoter activity by 60%.

Antisense oligodeoxyribonucleotide-directed cleavage of maternal mRNA in Xenopus oocytes and embryos

We have investigated the effect of specific antisense oligodeoxynucleotides (oligos) on endogenous histone H4 mRNA in Xenopus oocytes, eggs and embryos. In unfertilised eggs and non-matured oocytes, one 20-mer oligo (H4-1) mediated the RNAse H-like cleavage of up to 95% of H4 mRNA (which included polysomal mRNA), and cleavage was still obtained when the size of the oligo was reduced to a 10-mer; no cleavage was observed with 6- and 8-mers. The residual uncleaved mRNA appeared to be completely inaccessible to H4-1 since a second injection caused no further cleavage. A second 20-mer (H4-2) directed against a different region of H4 mRNA was much less effective (less than 5% cleavage). In fertilised embryos, injections of H4-1 and an oligo directed against the localised Vg1 mRNA caused less cleavage than in oocytes and also showed signs of inducing localised, non-specific mRNA cleavage. However we have been able to prepare fertilised embryos devoid of Vg1 mRNA by maturing and fertilising oligo-injected oocytes in vitro.

Decay of the oocyte-type heat shock response of Xenopus laevis

Xenopus oocytes have a complex heat shock response. During transition of the oocyte into fertilized egg, the heat shock response undergoes several qualitative and quantitative changes culminating in its complete extinction. Heat shock induces oocytes to synthesize four heat shock proteins (hsps): 83, 76, 70, and 57. After ovulation, two additional proteins (hsps 22 and 16) are inducible. The heat shock response of spawned eggs can be modified by changing the ionic configuration of the external medium and by adding pyruvate and oxaloacetate to the media. Since Xenopus eggs do not synthesize mRNA, these modifications to the external medium apparently alter the utilization of preexisting messenger RNAs in protein synthesis. Artificial activation terminates inducibility of hsps 76, 57, and 16 and diminishes the hsp 70 response. Two new heat shock proteins-66 and 48-are also inducible in artificially activated eggs. Fertilization, on the other hand, terminates the heat shock response; no hsps can be induced. However, hsp 70 appears to be made constitutively in fertilized eggs. RNA blot analyses reveal that oogenic hsp 70 messenger RNA is retained in eggs and early embryos. This messenger is apparently used for heat-induced synthesis of hsp 70 before fertilization and for constitutive synthesis of hsp 70 in zygotes.