Fidelity of transcription of Xenopus laevis globin genes injected into Xenopus laevis oocytes and unfertilized eggs

Intron-less RNA injected into the nucleus of Xenopus oocytes accesses a regulated translation control pathway

The translation of a capped, polyadenylated RNA after injection into the nucleus of Xenopus oocytes occurs only if the RNA contains an intron. A single point mutation in the splice donor site prevents translation. Intron-less RNA is exported efficiently to the cytoplasm and is held, undegraded, in a translationally inert state for several days. Translation can be activated by treating the oocytes with progesterone or by injecting antibodies that bind the FRGY2 class of messenger RNA binding proteins, p56 and p60, but these antibodies are only effective if delivered to the nucleus. Inhibitors of casein kinase II also activate translation whereas phosphatase inhibitors block progesterone-mediated activation of translation. These data suggest the presence of an RNA handling pathway in the nucleus of Xenopus oocytes which is regulated by casein kinase type II phosphorylation and which directs transcripts to be sequestered by p56/p60 or by closely related proteins. This pathway can be bypassed if the RNA contains an intron and it can be reversed by progesterone treatment. These data may have implications for understanding translational control during early development.

Evidence for torsional stress in transcriptionally activated chromatin

The existence of torsional stress in eukaryotic chromatin has been controversial. To determine whether it could be detected, we probed the structure of an alternating AT tract. These sequences adopt cruciform geometry when the DNA helix is torsionally strained by negative supercoiling. The single-strand-specific nuclease P1 was used to determine the structure of an alternating AT sequence upstream of the Xenopus beta-globin gene when assembled into chromatin in microinjected Xenopus oocytes. The pattern of cleavage by P1 nuclease strongly suggests that the DNA in this chromatin template is under torsional stress. The cruciform was detected specifically in the most fully reconstituted templates at later stages of chromatin assembly, suggesting that negative supercoiling is associated with chromatin maturation. Furthermore, the number of torsionally strained templates increased dramatically at the time when transcription of assembled chromatin templates began. Transcription itself has been shown to induce supercoiling, but the requisite negative supercoiling for cruciform extrusion by (AT)n in oocytes was not generated in this way since the characteristic P1 cutting pattern was retained even when RNA polymerase elongation was blocked with alpha-amanitin. Thus, torsional stress is associated with transcriptional activation of chromatin templates in the absence of ongoing transcription.

Evidence for torsional stress in transcriptionally activated chromatin

The existence of torsional stress in eukaryotic chromatin has been controversial. To determine whether it could be detected, we probed the structure of an alternating AT tract. These sequences adopt cruciform geometry when the DNA helix is torsionally strained by negative supercoiling. The single-strand-specific nuclease P1 was used to determine the structure of an alternating AT sequence upstream of the Xenopus beta-globin gene when assembled into chromatin in microinjected Xenopus oocytes. The pattern of cleavage by P1 nuclease strongly suggests that the DNA in this chromatin template is under torsional stress. The cruciform was detected specifically in the most fully reconstituted templates at later stages of chromatin assembly, suggesting that negative supercoiling is associated with chromatin maturation. Furthermore, the number of torsionally strained templates increased dramatically at the time when transcription of assembled chromatin templates began. Transcription itself has been shown to induce supercoiling, but the requisite negative supercoiling for cruciform extrusion by (AT)n in oocytes was not generated in this way since the characteristic P1 cutting pattern was retained even when RNA polymerase elongation was blocked with alpha-amanitin. Thus, torsional stress is associated with transcriptional activation of chromatin templates in the absence of ongoing transcription.

Xenopus transcription factors: key molecules in the developmental regulation of differential gene expression

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A chicken red cell inhibitor of transcription associated with the terminally differentiated state

When a red cell nuclear extract (RCE) from adult chickens was injected into Xenopus oocytes along with the chicken beta globin gene, transcript levels were dramatically reduced compared to injection of DNA alone. The inhibitory action of the RCE was not specific to the beta globin gene since the Herpes thymidine kinase and Xenopus 5S RNA gene transcript levels were similarly reduced. Transcriptional repression was observed even after passage of the RCE through oocyte cytoplasm to the nucleus. The inhibitory activity binds to DNA cellulose, which suggests that the inhibitor either binds to DNA or associates with DNA-binding proteins. Nuclease digestion of the chromatin assembled on injected beta globin DNA revealed that inhibition was not associated with local changes in chromatin structure. Extracts from 9-d chicken embryonic erythroid cells, in which the endogenous beta globin gene is actively expressed, did not inhibit transcription. The inhibitory activity is, therefore, restricted to transcriptionally quiescent, adult erythrocytes. Since the inhibitory effects were seen with both polymerase II and III directed genes, we speculate that the activity may be part of the extreme transcriptional repression which occurs in the terminally differentiated erythrocyte.

Transcription factor access is mediated by accurately positioned nucleosomes on the mouse mammary tumor virus promoter

A fragment of the mouse mammary tumor virus (MMTV) promoter was reconstituted from pure histones into a dinucleosome with uniquely positioned octamer cores. Core boundaries for the in vitro-assembled dinucleosome corresponded to the observed in vivo phasing pattern for long terminal repeat nucleosomes A and B. Nuclear factor 1 (NF1), a constituent of the MMTV transcription initiation complex, was excluded from the assembled dinucleosome, whereas the glucocorticoid receptor was able to bind. During transcription of MMTV in vivo, displacement of nucleosome B was necessary to permit assembly of the initiation complex. These results indicate that the nucleoprotein structure of the promoter can provide differential access to sequence-specific DNA-binding proteins and that active chromatin remodeling can occur during transcription activation.

Transcription factor access is mediated by accurately positioned nucleosomes on the mouse mammary tumor virus promoter

A fragment of the mouse mammary tumor virus (MMTV) promoter was reconstituted from pure histones into a dinucleosome with uniquely positioned octamer cores. Core boundaries for the in vitro-assembled dinucleosome corresponded to the observed in vivo phasing pattern for long terminal repeat nucleosomes A and B. Nuclear factor 1 (NF1), a constituent of the MMTV transcription initiation complex, was excluded from the assembled dinucleosome, whereas the glucocorticoid receptor was able to bind. During transcription of MMTV in vivo, displacement of nucleosome B was necessary to permit assembly of the initiation complex. These results indicate that the nucleoprotein structure of the promoter can provide differential access to sequence-specific DNA-binding proteins and that active chromatin remodeling can occur during transcription activation.

Xenopus Y-box transcription factors: molecular cloning, functional analysis and developmental regulation

We describe the cloning and characterization of two cDNAs from Xenopus laevis that encode sequence-specific DNA binding proteins called FRG Y1 and FRG Y2 (frog Y-box proteins 1 and 2). During oogenesis and embryogenesis, the genes encoding these proteins are differentially expressed. FRG Y1 mRNA is present in oocytes, embryos, and all adult tissues examined, whereas FRG Y2 mRNA is found only in testis and immature oocytes. The FRG Y1 and FRG Y2 proteins are shown to stimulate transcription from a promoter containing a Y box (CTGATTGGCCAA). This promoter element is found in both mammalian major histocompatibility complex class II and Xenopus germ-cell-specific genes. FRG Y1, FRG Y2, and a human Y-box binding protein are homologous and represent a distinct family of sequence-specific DNA binding proteins. We identify protamine-like regions that are present within this family of transcription factors, suggesting that they use unusual means of binding to DNA.

Activation and repression of a beta-globin gene in cell hybrids is accompanied by a shift in its temporal replication

To investigate whether a switch in the transcriptional activity of a gene is associated with a change in the timing of replication during the S phase, we examined the replication timing of the beta-globin genes in two different types of somatic cell hybrids. In mouse hepatoma (Hepa 1a) x mouse erythroleukemia (MEL) hybrid cells, the beta-globin gene from the MEL parent is transcriptionally inactivated and is later replicating than in the parental MEL cell line. In human fibroblast (GM3552) x MEL hybrid cells, the human beta-globin gene is transcriptionally activated, and all of the sequences within the human beta-globin domain (200 kilobases) we have examined appear to be earlier replicating than those in the parental fibroblast cell line. The chromatin configuration of the activated human beta-globin domain in the hybrids is relatively more sensitive to nucleases than that in the fibroblasts. Furthermore, major nuclease-hypersensitive sites that were absent in the chromatin flanking the distal 5' region of the human beta-globin gene cluster in the parental fibroblast cell line are present in the transcriptionally activated domain in the hybrid cell line. These results suggest that timing of replication of globin genes has been altered in these hybrid cells and thus is not fixed during the process of differentiation.

Activation and repression of a beta-globin gene in cell hybrids is accompanied by a shift in its temporal replication

To investigate whether a switch in the transcriptional activity of a gene is associated with a change in the timing of replication during the S phase, we examined the replication timing of the beta-globin genes in two different types of somatic cell hybrids. In mouse hepatoma (Hepa 1a) x mouse erythroleukemia (MEL) hybrid cells, the beta-globin gene from the MEL parent is transcriptionally inactivated and is later replicating than in the parental MEL cell line. In human fibroblast (GM3552) x MEL hybrid cells, the human beta-globin gene is transcriptionally activated, and all of the sequences within the human beta-globin domain (200 kilobases) we have examined appear to be earlier replicating than those in the parental fibroblast cell line. The chromatin configuration of the activated human beta-globin domain in the hybrids is relatively more sensitive to nucleases than that in the fibroblasts. Furthermore, major nuclease-hypersensitive sites that were absent in the chromatin flanking the distal 5' region of the human beta-globin gene cluster in the parental fibroblast cell line are present in the transcriptionally activated domain in the hybrid cell line. These results suggest that timing of replication of globin genes has been altered in these hybrid cells and thus is not fixed during the process of differentiation.

Replication of plasmid DNA in fertilized Xenopus eggs is sensitive to both the topology and size of the injected template

The behavior of various plasmid templates was examined following their microinjection into fertilized eggs of the frog Xenopus laevis using an assay that permits the examination of both replicated and unreplicated plasmids in single eggs. Our results show that both the size and the topology of the template drastically affect the fate of the injected plasmid. Only a small proportion of injected monomeric supercoiled plasmids underwent replication during 6 h of incubation, although not all injected cells supported replication. Nicked circles were less stable than supercoiled molecules, and we could not detect their replication. Linear monomeric molecules polymerized into large, randomly oriented multimers which were extensively, but not entirely, replicated. Similar results were obtained when linear templates were ligated into polymeric forms in vitro prior to injection. Thus large molecules or molecules which, due to their topology, could be converted into high molecular weight forms following injection were preferred templates for replication. On rare occasions tandemly repeated, high molecular weight DNA was generated following the injection of supercoiled plasmid monomers. This large DNA was shown to be almost entirely replicated.

Role for DNA replication in beta-globin gene activation

Transcriptional activation of the Xenopus laevis beta-globin gene requires the synergistic action of the simian virus 40 enhancer and DNA replication in DEAE-dextran-mediated HeLa cell transfections. Replication does not act through covalent modification of the template, since its requirement was not obviated by the prior replication of the transfected DNA in eucaryotic cells. Transfection of DNA over a 100-fold range demonstrates that replication does not contribute to gene activation simply increasing template copy number. Furthermore, in cotransfections of replicating and nonreplicating constructs, only replicating templates were transcribed. Replication is not simply a requirement of chromatin assembly, since even unreplicated templates generated nucleosomal ladders. Stimulation of beta-globin transcription by DNA replication, though less marked, was also observed in calcium phosphate transfections. We interpret these results as revealing a dynamic role for replication in gene activation.

Trans-activation of transcription, from promoters containing immunoglobulin gene octamer sequences, by myeloma cell mRNA in Xenopus oocytes

To study factors required for immunoglobulin gene transcription hybrid promoters were made by linking octamer elements to a Xenopus albumin gene construct containing only 50bp of the albumin gene promoter. When injected into oocytes these hybrid promoters directed transcription far less efficiently than the unmodified 50bp albumin gene promoter fragment. Activity of the hybrid promoter, but not the unmodified albumin promoter, could be stimulated by preinjection of poly(A)+ RNA from NS1 myeloma cells. This stimulation may be caused by translation of the NS1 poly(A)+ RNA into transcription factors that act on the octamer. Both the reduction in transcription caused by octamer insertion and the extent of the inducibility by NS1 RNA are greater when two, rather than one, octamers are inserted.

Role for DNA replication in beta-globin gene activation

Transcriptional activation of the Xenopus laevis beta-globin gene requires the synergistic action of the simian virus 40 enhancer and DNA replication in DEAE-dextran-mediated HeLa cell transfections. Replication does not act through covalent modification of the template, since its requirement was not obviated by the prior replication of the transfected DNA in eucaryotic cells. Transfection of DNA over a 100-fold range demonstrates that replication does not contribute to gene activation simply increasing template copy number. Furthermore, in cotransfections of replicating and nonreplicating constructs, only replicating templates were transcribed. Replication is not simply a requirement of chromatin assembly, since even unreplicated templates generated nucleosomal ladders. Stimulation of beta-globin transcription by DNA replication, though less marked, was also observed in calcium phosphate transfections. We interpret these results as revealing a dynamic role for replication in gene activation.

pBR322 DNA inhibits simian virus 40 gene expression in Xenopus laevis oocytes

SV40 DNA form I is expressed efficiently after its injection into the nuclei of Xenopus laevis oocytes, resulting in the synthesis of RNA and protein products of both viral late and early transcription units. However it was observed that injection of SV40 genes cloned into pBR322 or related plasmids yielded vastly reduced quantities of viral DNA and proteins. If SV40 DNA was cleaved from the plasmid, and then recircularized prior to microinjection, viral expression was regained. The inhibition by plasmid DNA was not confined to an effect in cis because coinjection of circular pBR322 DNA along with SV40 DNA, as separate entities, also blocked viral RNA and protein synthesis. As circular but not linear pBR322 DNA was actively transcribed by polymerase II in oocytes, even in the presence of SV40 DNA, it is likely that pBR322 competes for transcription factors required for viral gene expression. Injection of pBR322 as early as two hours after injection of SV40 DNA into the oocyte nucleus did not inhibit SV40 RNA synthesis, indicating that once initiated, SV40 transcription is stable and insensitive to the competition by plasmid DNA. A plasmid vector was developed that allows expression of SV40 DNA in Xenopus laevis oocytes.