DNAs of simian virus 40 and polyoma direct the synthesis of viral tumor antigens and capsid proteins in Xenopus oocytes

Kinetics of accumulation and processing of simian virus 40 RNA in Xenopus laevis oocytes injected with simian virus 40 DNA

We examined the kinetics of accumulation and processing of simian virus 40 (SV40) RNA in stage 6 oocytes of Xenopus laevis microinjected intranuclearly with SV40 DNA. The rates of synthesis and degradation, cellular distribution, size, and sequence specificity of radiolabeled SV40-specific and endogenous oocyte RNA were determined. The kinetics of accumulation of SV40 RNA were biphasic, with greater than 90% of the viral RNA turning over in the nucleus with a half-life of 20 to 40 min. Although most of the primary transcription products were multigenomic in length, some stable polyadenylated SV40-specific RNA similar in size and sequence to late 19S mRNA accumulated in the cytoplasm with time. Differences in strand preference, efficiencies of transcription termination and polyadenylation, and the splice sites used in the synthesis and processing of SV40 RNA in Xenopus oocytes and monkey cells were noted. However, these differences were quantitative, rather than qualitative, in nature. Consequently, they are probably due to regulatory rather than mechanistic differences between the two cell types. We therefore conclude that Xenopus oocytes may be a useful system for studying both mechanistic and cell type-specific regulatory aspects of mRNA biogenesis from cloned DNAs. However, since only a small percentage of the initially synthesized RNA ends up in stable mRNA, it will be important to determine whether mutants of cloned DNAs that produce abnormal amounts of stable mRNAs are altered in promotion and initiation of RNA synthesis, transcription termination, RNA processing, or the stability of the resultant mRNAs.

Template structural requirements for transcription in vivo by RNA polymerase II

Purified simian virus 40 (SV40) DNA is reconstituted into chromatin and transcribed by endogenous RNA polymerase II when microinjected into nuclei of Xenopus laevis oocytes. We have correlated the kinetics of chromatin reconstitution with that of accumulation of virus-specific RNA in this system. A delay of approximately 3 h was found in the appearance of appreciable numbers of both fully supercoiled molecules and transcriptionally active templates. SV40 mini-chromosomes, isolated from virus-infected monkey cells with 0.2 M NaCl, also exhibited this lag in onset of transcriptional activity when microinjected into oocytes. These findings indicate that neither purified SV40 DNA nor SV40 DNA containing a full complement of nucleosomes can function as a template for transcription in vivo before association with appropriate cellular nonhistone chromosomal factors has taken place. In addition, the gradual degradation of linear SV40 DNA in oocytes was not sufficient to account for the fact that it was much less transcriptionally active than circular SV40 DNA. Taken together, these results indicate that the conformational state of the DNA can affect its ability to function as a template for transcription in vivo by RNA polymerase II. In contrast, transcription by RNA polymerase III of purified, circularized cloned DNAs encoding genes for 5S rRNA was detectable long before the injected DNAs had time to reconstitute into chromatin. Therefore, the template structural requirements for transcription in vivo by RNA polymerases II and III are different.

Template structural requirements for transcription in vivo by RNA polymerase II

Purified simian virus 40 (SV40) DNA is reconstituted into chromatin and transcribed by endogenous RNA polymerase II when microinjected into nuclei of Xenopus laevis oocytes. We have correlated the kinetics of chromatin reconstitution with that of accumulation of virus-specific RNA in this system. A delay of approximately 3 h was found in the appearance of appreciable numbers of both fully supercoiled molecules and transcriptionally active templates. SV40 mini-chromosomes, isolated from virus-infected monkey cells with 0.2 M NaCl, also exhibited this lag in onset of transcriptional activity when microinjected into oocytes. These findings indicate that neither purified SV40 DNA nor SV40 DNA containing a full complement of nucleosomes can function as a template for transcription in vivo before association with appropriate cellular nonhistone chromosomal factors has taken place. In addition, the gradual degradation of linear SV40 DNA in oocytes was not sufficient to account for the fact that it was much less transcriptionally active than circular SV40 DNA. Taken together, these results indicate that the conformational state of the DNA can affect its ability to function as a template for transcription in vivo by RNA polymerase II. In contrast, transcription by RNA polymerase III of purified, circularized cloned DNAs encoding genes for 5S rRNA was detectable long before the injected DNAs had time to reconstitute into chromatin. Therefore, the template structural requirements for transcription in vivo by RNA polymerases II and III are different.

Kinetics of accumulation and processing of simian virus 40 RNA in Xenopus laevis oocytes injected with simian virus 40 DNA

We examined the kinetics of accumulation and processing of simian virus 40 (SV40) RNA in stage 6 oocytes of Xenopus laevis microinjected intranuclearly with SV40 DNA. The rates of synthesis and degradation, cellular distribution, size, and sequence specificity of radiolabeled SV40-specific and endogenous oocyte RNA were determined. The kinetics of accumulation of SV40 RNA were biphasic, with greater than 90% of the viral RNA turning over in the nucleus with a half-life of 20 to 40 min. Although most of the primary transcription products were multigenomic in length, some stable polyadenylated SV40-specific RNA similar in size and sequence to late 19S mRNA accumulated in the cytoplasm with time. Differences in strand preference, efficiencies of transcription termination and polyadenylation, and the splice sites used in the synthesis and processing of SV40 RNA in Xenopus oocytes and monkey cells were noted. However, these differences were quantitative, rather than qualitative, in nature. Consequently, they are probably due to regulatory rather than mechanistic differences between the two cell types. We therefore conclude that Xenopus oocytes may be a useful system for studying both mechanistic and cell type-specific regulatory aspects of mRNA biogenesis from cloned DNAs. However, since only a small percentage of the initially synthesized RNA ends up in stable mRNA, it will be important to determine whether mutants of cloned DNAs that produce abnormal amounts of stable mRNAs are altered in promotion and initiation of RNA synthesis, transcription termination, RNA processing, or the stability of the resultant mRNAs.

Tissue-specific trans-activation of the rabbit beta-globin promoter in Xenopus oocytes

Identification of transcription factors regulating tissue-specific gene expression implies functional tests in transcription systems. In spite of its practical advantages, the Xenopus oocyte has only rarely been used for trans-activation studies, because some critical parameters inherent to the system may cause artefacts. Depending on the amount of DNA injected, even tissue-specific genes may be spontaneously transcribed. To develop a reliable trans-activation assay, we used the erythroid-specific rabbit beta-globin gene and, for comparison, the constitutively transcribed viral thymidine kinase gene. The viral gene is active over a wide range of injected DNA (0.2-10 ng), and addition of nuclear proteins from various cell types does not stimulate but often inhibits this activity. When large amounts of DNA are injected (greater than 10 ng), transcription is inhibited by self competition. Addition of nuclear proteins now re-establishes activity probably through increasing the pool of general transcription factors. By contrast, spontaneous activity of the beta-globin promoter occurs only within a narrow range of injected DNA (0.2-1 ng). At higher DNA concentrations (greater than 5 ng) spontaneous transcription becomes negligible. The addition of nuclear proteins from nonerythroid cells extracts has no or only a weak stimulatory effect on the beta-globin promoter. Only nuclear proteins isolated from erythroid tissues, bone marrow and spleen, bring about a strong transcriptional activation. Co-injection with either the polyoma virus, or the oviduct-specific chicken lysozyme gene shows that the beta-globin promoter is selectively activated by factors present in erythroid cell extracts.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium channels expressed from rat brain cDNA have delayed rectifier properties

Injection into Xenopus oocytes of RNA synthesized in vitro using the rat brain cDNA RCK1 as a template or nuclear injection of the cDNA results in the expression of functional potassium channels. These channels exhibit properties similar to those of the non-inactivating delayed rectifier channel found in mammalian neurons and other excitable cells.

An excised SV40 intron accumulates and is stable in Xenopus laevis oocytes

Xenopus laevis oocytes injected with simian virus 40 (SV40) DNA synthesize abundant quantities of viral late region RNA. In a previous analysis of the 5' ends of oocyte SV40 late RNAs, it was observed that, in contrast to the majority of the late RNA species, an abundant class of viral late RNAs, whose 5' ends mapped at or near nucleotide 294, was not polyadenylated. The structure of this RNA class has now been characterized further. We have shown that this species consists of a class of small uncapped RNA molecules with heterogeneous 3' ends mapping between nucleotides 417 and 433. This corresponds well with the position of a 139-nucleotide intron within the leader region of late 16S RNA (nucleotides 294-433). The identification of this RNA class as an excised intron was strongly supported by the fact that it displayed anomalous mobilities on different percentage polyacrylamide gels, a property of lariat introns. Furthermore, incubation of oocyte RNA with a HeLa cell extract with lariat debranching activity converted the small RNA to a class that now migrated as less than or equal to 140 nucleotides in length in 8% gels, consistent with the size of the linear intraleader intron. Additional analysis of this RNA showed that it is primarily nuclear in localization and is probably the most stable viral RNA species in the oocyte. These data suggest that oocytes accumulate large quantities of the 16S intraleader intron because of their failure to debranch this RNA efficiently.

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.

Regulation of simian virus 40 gene expression in Xenopus laevis oocytes

Expression of the simian virus 40 (SV40) early and late regions was examined in Xenopus laevis oocytes microinjected with viral DNA. In contrast to the situation in monkey cells, both late-strand-specific (L-strand) RNA and early-strand-specific (E-strand) RNA could be detected as early as 2 h after injection. At all time points tested thereafter, L-strand RNA was synthesized in excess over E-strand RNA. Significantly greater quantities of L-strand, relative to E-strand, RNA were detected over a 100-fold range of DNA concentrations injected. Analysis of the subcellular distribution of [35S]methionine-labeled viral proteins revealed that while the majority of the VP-1 and all detectable small t antigen were found in the oocyte cytoplasm, most of the large T antigen was located in the oocyte nucleus. The presence of the large T antigen in the nucleus led us to investigate whether this viral product influences the relative synthesis of late or early RNA in the oocyte as it does in infected monkey cells. Microinjection of either mutant C6 SV40 DNA, which encodes a large T antigen unable to bind specifically to viral regulatory sequences, or deleted viral DNA lacking part of the large T antigen coding sequences yielded ratios of L-strand to E-strand RNA that were similar to those observed with wild-type SV40 DNA. Taken together, these observations suggest that the regulation of SV40 RNA synthesis in X. laevis oocytes occurs by a fundamentally different mechanism than that observed in infected monkey cells. This notion was further supported by the observation that the major 5' ends of L-strand RNA synthesized in oocytes were different from those detected in infected cells. Furthermore, only a subset of those L-strand RNAs were polyadenylated.

Intron sequences modulate feather keratin gene transcription in Xenopus oocytes

We have shown that chicken feather keratin genes are transcribed at extremely low levels in Xenopus oocytes. Primer extension reactions using carefully selected primers have shown that none of the detectable transcripts are spliced. When the single intron, located in the 5' untranslated region, was removed from the gene, we observed a 5-fold increase in transcript initiation from the in vivo "cap" site. Significantly, this 5-fold increase was also observed when the feather keratin intron was replaced with a similar sized fragment from pBR322. We conclude that the efficiency of accurate transcriptional initiation of chicken feather keratin genes in the oocyte is affected by DNA sequences within the intron.

Isolation and functional analysis of a human 70,000-dalton heat shock protein gene segment

A human 70-kDa heat shock protein (hsp70) gene segment has been isolated. The segment contains 3.15 kilobase pairs (kbp) of 5' nontranscribed sequence, an RNA leader of 119 bp, and a protein-coding region of 741 bp. The human protein sequence shows a high degree of homology to hsp70 sequences from other species. Expression experiments in Xenopus oocytes and mammalian cells indicate that a region that includes only 105 bp of 5' nontranscribed sequence contains all elements required for the efficient heat-controlled expression of the human gene. Two adjacent identical sequence elements, which are partly homologous to the Drosophila "heat shock consensus" sequence, are located 57 to 76 bp upstream from the capping site. Interestingly, the capping site itself is flanked by inverted repeat sequences.

Regulation of simian virus 40 gene expression in Xenopus laevis oocytes

Expression of the simian virus 40 (SV40) early and late regions was examined in Xenopus laevis oocytes microinjected with viral DNA. In contrast to the situation in monkey cells, both late-strand-specific (L-strand) RNA and early-strand-specific (E-strand) RNA could be detected as early as 2 h after injection. At all time points tested thereafter, L-strand RNA was synthesized in excess over E-strand RNA. Significantly greater quantities of L-strand, relative to E-strand, RNA were detected over a 100-fold range of DNA concentrations injected. Analysis of the subcellular distribution of [35S]methionine-labeled viral proteins revealed that while the majority of the VP-1 and all detectable small t antigen were found in the oocyte cytoplasm, most of the large T antigen was located in the oocyte nucleus. The presence of the large T antigen in the nucleus led us to investigate whether this viral product influences the relative synthesis of late or early RNA in the oocyte as it does in infected monkey cells. Microinjection of either mutant C6 SV40 DNA, which encodes a large T antigen unable to bind specifically to viral regulatory sequences, or deleted viral DNA lacking part of the large T antigen coding sequences yielded ratios of L-strand to E-strand RNA that were similar to those observed with wild-type SV40 DNA. Taken together, these observations suggest that the regulation of SV40 RNA synthesis in X. laevis oocytes occurs by a fundamentally different mechanism than that observed in infected monkey cells. This notion was further supported by the observation that the major 5' ends of L-strand RNA synthesized in oocytes were different from those detected in infected cells. Furthermore, only a subset of those L-strand RNAs were polyadenylated.

Regulation of adenovirus transcription by an E1a gene in microinjected Xenopus laevis oocytes

The regulation of adenovirus type 5 gene expression by the E1a gene product was examined in microinjected Xenopus laevis oocytes. Chimeric genes were constructed which included the promoter region of early adenovirus type 5 gene 3 and the structural sequence which codes for the bacterial enzyme chloramphenicol-3-O-acetyltransferase (CAT). A plasmid containing this chimeric gene as well as plasmids containing the E1a gene were coinjected into oocyte nuclei. The presence of the E1a gene was shown to increase CAT activity by up to 8.5-fold over basal levels. Synthesis of the functional product from the E1a gene requires the removal of intron sequences by RNA splicing. The E1a gene and a derivative that precisely lacks the intron were equally effective in increasing CAT activity, suggesting that splicing of the primary E1a transcript is efficiently accomplished in the oocyte nucleus. This was confirmed by directly examining the E1a mRNAs by the S1 mapping procedure. A protein extract from adenovirus type 5-infected HeLa cells enriched for the E1a protein may supplant the E1a plasmid in enhancing CAT activity. Synthesis of the CAT enzyme after gene injection is invariant in oocytes from the same frog, but oocytes from different frogs show a high degree of variability in their ability to synthesize the CAT enzyme. Microinjected X. laevis oocytes appear to be an extremely useful system to study the effects of protein elements on transcription.

Regulation of adenovirus transcription by an E1a gene in microinjected Xenopus laevis oocytes

The regulation of adenovirus type 5 gene expression by the E1a gene product was examined in microinjected Xenopus laevis oocytes. Chimeric genes were constructed which included the promoter region of early adenovirus type 5 gene 3 and the structural sequence which codes for the bacterial enzyme chloramphenicol-3-O-acetyltransferase (CAT). A plasmid containing this chimeric gene as well as plasmids containing the E1a gene were coinjected into oocyte nuclei. The presence of the E1a gene was shown to increase CAT activity by up to 8.5-fold over basal levels. Synthesis of the functional product from the E1a gene requires the removal of intron sequences by RNA splicing. The E1a gene and a derivative that precisely lacks the intron were equally effective in increasing CAT activity, suggesting that splicing of the primary E1a transcript is efficiently accomplished in the oocyte nucleus. This was confirmed by directly examining the E1a mRNAs by the S1 mapping procedure. A protein extract from adenovirus type 5-infected HeLa cells enriched for the E1a protein may supplant the E1a plasmid in enhancing CAT activity. Synthesis of the CAT enzyme after gene injection is invariant in oocytes from the same frog, but oocytes from different frogs show a high degree of variability in their ability to synthesize the CAT enzyme. Microinjected X. laevis oocytes appear to be an extremely useful system to study the effects of protein elements on transcription.

Isolation and translation of mRNA coding for the variant surface antigens of Paramecium

In the poly(A)+RNA isolated from the ciliate Paramecium primaurelia is found a discrete and abundant mRNA species of high molecular weight (corresponding to about 9,000 nucleotides). This mRNA species has size and abundance characteristics that identify it tentatively as the message coding for the variant cell-surface antigens. After microinjection of the high molecular weight mRNA into amphibian oocytes, polypeptides are synthesized that are immunoprecipitated specifically with antibodies directed against the homologous Paramecium antigen. On collecting the culture medium of oocytes microinjected with Paramecium mRNA, newly-synthesized complete antigen molecules (Mr approximately 300,000) can be recovered by immunoprecipitation.

Transcription of cloned Moloney murine leukemia proviral DNA injected into Xenopus laevis oocytes

We have microinjected genomic DNA clones containing the Moloney murine leukemia virus (M-MuLV) proviral genome and flanking mouse sequences from Mov-3, Mov-7 and Mov-10 mice into Xenopus laevis oocytes and analyzed the virus-specific transcription and translation products. These mouse strains carry a proviral genome copy of M-MuLV in their germ line at different chromosomal positions and differ from each other with respect to expression of the proviral genome. We show here that the different M-MuLV proviral genome copies were transcribed into virus-specific RNA with similar efficiencies. Transcription of viral RNA initiated correctly at the viral promoter in the 5' LTR, whereas the promoter in the 3' LTR was used with a much lower frequency, if at all, for initiation of RNA synthesis. Most of the virus-specific transcripts were smaller than authentic M-MuLV mRNA and confined to the oocyte nucleus. Using immunoprecipitation, we were not able to detect virus-specific proteins after injection of proviral DNA, whereas after injection of a comparable amount of M-MuLV mRNA viral protein was readily detectable.

Methylation of simian virus 40 Hpa II site affects late, but not early, viral gene expression

DNA methylation has been correlated with reduced gene expression in a number of studies, although evidence for a casual link between the two events has been lacking. Because microinjection of simian virus 40 (SV40) DNA into the nucleus of Xenopus laevis oocytes results in the synthesis of both early and late viral gene products, it was possible to test whether a specific methylation event can affect gene expression. The single SV40 Hpa II site at 0.72 SV40 map units was specifically methylated with Hpa II methylase. When this DNA was injected into oocytes, there was a marked reduction in the synthesis of the major late viral capsid protein VP-1, relative to the synthesis by an unmethylated control. However, production of the early proteins (the large and small tumor antigens) was not affected by Hpa II methylation. Therefore, methylation at a single site on the viral DNA located near the 5' end of the late region can specifically repress late gene expression. The possible mechanisms by which this repression is mediated are discussed.

Transcription of a Drosophila heat shock gene is heat-induced in Xenopus oocytes

Xenopus cells, like many other eukaryotic cells, respond to heat treatments by increasing the rate of synthesis of a few characteristic proteins, the heat shock proteins. Because of the generality of this response, it seemed possible to examine the expression of isolated heat shock genes in a heterologous system. Phage 122 DNA, containing two identical genes coding for the Drosophila 70,000-dalton heat shock protein (hsp70 genes), was microinjected into Xenopus oocyte nuclei. The Drosophila hsp70 genes are transcribed efficiently in heat-treated oocytes (35-37 degrees C) to give RNA of the correct size and sequence content. Transcription is sensitive to low levels of alpha-amanitin and therefore is carried out by RNA polymerase II. At normal temperatures (20-28 degrees C) essentially no Drosophila-specific RNA is formed. The isolated insert fragment of phage 122 also gives RNA of correct length in heat-treated oocytes which hybridizes to the coding segment of Drosophila hsp70 genes only. At normal temperatures, however, its rate of transcription is variable and only RNA heterogeneous in size is formed.

Translation and assembly of HLA-DR antigens in Xenopus oocytes injected with mRNA from a human B-cell line

HLA-DR antigens are polymorphic cell surface glycoproteins, expressed primarily in B lymphocytes and macrophages, which are thought to play an important role in the immune response. Two polypeptide chains, alpha and beta, are associated at the cell surface, and a third chain associates with alpha and beta intracellularly. RNA isolated from the human B-cell line Raji was injected in Xenopus laevis oocytes. Immunoprecipitates of translation products with several monoclonal antibodies revealed the presence of HLA-DR antigens similar to those synthesized in Raji cells. One monoclonal antibody was able to bind the beta chain after dissociation of the three polypeptide chains with detergent. The presence of all three chains was confirmed by two-dimensional gel electrophoresis. The glycosylation pattern of the three chains was identical to that observed in vivo, as evidenced in studies using tunicamycin, an inhibitor of N-linked glycosylation. The presence of alpha chains assembled with beta chains in equimolar ratio was further demonstrated by amino-terminal sequencing. An RNA fraction enriched for the three mRNAs, encoding alpha, beta, and intracellular chains, was isolated. This translation-assembly system and the availability of monoclonal antibodies make it possible to assay for mRNA encoding specific molecules among the multiple human Ia-like antigens.

Expression of the herpes thymidine kinase gene in Xenopus laevis oocytes: an assay for the study of deletion mutants constructed in vitro

When Xenopus laevis oocyte nuclei are injected with a recombinant plasmid containing the Herpes Simplex Virus (HSV) thymidine kinase (tk) gene, a 100-fold increase in tk enzymatic activity is observed. Three lines of evidence show that this increase in tk activity is a result of the expression of the HSV tk gene. First, the enzymatic activity is selectively inactivated by the IgG fraction of antiserum raised against HSV tk protein. Second, a polypeptide that comigrates with authentic HSV tk on polyacrylamide gels is synthesized uniquely by oocytes injected with the HSV tk gene. Third, the induced tk activity found in injected oocytes is capable of phosphorylating deoxycytidine, a substrate that is utilized by HSV tk but not by cellular tk. We have used these observations to establish an assay for examining the activity of mutated variants of the HSV tk gene. Two sets of deletion mutants of the tk gene were constructed in vitro. In one set varying amounts of 5' flanking and intragenic sequences are deleted. The other set is deleted at the 3' end of the gene. By testing the activity of each mutant in the oocyte injection assay we have delimited functional boundaries corresponding to the 5' and 3' termini of the HSV tk gene.

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.

Cloning of an origin of DNA replication of Xenopus laevis

DNA fragments of Xenopus laevis, the African frog, were cloned in the EcoRI site of the Escherichia coli plasmid pACYC189 and tested for ability to initiate and complete replication of the recombinant plasmid when injected into unfertilized eggs of X. laevis. After measurement of the [3H]-thymidine incorporation per egg for a number of recombinant plasmids, pSW14 and pSW9, which respectively contain a small segment (550 base pairs) and several kilobases of frog DNA, were selected for more extensive analysis. In spite of the small size of the segment in pSW14, it incorporates in 2 hr at least 3 times as much labeled thymidine as either pSW9 or the vector alone. The DNA synthesis in pSW14 was shown to be replication rather than repair synthesis, based on a buoyant density shift of the product when iododeoxyuridine was used for labeling. To determine the number of replications of pSW14, a novel method was employed. Because pSW14 is a head-to-head dimer of the vector with the Xenopus fragment inserted at an EcoRI site, the plasmid has three methylatable sites--two bracketing the Xenopus fragment and one opposite the fragment. By cotransformation of E. coli with pSW14 and pBR322 containing the EcoRI methylase gene, supercoiled pSW14 was methylated and injected into eggs with [3H]thymidine. Disappearance of modified EcoRI sites by semiconservative replication was followed by measuring the sensitivity to EcoRI endonuclease over time. The results showed that about 50% of the labeled, supercoiled DNA recovered from eggs after 4 hr was sensitive to EcoRI digestion, which indicates that most of the DNA that incorporated [3H]thymidine had replicated twice during the 4 hr in the unfertilized eggs of X. laevis. We conclude that pSW14 has a functional origin in the Xenopus DNA segment.

Kinetics of synthesis and processing of precursor polypeptides of murine leukemia virus in frog oocytes following microinjection of viral RNA

Microinjection of Rauscher murine leukemia viral RNA into living oocytes from Xenopus laevis, in contrast to cell-free systems, allowed detailed studies on the processing of newly synthesized viral precursor polypeptides. The viral messenger appeared to be stable for at least 5 days. Maximal rate of translation of 70-S virion RNA was observed 10 h after injection. The predominant translation products after a 1-h labeling period were three precursor polypeptides of Mr 77000, 75000 and 65000. Following longer labeling periods the most stable precursor polypeptide of Mr 65000 was most prominent. In addition, several intermediates of Mr 35000--60000 were observed. After about 24 h, mature viral core proteins appeared. The rate of synthesis of the 75000-Mr and 77000-Mr viral proteins decreased gradually after injection, suggesting that viral core polypeptides somehow regulated processing or synthesis of the group-specific antigen precursors. A heterogeneous group of 90000--95000-Mr polypeptides seemed to be post-translationally modified products of the 75000-Mr and 77000-Mr proteins. However, in this study no envelope-related polypeptides were synthesized, when viral RNA (70-S or 35-S) was injected into the cytoplasm or the nucleus of Xenopus oocytes.

Expression of a chicken chromosomal ovalbumin gene injected into frog oocyte nuclei

Ovalbumin is synthesized by Xenopus oocytes injected with a plasmid containing either the chicken chromosomal ovalbumin gene or a plasmid from which the 5' region of the chromosomal gene has been deleted. However, oocytes injected with a plasmid containing full-length ovalbumin cDNA do not synthesize ovalbumin, despite the fact that at least as much stable, ovalbumin-specific RNA is transcribed from the cDNA as from the chromosomal gene.

Order and intracellular location of the events involved in the maturation of a spliced tRNA

Microinjected frog oocytes were used to analyse the RNA processing steps which lead to the appearance of a mature cytoplasmic tRNAtyr molecule. The results show that removal of the intervening sequence from within a yeast tRNAtyr precursor, excision of extra 3' and 5' nucleotides, addition of a 3'-terminal CCA and modification of at least seven ribonucleotides all occur in the nucleus before the tRNAtyr is transported to the cytoplasm. Moreover, we find that the ribonucleotide modifications occur in a strict order which precisely correlates with the size alterations of the tRNAtyr precursor.

Intranuclear injection of anti-actin antibodies into Xenopus oocytes blocks chromosome condensation

The role of contractile proteins in the structural organisation of the interphase nucleus and of metaphase chromosomes is largely unknown. Actin has been found in interphase nuclei of different species, especially in association with condensed chromatin. In the germinal vesicle (nucleus) of Xenopus oocytes, actin has been localised in the nuclear gel supporting the chromosomes and the extrachromosomal nucleoli. It has been reported that the premeiotic lampbrush chromosomes in these germinal vesicles are positively stained for actin and tubulin by the immunoperoxidase technique. Moreover, the longitudinal contraction of these chromosomes is ATP dependent. Therefore it has been suggested that actin participates in the structural organisation of the highly specialised lampbrush chromosomes. However, actin is not a major component of the metaphase chromosome scaffold. The results reported here suggest that actin is involved in the condensation of Xenopus chromosomes.