Gpt Gene Research Articles

Mouse UDP-GlcNAc: dolichyl-phosphate N-acetylglucosaminephosphotransferase. Molecular cloning of the cDNA, generation of anti-peptide antibodies and chromosomal localization.

A cDNA encoding UDP-GlcNAc-dolichyl-phosphate N-acetylglucosaminephosphotransferase (GPT; EC 2.7.8.15), an enzyme that catalyses the first step in the synthesis of dolichol-linked oligosaccharides, was isolated from mRNA prepared from mouse mammary glands. The cDNA contains an open reading frame that codes for a protein of 410 amino acids with a predicted molecular mass of 46.472 kDa. Mouse GPT has two copies of a putative dolichol-recognition sequence that has so far been identified in all eukaryotic enzymes which interact with dolichol, and four consensus sites for asparagine-linked glycosylation. It shows a high degree of conservation with yeast and hamster GPTs at the amino acid level. The mouse GPT cDNA recognized a single mRNA species of about 2 kb in mouse mammary glands when used as a probe in Northern blot analysis. An antiserum raised against a 15-residue peptide, derived from the predicted amino acid sequence of the cloned mouse cDNA, specifically precipitated the activity of GPT from solubilized mouse mammary gland microsomes, and detected a protein of about 48 kDa on Western blot. This size is in good agreement with that predicted from the cDNA sequence, and also with that (46 and 50 kDa) of purified bovine GPT. With the use of a panel of mouse/hamster somatic-cell hybrids and a specific probe derived from the 3'-non-coding region of the mouse cDNA, the GPT gene was mapped to mouse chromosome 17.

Multivariate statistical analysis of mutational spectra of alkylating agents

A series of multivariate statistical methods were used to explore the current knowledge on the mutational spectra of alkylating agents (AA) in bacterial and mammalian cells. The data relative to lac I and gpt genes of Escherichia coli were considered. The analysis focused on the distribution of GC to AT transitions, which account for the majority of AA-induced mutations. The statistical analysis of 15 different mutational spectra obtained by various laboratories pointed to a number of biological factors involved in the mutational process. First of all, factor and cluster analyses demonstrated that the mutational profiles obtained in mammalian cells form a homogeneous cluster different from the cluster formed by the bacterial cell mutational spectra. S N1-type AAs give rise to classes of mutational spectra statistically different from the spectra induced by the S N2-type AAs. The analysis of the mutated sequences of both genes pointed to a correlation between mutation induction by S N1 AAs, which react through a positively charged alkylating intermediate, and the occurrence of mutations at guanines preceded 5′ by a purine. Moreover, our statistical analysis showed that the distribution of AA-induced mutations is not affected by the transcriptional activity of the target gene, but is strongly determined by the sequence specificity of AA-induced mutagenesis and by the structure of the target proteins. The agreement of our results with the findings of previous studies indicates that the multivariate data analysis methods are a sensitive and reliable tool for exploring the mechanisms underlying complex biological processes. The novelty of the present results lies in their quantitative character, and in the clarity of the graphical displays. We propose the use of this methodological approach to large bulk of information available on mutational spectra.

Thymidine-induced mutations in mammalian cells: sequence specificity and implications for mutagenesis in vivo.

Imbalances in the intracellular nucleotide precursor pools in mammalian cells can result in the induction of mutations during the DNA replication process. By using a shuttle vector system developed in our laboratory, we have analyzed the sequence specificity of mutations induced in mouse A9 cells by exposure of the cells to a high concentration of thymidine. The target for mutagenesis in these studies was the bacterial gpt gene stably integrated into the chromosomal DNA of the mouse cells. Previous studies in this laboratory had generated a large panel of xanthine guanine phosphoribosyl-transferase (EC 2.4.2.22)-negative mutant lines that possess single-base mutations within the gpt coding sequence. This study utilized four xanthine guanine phosphoribosyltransferase-negative mutant lines to assess the frequency of mutation induced by thymidine at guanine residues in four sequence contexts: the 5' and 3' guanine residues of a GG doublet, the middle guanine residue of a GGG triplet, and the 3' guanine residue of a GGGG quartet. The results of this study demonstrate that treatment of cultured cells with a high concentration of thymidine can result in G.C----A.T transition mutations that occur preferentially at the 3' guanine residue of a run of two or more adjacent guanines. Guanine residues flanked on their 3' side by other guanine residues are severalfold less mutable by thymidine than are guanine residues flanked on their 3' side by a different base. This study demonstrates a sequence-specific mode for thymidine-induced mutations and suggests implications for mutagenesis in vivo.

Co-expression of insulin and somatostatin genes in pancreatic endocrine cells selected for their high level of insulin gene transcription.

RW cells are pancreatic endocrine RIN cells that have been stably transfected with a chimeric gene that places the expression of the dominant selection gpt gene under the control of the insulin gene regulatory sequences. These RW cells were examined for hormone content using immunocytochemistry. This analysis shows that: first, there are cells that are negative for insulin although they were cultured under selective pressure. Second, there is a higher proportion of somatostatin-producing cells than in the parental RIN cells; these somatostatin cells form two populations: one of cells containing only somatostatin and, surprisingly, one made of cells containing both insulin and somatostatin. Thus: (1) expression of the transfected and endogenous insulin regulatory sequences is not regulated in a coordinate fashion; (2) the presence of both hormones in the same cell suggests that the regulation of the expression of insulin and somatostatin genes and the differentiation pathway of the two respective cell types may be closely related.

Analysis of sequence specificity of 5-bromodeoxyuridine-induced reversion in cells containing multiple copies of a mutant gpt gene.

For studies on molecular mechanisms of mutagenesis, it would be advantageous to transfer mutant genes with specific alterations into mammalian cells and use the transformed cells in reversion analyses. In the present paper, we describe an efficient method for analyzing reversion events occurring in cells that possess multiple copies of a mutational target gene. This method involves amplification of the chromosomally integrated target genes with the polymerase chain reaction (PCR) and restriction endonuclease digestion of the amplified product. Single reversion events that either create or destroy restriction endonuclease recognition sequences that encompass the site of the original mutation can be identified in a background of 10-20 copies of the gene that retain the mutant sequence. Using this method, we have analyzed revertants induced by 5-bromodeoxyuridine (BrdU) in a Chinese hamster ovary cell line that possesses multiple copies of a mutant bacterial gpt gene containing a specific alteration. The results of this study not only demonstrate the effectiveness of this method for analyzing reversion of a single gene copy in transfectants possessing multiple copies of a mutant target gene, but also demonstrate that the sequence specificity for BrdU-induced mutations is the same in Chinese hamster cells as previously observed with mouse cells.

N-methyl- N-nitrosourea-induced mutations in human cells : Effects of the transcriptional activity of the target gene

In this study we addressed the question as to whether the mutagenesis by methylating agents is affected by the transcriptional activity of the damaged gene. An Epstein-Barr virus (EBV)-derived shuttle vector system was developed where the genetic target for mutation analysis, the bacterial gpt gene, is under the control of an eukaryotic inducible promoter in plasmid pF1-EBV and lacks the eukaryotic promoter in plasmid pF2-EBV. Two human cell lines that episomically maintain these shuttle vectors were established. In clone 6NT cells, which contain pF1-EBV plasmid, the gpt gene is actively transcribed and the transcription rate is regulated by zinc ions. In clone 3 cells, which harbor pF2-EBV plasmid, the gpt gene is not transcribed. Following treatment of both cell lines with the potent alkylating carcinogen N-methyl- N-nitrosourea (MNU), G · C to A · T transitions were the major mutagenic event, consistent with the miscoding potential of O 6 -methylguanine. The mutations were predominantly generated in the non-transcribed DNA strand of the active gpt gene. The same strand-bias was observed when the gpt gene was transcriptionally inactive, indicating that MNU-induced strand-specific formation of mutations is not due to transcription. Our data identify as major determinants of this phenomenon the sequence-specificity of MNU mutagenesis and the conformational properties of the target protein. Differences in mutation distribution were observed between the transcriptionally active and inactive gpt gene. This finding suggests that the organization of active genes in chromatin might modulate DNA alkylation and/or DNA repair.

Fidelity of replication of the leading and the lagging DNA strands opposite N-methyl-N-nitrosourea-induced DNA damage in human cells.

Semi-conservative replication of double-stranded DNA in eukaryotic cells is an asymmetric process involving leading and lagging strand synthesis and different DNA polymerases. We report a study to analyze the effect of these asymmetries when the replication machinery encounters alkylation-induced DNA adducts. The model system is an EBV-derived shuttle vector which replicates in synchrony with the host human cells and carries as marker gene the bacterial gpt gene. A preferential distribution of N-methyl-N-nitrosourea (MNU)-induced mutations in the non transcribed DNA strand of the shuttle vector pF1-EBV was previously reported. The hypermutated strand was the leading strand. To test whether the different fidelity of DNA polymerases synthesizing the leading and the lagging strands might contribute to MNU-induced mutation distribution the mutagenesis study was repeated on the shuttle vector pTF-EBV which contains the gpt gene in the inverted orientation. We show that the base substitution error rates on an alkylated substrate are similar for the replication of the leading and lagging strands. Moreover, we present evidence that the fidelity of replication opposite O6-methylguanine adducts of both the leading and lagging strands is not affected by the 3' flanking base. The preferential targeting of mutations after replication of alkylated DNA is mainly driven by the base at the 5' side of the G residues.

Non-phenotypic selection of N-methyl-N-nitrosourea-induced mutations in human cells

The distribution of mutations in a particular gene as detected by a selective mutation assay could be affected by the structural properties of the target protein. To investigate this, we have analysed N-methyl-N-nitrosourea (MNU)-induced mutations in two restriction recognition sequences of a target gene for mutation analysis and compared these data with what previously observed in a phenotypic mutation assay. DNA base changes in the Ncil and EcoRV sites of the gpt gene maintained in human cells by a shuttle vector system were measured by restriction fragment length polymorphism/polymerase chain reaction (RFLP/PCR) technique. After MNU-treatment of human cells, mutations were detected in the Ncil recognition sequence but not in the EcoRV site. DNA sequencing analysis revealed that all Ncil-resistant mutations were GC to AT transitions located over four bases of the Ncil recognition sequence. Only one of these mutations drastically affected the functionality of the GPT protein. The Ncil-resistant mutations were randomly distributed in both DNA strands of the gpt gene and were preferentially targeted at guanine residues flanked 5' by a guanine. Our results indicate that the structure of the GPT protein is the main contributor to the strand-specificity of MNU-induced mutations previously reported by using a phenotypic mutation assay. The potential use of the RFLP/PCR technique as a general tool for mutation detection is also discussed.

Non-phenotypic selection of N-methyl-N-nitrosourea-induced mutations in human cells.

The distribution of mutations in a particular gene as detected by a selective mutation assay could be affected by the structural properties of the target protein. To investigate this, we have analysed N-methyl-N-nitrosourea (MNU)-induced mutations in two restriction recognition sequences of a target gene for mutation analysis and compared these data with what previously observed in a phenotypic mutation assay. DNA base changes in the Ncil and EcoRV sites of the gpt gene maintained in human cells by a shuttle vector system were measured by restriction fragment length polymorphism/polymerase chain reaction (RFLP/PCR) technique. After MNU-treatment of human cells, mutations were detected in the Ncil recognition sequence but not in the EcoRV site. DNA sequencing analysis revealed that all Ncil-resistant mutations were GC to AT transitions located over four bases of the Ncil recognition sequence. Only one of these mutations drastically affected the functionality of the GPT protein. The Ncil-resistant mutations were randomly distributed in both DNA strands of the gpt gene and were preferentially targeted at guanine residues flanked 5' by a guanine. Our results indicate that the structure of the GPT protein is the main contributor to the strand-specificity of MNU-induced mutations previously reported by using a phenotypic mutation assay. The potential use of the RFLP/PCR technique as a general tool for mutation detection is also discussed.

Cloning and nucleotide sequence of the guanine phosphoribosyl-transferase ( gpt) gene of Salmonella typhimurium

Cloning and nucleotide sequence of the guanine phosphoribosyl-transferase ( gpt) gene of Salmonella typhimurium

DNA sequence changes in mutations induced by ultraviolet light in the gpt gene on the chromosome of Escherichia coli uvr + and uvrA cells

Sequence changes in mutations induced by ultraviolet light are reported for the chromosomal Escherichia coli gpt gene in almost isogenic E. coli uvr+ and excision-deficient uvrA cells. Differences between the mutagenic spectra are ascribed to preferential removal of photoproducts in the transcribed strand by excision repair in uvr+ cells. This conclusion is confirmed by analysis of published results for genes in both uvr+ and uvr- cells, showing a similar selective removal of mutagenic products from the transcribed strand of the E. coli lacI gene and of the lambda phage cI repressor gene. Comparison of these data with published results for ultraviolet mutagenesis of gpt on a chromosome in Chinese hamster ovary cells showed that a mutagenic hot spot in mammalian cells is not present in E. coli; the possibility is suggested that the hot spot might arise from localized lack of excision repair. Otherwise, mutagenesis in hamster cells appeared similar to that in E. coli uvr+ cells, except there appears to be a smaller fraction of single-base additions and deletions (frameshifts) in mammalian than in bacterial cells. Phenotypes of 6-thioguanine-resistant E. coli showed there is a gene (or genes) other than gpt involved in the utilization of thioguanine by bacteria.

Extracellular vaccinia virus formation and cell-to-cell virus transmission are prevented by deletion of the gene encoding the 37,000-Dalton outer envelope protein

There are two types of infectious vaccinia virus particles: intracellular naked virions and extracellular enveloped virions (EEV). To determine the biological role of the enveloped form of vaccinia virus, we produced and characterized a mutant that is defective in EEV formation. The strategy involved replacement by homologous recombination of the gene F13L, encoding a 37,000-Da protein (VP37) that is specific for the outer envelope of EEV, with a selectable antibiotic resistance marker, the Escherichia coli gpt gene. Initial experiments, however, suggested that such a mutation was lethal or prevented plaque formation. By employing a protocol consisting of high-multiplicity passages of intracellular virus from the transfected cells and then limiting dilution cloning, we succeeded in isolating the desired mutant, which was defective in production of plaques and extracellular virus but made normal amounts of intracellular naked virions. Electron microscopic examination indicated that the mutant virus particles, unlike wild type, were neither wrapped with Golgi-derived membranes nor associated with the cell surface. The absence of VP37 did not prevent the transport of the viral hemagglutinin to the plasma membrane but nevertheless abrogated both low-pH- and antibody-mediated cell fusion. These results indicate that VP37 is required for EEV formation and also plays a critical role in the local cell-to-cell transmission of vaccinia virus, perhaps via enveloped virions attached to or released from the cell membrane. By contrast, a mutated virus with a deletion of the K4L open reading frame, which is a homolog of the VP37 gene, was not defective in formation of plaques or EEV.

Transactivation of pancreas-specific gene sequences in somatic cell hybrids.

Enhancer/promoter elements from two pancreas-specific genes, those encoding amylase and elastase, were ligated to the bacterial GPT gene. The resulting construct can be used to select for expression of gene products which activate these pancreas-specific promoters in hybrid cells. The selectable GPT construct was stably transferred into several cell lines either directly or by cotransfection with pSV2Neo. GPT was expressed when transferred to pancreatic cell lines but not when transferred to GPT-fibroblast (L) cells or hepatoma cells. When the transformed L cells and hepatoma cells were fused with pancreatic cell lines, GPT was activated in the hybrid cells. Endogenous pancreas-specific genes from the L-cell and hepatoma parents were also activated in the hybrids. In addition, a pancreas-specific nuclear protein, PTF1, was produced in pancreatic and hybrid cells, correlating with GPT expression. The transformed L cells and hepatoma cells thus contained a nonexpressed construct which could be activated in trans by factors present in pancreatic cells. The hepatoma hybrid also continued to produce albumin, demonstrating the coexpression of liver and pancreas-specific genes in the hybrid-cell population. Cell lines carrying the amylase/elastase/GPT construct may be useful as a selection system for cloning of pancreatic transcription activators.

Transactivation of Pancreas-Specific Gene Sequences in Somatic Cell Hybrids

Enhancer/promoter elements from two pancreas-specific genes, those encoding amylase and elastase, were ligated to the bacterial GPT gene. The resulting construct can be used to select for expression of gene products which activate these pancreas-specific promoters in hybrid cells. The selectable GPT construct was stably transferred into several cell lines either directly or by cotransfection with pSV2Neo. GPT was expressed when transferred to pancreatic cell lines but not when transferred to GPT-fibroblast (L) cells or hepatoma cells. When the transformed L cells and hepatoma cells were fused with pancreatic cell lines, GPT was activated in the hybrid cells. Endogenous pancreas-specific genes from the L-cell and hepatoma parents were also activated in the hybrids. In addition, a pancreas-specific nuclear protein, PTF1, was produced in pancreatic and hybrid cells, correlating with GPT expression. The transformed L cells and hepatoma cells thus contained a nonexpressed construct which could be activated in trans by factors present in pancreatic cells. The hepatoma hybrid also continued to produce albumin, demonstrating the coexpression of liver and pancreas-specific genes in the hybrid-cell population. Cell lines carrying the amylase/elastase/GPT construct may be useful as a selection system for cloning of pancreatic transcription activators.

Sequence analysis, expression, and deletion of a vaccinia virus gene encoding a homolog of profilin, a eukaryotic actin-binding protein

A 4,500-bp BamHI fragment, located within the HindIII A segment of the vaccinia virus genome, was found to contain eight potential coding regions for polypeptides of 78 to 346 amino acids. The open reading frames with 133, 346, and 125 codons were homologous to profilin (an actin-binding protein), 3-beta-hydroxysteroid dehydrogenase, and Cu-Zn superoxide dismutase, respectively. Sequence alignments indicated that the vaccinia virus and mammalian profilins were more closely related to each other than to known profilins of other eukaryotes. The expression and possible role of the profilin homolog in the virus replicative cycle were therefore investigated. Antibody raised to Escherichia coli expressed vaccinia virus profilin was used to demonstrate the synthesis of the 15-kDa polypeptide at late times after vaccinia virus infection of mammalian cells. The protein accumulated in the cytoplasm, but only trace amounts remained associated with highly purified virions. The isolation of vaccinia virus mutants (in strains WR and IHD-J), with nearly the entire profilin gene replaced by the E. coli gpt gene, indicated that the protein is not essential for infectivity. The characteristic vaccinia virus-induced changes in actin fibers, seen by fluorescence microscopy, occurred in cells infected with the mutant. Moreover, the virus-encoded profilin homolog was not required for actin-associated events, including intracellular virus movement to the periphery of the cell, formation of specialized microvilli, or release of mature virions, as shown by electron microscopy and yields of infectious intra- and extracellular virus.

Effects of flanking base sequences on 5-bromodeoxyuridine mutagenesis in mammalian cells

The molecular mechanisms of incorporation-dependent, 5-bromodeoxyuridine (BrdU)-induced mutagenesis were analyzed in murine A9 cells that possess a single copy of the Escherichia coli gpt gene integrated into the chromosomal DNA as part of a shuttle vector. Four independently derived GPT- mutants with single base changes within the integrated gpt gene were utilized in BrdU-induced reversion analyses to test the relative mutability of guanine residues in four different settings: the 5' and 3' guanine residues of a GG doublet, the 3' guanine residue of a GGGG quartet, and the middle guanine residue of a GGG triplet. Two of the mutant lines possessed GG doublet sequences in which a GC----AT transition at either guanine residue of the doublet leads to restoration of GPT enzyme activity without restoring wild-type DNA sequence. Both lines were shown to be effectively reverted by BrdU incorporation-dependent mutagenesis, and sequencing of the gpt genes from numerous independently derived revertants of both lines demonstrated that greater than 90% of the revertants arose due to GC----AT transitions at the 3' guanine residue of the doublet. BrdU-induced reversion of two additional GPT- mutant lines demonstrated that the 3' guanine residue of a GGGG quartet is efficiently mutated, while the middle guanine residue of a GGG triplet sequence is at least 10-fold less mutable by BrdU incorporation-dependent mutagenesis than the 3' guanine residue of a GG doublet or GGGG quartet. All four mutant lines tested were equally revertible by treatment with the alkylating agent ethyl methane sulfonate. The results from this study define a sequence-specific mechanism for BrdU-induced, incorporation-dependent mutagenesis and demonstrate the use of reversion analysis for the determination of sequence specific effects at precise sites within a gene.

Mutagenic specificity of N′-methyl-N′-nitro-N-nitrosoguanidine in the gpt gene on a chromosome of Chinese hamster ovary cells and of Escherichia coli cells

DNA base sequence changes induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) mutagenesis have been determined for the Escherichia coli gpt gene stably incorporated in a chromosome of Chinese hamster ovary cells and in the chromosome of both growing and starving E. coli cells, instead of on a plasmid as in most previous studies. In the three cases, nearly all mutations were G:C to A:T transitions, with a 2- to 4-fold higher mutation rate, compared to other sites, at guanines flanked on the 5' side by another guanine. Mutagenic hot spots in these experiments were less prominent than in published results for MNNG mutagenesis of gpt and of other genes. A suggested explanation involves repair of O6meG. At low levels of mutagenic products, most are repaired and even small differences in the repair rates leads to large differences in the relative amounts of residual O6meG at various sites; in contrast, at high levels of mutagenic products there is little effect of repair on the distribution.

Extinction of the human insulin gene expression in insulinoma X fibroblast somatic cell hybrids involves cis‐acting DNA elements

Insulin gene expression in rat insulinoma (RIN) cells is extinct in RIN x fibroblast hybrids and can reappear upon loss of DNA contributed by the fibroblast parent. (Besnard et al., Exp. Cell Res. 185:101-108, 1989). In the present study, we looked for the role of 5'-flanking sequences of the human insulin gene in the negative control observed in the hybrids. RIN cells were transformed with composite genes which consisted of the coding sequence of the gpt gene placed under the control of 5'-flanking regions of the human insulin gene (Ins.gpt gene). Upon hybridization of these cells with mouse fibroblasts, the expression of both Ins.gpt and endogenous rat insulin genes were suppressed together. The results obtained indicate that cis-acting DNA elements are involved in the negative control of the gene. These elements are located in a fragment spread from -258 to +241 of the transcription origin of the human insulin gene.

Reversible conversion of cells between cancerous and normal states.

A comparison of the chromosomal organisation of various genes in clusters has shown that when two genes lie on the same DNA strand and have an intergenic distance shorter than a defined length, the transcriptional activities of one or both genes are, in most cases, absent or greatly reduced. Based on this observation, we used cell lines (derived from NIH/3T3 cells) in which human mutated c-H-ras was closely linked to another gene, e.g. E. coli xanthine-guanine phosphoribosyltransferase (gpt) gene with the SV40 promoter, on the same DNA strand of the chromosome to examine the possibility that oncogene expression suffers interference from a cis-acting gene-to-gene interaction and as a result, cell transformation and the cancerous nature of cells can be altered. We present evidence that the cancerous nature of cells is readily converted to the“normal”state by the enforced expression of the neighbouring gpt gene and vice versa.

Molecular analysis of an IS200 insertion in the gpt gene of Salmonella typhimurium LT2

A strain of Salmonella typimurium LT2 has been isolated which carries an insertion of approximately 700 bp in the gpt gene. The insertion in the gpt gene was shown to be the Salmonella-specific element IS200. The mutation in strain CR1 arose without selection during storage and is only the second phenotypically identified mutation caused by the insertion of IS200.