Genomic cloning and preliminary characterization of the human thymidine kinase gene

Cytotoxicity of trifluridine correlates with the thymidine kinase 1 expression level

Trifluridine (FTD), a tri-fluorinated thymidine analogue, is a key component of the oral antitumor drug FTD/TPI (also known as TAS-102), which is used to treat refractory metastatic colorectal cancer. Thymidine kinase 1 (TK1) is thought to be important for the incorporation of FTD into DNA, resulting in DNA dysfunction and cytotoxicity. However, it remains unknown whether TK1 is essential for FTD incorporation into DNA and whether this event is affected by the expression level of TK1 because TK1-specific-deficient human cancer cell lines have not been established. Here, we generated TK1-knock-out human colorectal cancer cells using the CRISPR/Cas9 genome editing system and validated the specificity of TK1 knock-out by measuring expression of AFMID, which is encoded on the same locus as TK1. Using TK1-knock-out cells, we confirmed that TK1 is essential for cellular sensitivity to FTD. Furthermore, we demonstrated a correlation between the TK1 expression level and cytotoxicity of FTD using cells with inducible TK1 expression, which were generated from TK1-knock-out cells. Based on our finding that the TK1 expression level correlates with sensitivity to FTD, we suggest that FTD/TPI might efficiently treat cancers with high TK1 expression.

5-Substituted-2′-deoxyuridines as anti-HSV-1 Agents: Synthesis and Structure Activity Relationship

Nucleoside and pyrophosphate analogues are currently in use to treat infection with Human herpesvirus 1 (HSV-1). Both series of compounds exert their activity by inhibition of the viral DNA polymerase either directly, or after anabolic phosphorylation. As the X-ray structure of the viral-specific DNA polymerase is not known, it is difficult to design a nucleoside or non-nucleoside antiviral agent which specifically inhibits this enzyme. Therefore, alternative strategies have relied on extensive structure activity relationship studies of anti-HSV-1 agents in an endeavour to understand the essential structural requirements for activity and hence the design of drugs with increased selectivity. A virus-specific enzyme which plays a crucial role in the selective activation of nucleoside analogues is thymidine kinase. Present knowledge regarding the specificity of herpesvirus thymidine kinase for its 5-substituted-2′-deoxyuridine substrates is reviewed herein.

Redistribution of nucleoside transporters to the cell membrane provides a novel approach for imaging thymidylate synthase inhibition by positron emission tomography

Thymidylate synthase (EC 2.1.1.45) is a key enzyme for the de novo synthesis of DNA and as such a target for anticancer drug development. There is a need to develop noninvasive methods for assessing thymidylate synthase inhibition in tumors. The aim of this study was to assess the potential of 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) positron emission tomography (PET) for early measurement of thymidylate synthase inhibition and to elucidate the cellular mechanisms involved. Radiation-induced fibrosarcoma-1 tumor-bearing mice were injected with a single i.p. dose of the thymidylate synthase inhibitor 5-fluorouracil (5-FU; 165 mg/kg) and imaged by [(18)F]FLT-PET at 1 to 2 hours after treatment. Deoxyuridine, thymidine kinase 1 (cytoplasmic thymidine kinase; EC2.7.1.21), and ATP levels in excised tumors were measured. Cellular assays for membrane transport were also done. There was a 1.8-fold increase in the 60-minute [(18)F]FLT tumor/heart radioactivity ratio in drug-treated mice compared with vehicle controls (P = 0.0016). Plasma and tumor deoxyuridine levels increased significantly but thymidine kinase and ATP levels were unchanged. Whole-cell assays implicated a (low level) functional role for the type-1 equilibrative nucleoside transporter (ENT). There was an increase in type-1 ENT-binding sites per cell from 49,110 in untreated cells to 73,142 (P = 0.03) in cells treated with 10 microg/mL 5-FU for 2 hours, without a change in transporter affinity (P = 0.41). We conclude that [(18)F]FLT-PET can be used to measure thymidylate synthase inhibition as early as 1 to 2 hours after treatment with 5-FU by a mechanism involving redistribution of nucleoside transporters to the plasma membrane.

Mice deficient for cytosolic thymidine kinase gene develop fatal kidney disease

The thymidine kinase (Tk) gene codes for a cytosolic protein involved in the pyrimidine nucleotide salvage pathway. A functional Tk gene is not necessary for cells in culture, and a naturally occurring Tk deficient phenotype has not been described in humans or animal models. In order to determine the biological significance of the Tk gene, we created Tk(-/-) knockout (KO) mice through homologous recombination in mouse embryonic stem cells. Tk KO mice have shortened life spans compared with their wild-type or Tk heterozygous (HET) siblings. All Tk KO mice develop sclerosis of kidney glomeruli and die before one year of age of kidney failure. Among other changes in KO animals, the most consistent is a switch from exclusively mucous secretion to predominantly serous secretion in the sublingual salivary gland. HET parents can produce KO mice at a frequency approaching Mendelian inheritance. Other observations in KO animals include an elevated level of serum thymidine, a significant decrease in the cloning efficiency of splenic lymphocytes, an increase in the frequency of hypoxanthine guanine phosphoribosyl transferase gene mutant lymphocytes, and histological alteration in the lymphoid structure of the spleen. In addition, KO animals sporadically exhibit inflammation of the arteries, which taken together with the lymphocyte and spleen abnormalities, suggest an abnormal immune system. Alterations in Tk KO mice indicate that the pyrimidine nucleotide salvage pathway is indispensable in vivo.

Different capacities for recombination in closely related human lymphoblastoid cell lines with different mutational responses to X-irradiation

WIL2-NS and TK6 are two distinct human lymphoblast cell lines derived from a single male donor. WIL2-NS cells are significantly more resistant to the cytotoxic effects of X-irradiation but considerably more sensitive to induced mutation. In an effort to determine the mechanistic basis for these differences, we analyzed the physical structures of thymidine kinase (tk)-deficient mutants isolated after X-ray treatment of tk heterozygotes derived from TK6 and the more mutable WIL2-NS. Southern analysis showed that while 84% of TK6-derived mutants had arisen by loss of heterozygosity (LOH), all 106 mutants from WIL2-NS derivatives arose with LOH at tk and all but one showed LOH at other linked loci on chromosome 17. We adapted a fluorescence in situ hybridization technique to distinguish between LOH due to deletion, which results in retention of only one tk allele, and LOH due to a mechanism involving the homologous chromosome (e.g., recombination), which results in the retention of two alleles. Among the LOH mutants derived that were analyzed in this way, 9 of 26 from WIL2-NS and 11 of 17 from TK6 cell lines arose by deletion. The remaining mutants retained two copies of the tk gene and thus arose by a mechanism involving the homologous allele. Since many of these mutants arising by a homologous mechanism retained partial heterozygosity of chromosome 17, they must have arisen by recombination or gene conversion, and not chromosome loss and reduplication. Finally, the recombinational capacities of WIL2-NS and TK6 were compared in transfection assays with plasmid recombination substrates. Intermolecular recombination frequencies were greater in WIL2-NS than in TK6. These data are consistent with a model suggesting that a recombinational repair system is functioning at a higher level in WIL2-NS than in TK6; the greater mutability of the tk locus in WIL2-NS results from more frequent inter- and intramolecular recombination events.

Different capacities for recombination in closely related human lymphoblastoid cell lines with different mutational responses to X-irradiation

WIL2-NS and TK6 are two distinct human lymphoblast cell lines derived from a single male donor. WIL2-NS cells are significantly more resistant to the cytotoxic effects of X-irradiation but considerably more sensitive to induced mutation. In an effort to determine the mechanistic basis for these differences, we analyzed the physical structures of thymidine kinase (tk)-deficient mutants isolated after X-ray treatment of tk heterozygotes derived from TK6 and the more mutable WIL2-NS. Southern analysis showed that while 84% of TK6-derived mutants had arisen by loss of heterozygosity (LOH), all 106 mutants from WIL2-NS derivatives arose with LOH at tk and all but one showed LOH at other linked loci on chromosome 17. We adapted a fluorescence in situ hybridization technique to distinguish between LOH due to deletion, which results in retention of only one tk allele, and LOH due to a mechanism involving the homologous chromosome (e.g., recombination), which results in the retention of two alleles. Among the LOH mutants derived that were analyzed in this way, 9 of 26 from WIL2-NS and 11 of 17 from TK6 cell lines arose by deletion. The remaining mutants retained two copies of the tk gene and thus arose by a mechanism involving the homologous allele. Since many of these mutants arising by a homologous mechanism retained partial heterozygosity of chromosome 17, they must have arisen by recombination or gene conversion, and not chromosome loss and reduplication. Finally, the recombinational capacities of WIL2-NS and TK6 were compared in transfection assays with plasmid recombination substrates. Intermolecular recombination frequencies were greater in WIL2-NS than in TK6. These data are consistent with a model suggesting that a recombinational repair system is functioning at a higher level in WIL2-NS than in TK6; the greater mutability of the tk locus in WIL2-NS results from more frequent inter- and intramolecular recombination events.

Gene dosage and expression, and enzyme activity of thymidine kinase and thymidylate synthase in xenografted colorectal adenocarcinomas

Cytogenetic studies performed on human colorectal tumors have revealed 2 specific patterns of chromosomal anomalies. The major pattern, known as the monosomic type (MT), is characterized by the loss or deletion of chromosomes 18, 17 (short arm 17p) and, less frequently, 1p, 4, 15, 5 (long arm 5q) and 21. The other one, known as the trisomic type (TT), is characterized by the gain of several chromosomes: 7, 12, X, 5 and 8. Losses of chromosome 18 and of the 17p arm never coexist in TT tumors. It was observed that many chromosome losses or deletions involved genes encoding for enzymes of the de novo pathways of nucleotide synthesis. In contrast, gains involved genes encoding for enzymes of the salvage pathways of the same metabolism. This led to the hypothesis that chromosome imbalances corresponded to those of nucleotide synthesis in tumor cells. Such an interrelation was confirmed by the dosage of thymidylate synthase (TS) and thymidine kinase (TK) activities in a series of colorectal grafted tumors. This study has been expanded to a larger series of xenografted tumors (23 cases) in which both TS and TK activities were studied, in parallel with an analysis of mRNA, by Northern blotting. The amount of mRNA was found to correlate with the number of gene copies calculated from cytogenetic data, indicating a direct gene-dosage effect. It also correlated with enzyme activities, but less strongly. This suggests the existence of an efficient post-transcriptional regulation, in particular for TS, whose level of expression varies over a wide range. Such variations may explain the diversity of responses to chemotherapy.

Partial complementation of the Fanconi anemia defect upon transfection by heterologous DNA. Phenotypic dissociation of chromosomal and cellular hypersensitivity to DNA cross-linking agents

Transfectants obtained by mouse DNA-mediated gene transfer in Fanconi anemia (FA) primary fibroblasts from the genetic complementation groups A and B were examined for the frequencies of chromosomal aberrations and cytotoxicity following treatments by cross-linking agents. Cells from group A (FA 150), which is the most sensitive to such agents, are partially corrected for both the chromosomal and cellular hypersensitivity to 8-methoxypsoralen photoaddition. In contrast, after treatment with mitomycin C (MMC), only the chromosomal sensitivity is re-established to a near normal level. The opposite is true for FA group B cells (FA 145), i.e. cell survival to MMC is partially corrected, whereas the frequency of MMC-induced chromosomal aberration remains close to that of the untransfected cells. The partial phenotypic correction of the two end points examined is interpreted as indicating either a gene dosage effect or the necessity of introducing more than one gene type in order to achieve complete recovery of a normal phenotype. The phenotypic dissociation between the clastogenic and cellular hypersensitivity to cross-linking agents may offer the opportunity of isolating separately the responsible gene(s) by conventional rescue techniques.

S-phase-specific regulation by deletion mutants of the human thymidine kinase promoter

The levels of thymidine kinase (TK; EC 2.7.1.21) mRNA were determined in nine established cell lines derived from TK-ts13, a temperature-sensitive mutant cell line that arrests in late G1 phase of the cell cycle at the restrictive temperature. The derivative cell lines carried either a cDNA or a minigene of human TK under the control of TK promoters of different lengths. A tenth cell line carried a human TK cDNA under the control of a simian virus 40 promoter. Two different assays were used to determine the S-phase-specific regulation of human TK mRNA levels in quiescent cells stimulated to proliferate. Results from these two assays indicated that (i) the first two introns of the human TK gene had no effect on the S-phase-specific regulation of TK mRNA levels, although the presence of introns increased the amount of TK mRNA; (ii) similar amounts of TK mRNA were present in cells containing constructs with an 83-base-pair (bp) promoter as with other TK promoters comprising up to approximately 4000 bp of 5' flanking sequence; (iii) a 456-bp promoter was fully S-phase-regulated, whereas the 83-bp promoter was only partially regulated; (iv) a 63-bp promoter was much less regulated than an 83-bp promoter; and (v) the crucial element in the 20-bp fragment comprising bp -83 to -64 has been localized, by site-directed mutagenesis, to the CCAAT element at -70.

Isolation of Chinese hamster ovary cell lines expressing human acyl-coenzyme A/cholesterol acyltransferase activity

We have previously reported the isolation of Chinese hamster ovary cell mutants deficient in acylcoenzyme A/cholesterol acyltransferase (ACAT) activity (Cadigan, K. M., J. G. Heider, and T. Y. Chang. 1988, J. Biol. Chem. 263:274-282). We now describe a procedure for isolating cells from these mutants that have regained the ability to synthesize cholesterol esters. The protocol uses the fluorescent stain Nile red, which is specific for neutral lipids such as cholesterol ester. After ACAT mutant populations were subjected to chemical mutagenesis or transfected with human fibroblast whole genomic DNA, two revertants and one primary transformant were isolated by virtue of their higher fluorescent intensities using flow cytofluorimetry. Both the revertants and transformant have regained large amounts of intracellular cholesterol ester and ACAT activity. However, heat inactivation experiments revealed that the enzyme activity of the transformant had heat stability properties identical to that of human fibroblasts, while the ACAT activities of the revertants were similar to that of other Chinese hamster ovary cell lines. These results suggest that the molecular lesion in the ACAT mutants resides in the structural gene for the enzyme, and the transformant has corrected this defect by acquiring and stably expressing a human gene encoding the ACAT polypeptide. Secondary transformants were isolated by transfection of ACAT mutant cells with primary transformant genomic DNA. Genomic Southern analysis of the secondary transformants using a probe specific for human DNA revealed several distinct restriction fragments common to all the transformants which most likely comprise part or all of the human ACAT gene. The cell lines described here should facilitate the cloning of the gene encoding the human ACAT enzyme.

Lack of dose-rate effect for mutation induction by gamma-rays in human TK6 cells

Survival and mutation induction in a human lymphoblastoid cell line ('TK6'), after acute X- and low dose rate continuous gamma-irradiation, were investigated using the hypoxanthine guanine phosphoribosyl transferase (HGPRT)- and the thymidine kinase (TK)-mutation assay. The surviving fraction after acute exposure decreased exponentially (D0 = 0.47 Gy). The HGPRT- and TK-mutation frequencies after acute X- and continuous gamma-irradiation (2.7 and 27 mGy/h) showed linear responses and no dose-rate dependence.

Deletion and hypermethylation of thymidine kinase gene in V79 Chinese hamster cells resistant to bromodeoxyuridine

Previous studies on V79 Chinese hamster cells have shown that bromodeoxyuridine (BrdU) -resistant variants deficient in thymidine kinase (TK) activity arise by a multistep process which is initiated by a random event and progresses gradually during serial culture in the presence of the drug. In order to determine the molecular basis for the loss of TK activity in these cells, the TK gene was isolated from a lambda phage library of genomic V79 DNA, using a fragment of the human TK gene as a probe. One phage isolated contained the entire TK gene in a 15-kb insert, as demonstrated by the ability of the phage DNA to transform Ltk- mouse cells to the TK+ phenotype. Five fragments spanning the entire gene were then subcloned into the plasmid pUC12 for DNA methylation studies. With these probes it was shown by hybridization analysis that the copy number of the TK gene in V79 cells is about four times the copy number in CHO cells and Chinese hamster liver cells. Southern hybridization analysis of the DNA from first-stage variants partially resistant to BrdU indicated that partial resistance was accompanied by deletion of a number of copies of the TK gene in V79 cells. However, the subsequent gradual transition to full BrdU resistance and full loss of TK activity was correlated with a gradual hypermethylation of sites in the 5' region of the TK gene, with no further change in gene copy number.

Identification of DNA sequences required for mouse APRT gene expression

The mouse aprt promoter contains four GC boxes, which bind transcription factor Spl in vitro, and lacks both TATA and CCAAT boxes. Removal of the two most distal GC boxes of this promoter had little effect on APRT enzyme levels produced in a transient expression assay. Deletion of the distal three GC boxes resulted in a 50% reduction, and deletion of all GC boxes resulted in essentially complete loss of APRT activity. There are two predominant transcription start sites which are located within the region containing the GC boxes. The promoter behaved as a relatively strong promoter when compared to the RSV LTR promoter in a transient CAT assay, and operated in one orientation only. No upstream anti-sense transcripts were detected in either mouse CAK or liver cells, confirming that the mouse aprt promoter, unlike some other GC-rich promoters appears not to support bidirectional transcription.

Role of the promoter in the regulation of the thymidine kinase gene

To identify the regulatory elements of the human thymidine kinase (TK) gene, we have established stable cell lines carrying different chimeric constructs of the TK gene. Our results can be summarized as follows. (i) When the TK coding sequence is under the control of the calcyclin promoter (a promoter that is activated when G0 cells are stimulated by growth factors), TK mRNA levels are higher in G1-arrested cells than in proliferating cells; (ii) when the TK coding sequence is under the control of the promoter of heat shock protein HSP70, steady-state levels of TK mRNA are highest after heat shock, regardless of the position of the cells in the cell cycle; (iii) the bacterial CAT gene under the control of the human TK promoter is maximally expressed in the S phase; (iv) the TK cDNA driven by the simian virus 40 promoter is also maximally expressed in the S phase; and (v) TK enzyme activity is always at a maximum in the S phase, even when the levels of TK mRNA are highest in nonproliferating cells. We conclude that although the TK coding sequence may also play some role, the TK promoter has an important role in the cell cycle regulation of TK mRNA levels.

Nuclear posttranscriptional processing of thymidine kinase mRNA at the onset of DNA synthesis

The posttranscriptional regulatory mechanism(s) underlying thymidine kinase (TK) mRNA accumulation was investigated in BALB/c 3T3 cells during their progression from G0 into S phase of the cell cycle. Very little TK mRNA could be detected in either the nuclear or the cytoplasmic compartment from cells harvested in G0 or G1. At the onset of S phase, however, the level of nuclear TK mRNA precursors and mature TK mRNAs increased dramatically. The high molecular weight TK heterogeneous nuclear RNA species detected in the nuclei of S-phase cells were polyadenylylated and hybridized to intron sequences derived from the TK gene. A series of high molecular weight precursors could be chased to lower molecular weight species in the presence of actinomycin D, suggesting an ordered removal of intron sequences with the kinetics of a precursor-product relationship. These results demonstrate a striking change in the nuclear posttranscriptional processing of TK heterogeneous nuclear RNA at the G1-S boundary and, furthermore, define a model system for the examination of RNA-processing events in vivo.

Role of the promoter in the regulation of the thymidine kinase gene

To identify the regulatory elements of the human thymidine kinase (TK) gene, we have established stable cell lines carrying different chimeric constructs of the TK gene. Our results can be summarized as follows. (i) When the TK coding sequence is under the control of the calcyclin promoter (a promoter that is activated when G0 cells are stimulated by growth factors), TK mRNA levels are higher in G1-arrested cells than in proliferating cells; (ii) when the TK coding sequence is under the control of the promoter of heat shock protein HSP70, steady-state levels of TK mRNA are highest after heat shock, regardless of the position of the cells in the cell cycle; (iii) the bacterial CAT gene under the control of the human TK promoter is maximally expressed in the S phase; (iv) the TK cDNA driven by the simian virus 40 promoter is also maximally expressed in the S phase; and (v) TK enzyme activity is always at a maximum in the S phase, even when the levels of TK mRNA are highest in nonproliferating cells. We conclude that although the TK coding sequence may also play some role, the TK promoter has an important role in the cell cycle regulation of TK mRNA levels.

Expression of interleukins in L cells transfected with human DNA

We have obtained, by transfection of mouse L cells with total human DNA, clones that constitutively secreted human interleukins IL1, IL2 and B-cell grown-factor activities, as assessed by specific biological assays. Southern analysis with IL2 and IL1 beta cDNAs confirmed the integration of the corresponding human genes in the genome of recipient mouse cells and showed their amplification and rearrangement. All the four IL2-secreting clones integrated in the mouse genome the human IL2 gene. Three out of the sixteen IL1-producing clones contained the IL1 beta gene. The IL1 activity secreted by the remaining clones exhibited a molecular mass of 17 kDa similar to that reported for mature human macrophage IL1. Our results demonstrate that DNA-mediated gene transfer may represent a suitable tool for the production of human growth and differentiation factors and the cloning of their genes.

Control of thymidine kinase mRNA during the cell cycle

To investigate the mechanism which controls the onset of DNA synthesis, we examined the regulation of thymidine kinase (TK) and its mRNA in the cell cycle. TK activity provides a useful marker for the onset of the S phase in mammalian cells. The present analysis of regulation of TK mRNA in BALB/c 3T3 cells showed that (i) the increase in TK activity depended on the availability of TK mRNA, (ii) the level of TK mRNA between G0 and S increased more than 20-fold, (iii) the rate of run-on TK transcription increased at most 2- to 4-fold between the G0 and S phases, (iv) the half-life of TK mRNA was greater than 8 to 12 h in the S and M phases and decreased as cells entered quiescence, (v) the TK mRNA increase was fully blocked by inhibition of protein synthesis by only 60%, (vi) this inhibition was completely effective for up to about 10 h following serum addition and progressively much less effective when the drugs were added later. These results suggest that the appearance of TK mRNA at the beginning of the S phase in serum-stimulated 3T3 cells is controlled not only by the rate of gene transcription but importantly also by the decreased rate of mRNA degradation. Similar mechanisms may be involved in regulation of the onset of DNA synthesis and the increase in TK mRNA since both are controlled in a manner consistent with a requirement for a labile protein.

Control of thymidine kinase mRNA during the cell cycle

To investigate the mechanism which controls the onset of DNA synthesis, we examined the regulation of thymidine kinase (TK) and its mRNA in the cell cycle. TK activity provides a useful marker for the onset of the S phase in mammalian cells. The present analysis of regulation of TK mRNA in BALB/c 3T3 cells showed that (i) the increase in TK activity depended on the availability of TK mRNA, (ii) the level of TK mRNA between G0 and S increased more than 20-fold, (iii) the rate of run-on TK transcription increased at most 2- to 4-fold between the G0 and S phases, (iv) the half-life of TK mRNA was greater than 8 to 12 h in the S and M phases and decreased as cells entered quiescence, (v) the TK mRNA increase was fully blocked by inhibition of protein synthesis by only 60%, (vi) this inhibition was completely effective for up to about 10 h following serum addition and progressively much less effective when the drugs were added later. These results suggest that the appearance of TK mRNA at the beginning of the S phase in serum-stimulated 3T3 cells is controlled not only by the rate of gene transcription but importantly also by the decreased rate of mRNA degradation. Similar mechanisms may be involved in regulation of the onset of DNA synthesis and the increase in TK mRNA since both are controlled in a manner consistent with a requirement for a labile protein.

Isolation of the CD7 gene from the DNA of transfected L cells

Using DNA from L cells which expressed high levels of the CD7 (Leu-9 or HuLy-m2) antigen obtained after two cycles of transfection, a genomic library was constructed in the lambda phage Charon 4A. Recombinant clones containing the gene coding for this antigen were identified by first screening the library with both the HSV-tk gene and a probe detecting the human repetitive (Alu) sequences. DNA from 10 tk+ and 12 Alu+ recombinant clones was used to transfect L cells which were analyzed for the cell-surface expression of CD7 either early (48-72 h posttransfection) or later when hypoxanthine aminopterin thymidine-resistant colonies were obtained. Transfection with either Alu+ or tk+ recombinant phages led to transient early expression of CD7, and stable CD7+ transfectants were also established. Thus the CD7 gene has been isolated in a number of clones in association with either the Alu repetitive sequence or with the HSV-tk gene; the insert size in one of the genomic clones was 13.5 kb.

Genetic analysis of the human thymidine kinase gene promoter

The promoter of the human thymidine kinase gene was defined by DNA sequence and genetic analyses. Mutant plasmids with deletions extending into the promoter region from both the 5' and 3' directions were constructed. The mutants were tested in a gene transfer system for the ability to transform TK- cells to the TK+ phenotype. This analysis delimited the functional promoter to within an 83-base-pair region upstream of the mRNA cap site. This region contains sequences common to other eucaryotic promoters including G X C-rich hexanucleotides, a CAAT box, and an A X T-rich region. The CAAT box is in an inverted orientation and is part of a 9-base-pair sequence repeated twice in the promoter region. Comparison of the genomic sequence with the cDNA sequence defined the first exon of the thymidine kinase gene.

Structure and expression of the Chinese hamster thymidine kinase gene

My colleagues and I have cloned a nearly full-length Chinese hamster thymidine kinase (TK) cDNA in a lambda gt10 vector and characterized this cDNA by nucleotide sequencing. The hamster TK protein is encoded in this cDNA by a 702-base-pair open reading frame which specifies a 25,625-dalton protein closely homologous to the previously described human and chicken TK proteins. Using cDNA nucleotide sequence data in conjunction with sequence data derived from selected subclones of the hamster TK gene recombinant phage lambda HaTK.5, we have resolved the structure of the TK gene, finding the 1,219 base pairs of the cDNA sequence to be distributed through 11.2 kilobases of genomic DNA in at least seven exon segments. In addition, we have constructed a variety of Chinese hamster TK minigenes and exonuclease III-S1 derivatives of these genes which have permitted us to define the limits of the Chinese hamster TK gene promoter and demonstrate that efficient TK transformation of Ltk- cells by TK minigenes depends on the presence of both TK intervening sequences and sequences 3' to the site of mRNA polyadenylation.

Genetic determinants of growth phase-dependent and adenovirus 5-responsive expression of the Chinese hamster thymidine kinase gene are contained within thymidine kinase mRNA sequences

We have constructed a chimeric thymidine kinase (TK) minigene, pHe delta 6Ha, which combines the complete coding and 3' noncoding regions of a Chinese hamster TK cDNA with the promoter region and 5' untranslated region of the TK gene of herpes simplex virus type 1. We have transformed rat 4 cells to Tk+ with this gene and analyzed the pattern of TK gene expression in these transformants under various conditions of in vitro cell culture. We find that TK gene expression in these Tk+ transformants is growth phase dependent, responsive to adenovirus 5 infection, and indistinguishable in character under a variety of cell culture conditions from the pattern of TK gene expression in rat 4 cells transformed to Tk+ with the genomic Chinese hamster TK gene clone lambda HaTK.5. We are led to the conclusion that the genetic elements which mediate growth phase-dependent TK gene expression are contained entirely within the sequences of the mature cytoplasmic hamster TK mRNA.

Molecular cloning and structural analysis of murine thymidine kinase genomic and cDNA sequences

Two functional cytosolic thymidine kinase (tk) cDNA clones were isolated from a mouse L-cell library. An RNA blot analysis indicated that one of these clones contains a nearly full-length tk sequence and that LTK- cells contain little or no TK message. The nucleotide sequences of both clones were determined, and the functional mouse tk cDNA contains 1,156 base pairs. An analysis of the sequence implied that there is an untranslated 32-nucleotide region at the 5' end of the mRNA, followed by an open reading frame of 699 nucleotides. The 3' untranslated region is 422 nucleotides long. Thus, the gene codes for a protein containing 233 amino acids, with a molecular weight of 25,873. A comparison of the coding sequences of the mouse tk cDNA with the human and chicken tk genes revealed about 86 and 70% homology, respectively. We also isolated the tk gene from a mouse C57BL/10J cosmid library. The structural organization was determined by restriction mapping, Southern blotting, and heteroduplex analysis of the cloned sequences, in combination with a mouse tk cDNA. The tk gene spans approximately 11 kilobases and contains at least five introns. Southern blot analysis revealed that this gene is deleted in mouse LTK- cells, consistent with the inability of these cells to synthesize TK message. This analysis also showed that tk-related sequences are present in the genomes of several mouse strains, as well as in LTK- cells. These segments may represent pseudogenes.

Genetic analysis of the human thymidine kinase gene promoter

The promoter of the human thymidine kinase gene was defined by DNA sequence and genetic analyses. Mutant plasmids with deletions extending into the promoter region from both the 5' and 3' directions were constructed. The mutants were tested in a gene transfer system for the ability to transform TK- cells to the TK+ phenotype. This analysis delimited the functional promoter to within an 83-base-pair region upstream of the mRNA cap site. This region contains sequences common to other eucaryotic promoters including G X C-rich hexanucleotides, a CAAT box, and an A X T-rich region. The CAAT box is in an inverted orientation and is part of a 9-base-pair sequence repeated twice in the promoter region. Comparison of the genomic sequence with the cDNA sequence defined the first exon of the thymidine kinase gene.

Genetic determinants of growth phase-dependent and adenovirus 5-responsive expression of the Chinese hamster thymidine kinase gene are contained within thymidine kinase mRNA sequences

We have constructed a chimeric thymidine kinase (TK) minigene, pHe delta 6Ha, which combines the complete coding and 3' noncoding regions of a Chinese hamster TK cDNA with the promoter region and 5' untranslated region of the TK gene of herpes simplex virus type 1. We have transformed rat 4 cells to Tk+ with this gene and analyzed the pattern of TK gene expression in these transformants under various conditions of in vitro cell culture. We find that TK gene expression in these Tk+ transformants is growth phase dependent, responsive to adenovirus 5 infection, and indistinguishable in character under a variety of cell culture conditions from the pattern of TK gene expression in rat 4 cells transformed to Tk+ with the genomic Chinese hamster TK gene clone lambda HaTK.5. We are led to the conclusion that the genetic elements which mediate growth phase-dependent TK gene expression are contained entirely within the sequences of the mature cytoplasmic hamster TK mRNA.

Structure and expression of the Chinese hamster thymidine kinase gene

My colleagues and I have cloned a nearly full-length Chinese hamster thymidine kinase (TK) cDNA in a lambda gt10 vector and characterized this cDNA by nucleotide sequencing. The hamster TK protein is encoded in this cDNA by a 702-base-pair open reading frame which specifies a 25,625-dalton protein closely homologous to the previously described human and chicken TK proteins. Using cDNA nucleotide sequence data in conjunction with sequence data derived from selected subclones of the hamster TK gene recombinant phage lambda HaTK.5, we have resolved the structure of the TK gene, finding the 1,219 base pairs of the cDNA sequence to be distributed through 11.2 kilobases of genomic DNA in at least seven exon segments. In addition, we have constructed a variety of Chinese hamster TK minigenes and exonuclease III-S1 derivatives of these genes which have permitted us to define the limits of the Chinese hamster TK gene promoter and demonstrate that efficient TK transformation of Ltk- cells by TK minigenes depends on the presence of both TK intervening sequences and sequences 3' to the site of mRNA polyadenylation.

Molecular cloning and structural analysis of murine thymidine kinase genomic and cDNA sequences

Two functional cytosolic thymidine kinase (tk) cDNA clones were isolated from a mouse L-cell library. An RNA blot analysis indicated that one of these clones contains a nearly full-length tk sequence and that LTK- cells contain little or no TK message. The nucleotide sequences of both clones were determined, and the functional mouse tk cDNA contains 1,156 base pairs. An analysis of the sequence implied that there is an untranslated 32-nucleotide region at the 5' end of the mRNA, followed by an open reading frame of 699 nucleotides. The 3' untranslated region is 422 nucleotides long. Thus, the gene codes for a protein containing 233 amino acids, with a molecular weight of 25,873. A comparison of the coding sequences of the mouse tk cDNA with the human and chicken tk genes revealed about 86 and 70% homology, respectively. We also isolated the tk gene from a mouse C57BL/10J cosmid library. The structural organization was determined by restriction mapping, Southern blotting, and heteroduplex analysis of the cloned sequences, in combination with a mouse tk cDNA. The tk gene spans approximately 11 kilobases and contains at least five introns. Southern blot analysis revealed that this gene is deleted in mouse LTK- cells, consistent with the inability of these cells to synthesize TK message. This analysis also showed that tk-related sequences are present in the genomes of several mouse strains, as well as in LTK- cells. These segments may represent pseudogenes.

Induction of cellular thymidine kinase occurs at the mRNA level

The thymidine kinase (TK) gene has been isolated from human genomic DNA. The gene was passaged twice by transfection of LTK- cells with human chromosomal DNA, and genomic libraries were made in lambda Charon 30 from a second-round TK+ transformant. When the library was screened with a human Alu probe, seven overlapping lambda clones from the human TK locus were obtained. None of the seven contained a functional TK gene as judged by transfection analysis, but several combinations of clones gave rise to TK+ colonies when cotransfected into TK- cells. A functional cDNA clone encoding the human TK gene has also been isolated. Using this cDNA clone as a probe in restriction enzyme/blot hybridization analyses, we have mapped the coding sequences and direction of transcription of the gene. We have also used a single-copy subclone from within the coding region to monitor steady-state levels of TK mRNA in serum-stimulated and simian virus 40-infected simian CV1 tissue culture cells. Our results indicate that the previously reported increase in TK enzyme levels seen after either treatment is paralleled by an equivalent increase in the steady-state levels of TK mRNA. In the case of simian virus 40-infected cells, the induction was delayed by 8 to 12 h, which is the length of time after infection required for early viral protein synthesis. In both cases, induction of TK mRNA coincides with the onset of DNA synthesis, but virally infected cells ultimately accumulate more TK mRNA than do serum-stimulated cells.

Induction of cellular thymidine kinase occurs at the mRNA level

The thymidine kinase (TK) gene has been isolated from human genomic DNA. The gene was passaged twice by transfection of LTK- cells with human chromosomal DNA, and genomic libraries were made in lambda Charon 30 from a second-round TK+ transformant. When the library was screened with a human Alu probe, seven overlapping lambda clones from the human TK locus were obtained. None of the seven contained a functional TK gene as judged by transfection analysis, but several combinations of clones gave rise to TK+ colonies when cotransfected into TK- cells. A functional cDNA clone encoding the human TK gene has also been isolated. Using this cDNA clone as a probe in restriction enzyme/blot hybridization analyses, we have mapped the coding sequences and direction of transcription of the gene. We have also used a single-copy subclone from within the coding region to monitor steady-state levels of TK mRNA in serum-stimulated and simian virus 40-infected simian CV1 tissue culture cells. Our results indicate that the previously reported increase in TK enzyme levels seen after either treatment is paralleled by an equivalent increase in the steady-state levels of TK mRNA. In the case of simian virus 40-infected cells, the induction was delayed by 8 to 12 h, which is the length of time after infection required for early viral protein synthesis. In both cases, induction of TK mRNA coincides with the onset of DNA synthesis, but virally infected cells ultimately accumulate more TK mRNA than do serum-stimulated cells.

Human thymidine kinase gene: molecular cloning and nucleotide sequence of a cDNA expressible in mammalian cells

A cDNA containing the entire coding region of the human thymidine kinase gene has been molecularly cloned. The cDNA is under the control of a simian virus 40 promoter and is expressible in mammalian cells. The complete nucleotide sequence of the human thymidine kinase cDNA has been determined. The cDNA is 1,421 base pairs in length and has a large open reading frame of 702 base pairs capable of specifying a protein with a molecular weight of 25,504. Genomic Southern blotting experiments show that sequences homologous to the human thymidine kinase cDNA are conserved among many vertebrates, including prosimians (lemur), tree shrews, rats, mice, and chickens. Direct comparison of the nucleotide sequences of the human thymidine kinase cDNA and the chicken thymidine kinase gene reveals ca. 70% overall homology. This homology is extended further at the amino acid sequence level, with greater than 74% amino acid residues matched between the human and chicken thymidine kinase proteins.

Human thymidine kinase gene: molecular cloning and nucleotide sequence of a cDNA expressible in mammalian cells

A cDNA containing the entire coding region of the human thymidine kinase gene has been molecularly cloned. The cDNA is under the control of a simian virus 40 promoter and is expressible in mammalian cells. The complete nucleotide sequence of the human thymidine kinase cDNA has been determined. The cDNA is 1,421 base pairs in length and has a large open reading frame of 702 base pairs capable of specifying a protein with a molecular weight of 25,504. Genomic Southern blotting experiments show that sequences homologous to the human thymidine kinase cDNA are conserved among many vertebrates, including prosimians (lemur), tree shrews, rats, mice, and chickens. Direct comparison of the nucleotide sequences of the human thymidine kinase cDNA and the chicken thymidine kinase gene reveals ca. 70% overall homology. This homology is extended further at the amino acid sequence level, with greater than 74% amino acid residues matched between the human and chicken thymidine kinase proteins.

The nucleotide sequence of the chicken thymidine kinase gene and the relationship of its predicted polypeptide to that of the vaccinia virus thymidine kinase

The entire DNA nucleotide sequence of a 3.0 kilobase pair Hind III fragment containing the chicken cytoplasmic thymidine kinase gene was determined. Oligonucleotide linker insertion mutations distributed throughout this gene and having known effects upon gene activity ( Kwoh , T.J., Zipser , D., and Wigler , M. 1983. J. Mol. Appl. Genet. 2, 191-200), were used to access regions of the Hind III fragment for sequencing reactions. The complete nucleotide sequence, together with the positions of the linker insertion mutations within the sequence, allows us to propose a structure for the chicken thymidine kinase gene. The protein coding sequence of the gene is divided into seven small segments (each less than 160 base pairs) by six small introns (each less than 230 base pairs). The proposed 244 amino acid polypeptide encoded by this gene bears strong homology to the vaccinia virus thymidine kinase. No homology with the thymidine kinases of the herpes simplex viruses was found.

Direct isolation of the functional human thymidine kinase gene with a cosmid shuttle vector

We have developed a new recombinant DNA cloning system to isolate directly the functional unit of the human thymidine kinase (TK) gene. The system utilizes a cosmid vector that can shuttle cloned DNA sequences between bacteria and mammalian cells. A complete human cosmid library was constructed and DNA from the total library was transfected to mouse L cells deficient in TK (LTK-) by calcium phosphate precipitation. The transfected cells were then selected with hypoxanthine/aminopterin/thymidine (HAT) medium, and one HAT-resistant cell clone was isolated. This cell line became resistant to HAT selection by acquiring the TK gene derived from the human cosmid library. As the cosmid vector contains the cohesive ends of the bacteriophage, we could directly retrieve the human DNA sequences from the transformed mouse L cells. Total DNA from the transformed TK+ L cells was packaged in vitro with lysogenic bacterial extracts and used to infect Escherichia coli. One of the two recombinant cosmids isolated contained a 43.8-kilobase human DNA insert and was capable of converting TK- L cells to the TK+ phenotype in both acute and stable transformation assays. Thus, we have isolated the functional human TK gene in this recombinant cosmid. The gene was further localized on a 14.5-kilobase BamHI DNA fragment, and it transcribed a mature mRNA of about 1,500 nucleotides. This method of gene isolation has several special features: (i) an intact structural gene can be cloned directly based on its function without knowledge of its amino acid or nucleotide sequence; (ii) the functional gene sequences can be recovered faster and more efficiently than with the usual DNA transfection method; and (iii) in conjunction with cell-sorting techniques, this method can be used to clone genes encoding cell surface markers.