Biochemical transformation of thymidine kinase (TK)-deficient mouse cells by herpes simplex virus type 1 DNA fragments purified from hybrid plasmids

Phage particle-mediated gene transfer to cultured mammalian cells

Recombinant phage particles carrying the thymidine kinase (TK) gene of herpes simplex virus type 1, coprecipitated with calcium phosphate, efficiently transformed mouse Ltk- cells to the TK+ phenotype. The conditions necessary to achieve high efficiency of transfer of the TK gene by phage particle-mediated gene transfer were investigated. Of the parameters examined, the pH of the buffer used for coprecipitation of phage particles with calcium phosphate, the length of time of coprecipitation, and the length of the adsorption period were found to alter the transfer efficiency significantly. The optimal pH was 6.87 at 25 degrees C. The other optimal values for these parameters were as follows: coprecipitation time, 7 to 20 min; adsorption time, 18 to 30 h. Treatment with dimethyl sulfoxide, glycerol, or sucrose did not enhance gene transfer. The optimal conditions yielded about 1 transformant per 10(5) phage particles per 10(6) cells without carrier DNA. An increase in the dosage of phage particles, up to at least 5 x 10(7) phage particles per 100-mm dish, resulted in a linear increase in the number of transformants. Addition of carrier phage, up to 10(10) phage particles per dish, did not significantly affect the number of transformants.

Phage particle-mediated gene transfer to cultured mammalian cells

Recombinant phage particles carrying the thymidine kinase (TK) gene of herpes simplex virus type 1, coprecipitated with calcium phosphate, efficiently transformed mouse Ltk- cells to the TK+ phenotype. The conditions necessary to achieve high efficiency of transfer of the TK gene by phage particle-mediated gene transfer were investigated. Of the parameters examined, the pH of the buffer used for coprecipitation of phage particles with calcium phosphate, the length of time of coprecipitation, and the length of the adsorption period were found to alter the transfer efficiency significantly. The optimal pH was 6.87 at 25 degrees C. The other optimal values for these parameters were as follows: coprecipitation time, 7 to 20 min; adsorption time, 18 to 30 h. Treatment with dimethyl sulfoxide, glycerol, or sucrose did not enhance gene transfer. The optimal conditions yielded about 1 transformant per 10(5) phage particles per 10(6) cells without carrier DNA. An increase in the dosage of phage particles, up to at least 5 x 10(7) phage particles per 100-mm dish, resulted in a linear increase in the number of transformants. Addition of carrier phage, up to 10(10) phage particles per dish, did not significantly affect the number of transformants.

Identification of sequences in the herpes simplex virus thymidine kinase gene required for efficient processing and polyadenylation

The herpes simplex virus (HSV) type 1 thymidine kinase gene (tk) was resected from its 3' end with BAL 31 exonuclease. Two sets of plasmids were isolated that lacked information distal to the two copies of the hexanucleotide 5'-AATAAA-3' located at the 3' end of the HSV tk gene. The presence of a simian virus 40 origin of DNA replication in each plasmid facilitated analysis of patterns of transcription in transfected Cos-1 monkey cells. Transcription analyses were performed with an S1 nuclease protection assay. Efficient processing and polyadenylation at the normal site still occurred when all sequences more than 44 or 46 base pairs (bp) downstream from the first AATAAA were removed (pTK311R/SV010 and pTK209R/SV010). Removal of an additional 7 bp (pTK312R/SV010) decreased the amount of tk mRNA processed at that normal site, and tk mRNA polyadenylated at a cryptic site within pBR322 sequences began to appear. The normal processing and polyadenylation site was not used at all when an additional 12 bp was removed (pTK314R/SV010); the small amount of tk mRNA produced was processed and polyadenylated at the cryptic pBR322 site. The region of the tk gene critical for efficient processing and polyadenylation of tk mRNA is located 20 to 38 bp downstream from the first AATAAA, distal to the polyadenylation site, and as RNA can form a stem-loop structure containing AAUAAA. Similar G + T-rich elements were located in DNA fragments which substitute efficiently for the HSV tk processing and polyadenylation signal and were not found in AATAAA-containing DNA fragments which substitute inefficiently for the HSV tk signal.

Induction of thymidine kinase activity by viruses with group B DNA genomes: bovine cytomegalovirus (bovine herpesvirus 4)

The bovine herpesvirus type 4 (BHV-4) group has a slow replication cycle, a narrow host range, and cytopathogenic effects characteristic of cytomegaloviruses (CMV), but a Group B genome structure similar to that of lymphotropic Herpesvirus saimiri (HVS). Reference BHV-4 strain DN599 and BHV-4 strains N124 and FHV-2 induced in the cytosol fraction of thymidine kinase-negative (TK-) rabbit skin (RAB-BU) cell mutants a novel TK activity. The BHV-4-induced thymidine kinase (TK) differed from the principal cytosol TK of mock-infected cells in PAGE mobility (Rm) under non-denaturing conditions and in the capacity to efficiently substitute CTP for ATP as a phosphate donor. The BHV-4 thymidine phosphorylating activity could also be distinguished from many common herpesvirus-induced TKs because it lacked iododeoxycytidine phosphorylating activity. Iododeoxyuridine, trifluorothymidine and bromovinyldeoxyuridine inhibited [3H]thymidine (0.01 mM) phosphorylation by the BHV-4 enzyme in a dose-dependent manner, but arabinosylthymine and 2'-fluoro-5-methyl-arabinosyluracil (FMAU) were poor inhibitors of [3H]thymidine phosphorylation, and acyclovir and (dihydroxy-2-propoxymethyl)guanine (DHPG) did not inhibit at all at 60 and 40 times the concentrations of [3H]thymidine, respectively.

Identification of sequences in the herpes simplex virus thymidine kinase gene required for efficient processing and polyadenylation

The herpes simplex virus (HSV) type 1 thymidine kinase gene (tk) was resected from its 3' end with BAL 31 exonuclease. Two sets of plasmids were isolated that lacked information distal to the two copies of the hexanucleotide 5'-AATAAA-3' located at the 3' end of the HSV tk gene. The presence of a simian virus 40 origin of DNA replication in each plasmid facilitated analysis of patterns of transcription in transfected Cos-1 monkey cells. Transcription analyses were performed with an S1 nuclease protection assay. Efficient processing and polyadenylation at the normal site still occurred when all sequences more than 44 or 46 base pairs (bp) downstream from the first AATAAA were removed (pTK311R/SV010 and pTK209R/SV010). Removal of an additional 7 bp (pTK312R/SV010) decreased the amount of tk mRNA processed at that normal site, and tk mRNA polyadenylated at a cryptic site within pBR322 sequences began to appear. The normal processing and polyadenylation site was not used at all when an additional 12 bp was removed (pTK314R/SV010); the small amount of tk mRNA produced was processed and polyadenylated at the cryptic pBR322 site. The region of the tk gene critical for efficient processing and polyadenylation of tk mRNA is located 20 to 38 bp downstream from the first AATAAA, distal to the polyadenylation site, and as RNA can form a stem-loop structure containing AAUAAA. Similar G + T-rich elements were located in DNA fragments which substitute efficiently for the HSV tk processing and polyadenylation signal and were not found in AATAAA-containing DNA fragments which substitute inefficiently for the HSV tk signal.

Nucleotide sequence of the marmoset herpesvirus thymidine kinase gene and predicted amino acid sequence of thymidine kinase polypeptide

The nucleotide sequence of a 2549-bp DNA fragment containing the entire coding region of the marmoset herpesvirus (MarHV) thymidine kinase gene (tk) and the flanking sequences was determined by the dideoxynucleotide chain termination method. The MarHV thymidine kinase polypeptide predicted from the nucleotide sequence contained 376 amino acids and had a molecular weight of 41,281. The sequencing data also reveal that the coding portion of another MarHV gene probably begins only 292 nucleotides downstream from the stop codon of the MarHV tk gene. There was relatively little nucleotide sequence homology between the MarHV tk gene and that of the herpes simplex virus (HSV) types 1 and 2 tk genes. Comparisons of the predicted amino acid sequences of the MarHV thymidine kinase polypeptide with that of the HSV-1 and HSV-2 thymidine kinase polypeptides, however, revealed clear, but interrupted, homology within several regions of the polypeptide chains. Amino acid sequence homology was particularly striking at residues 10 to 27 of the MarHV thymidine kinase polypeptide and residues 49 to 66 of the HSV-1 and HSV-2 thymidine kinase polypeptides. These same amino acid residues exhibit noticeable sequence homology to the mitochondrial beta subunit ATPase, oncogene p21 protein, adenylate kinase, and to other nucleotide-binding proteins. It has been proposed that the indicated regions of homology are elements of a nucleotide-binding pocket in ATPase, p21, and adenylate kinase, raising the possibility that amino acid residues 15 to 25 of the MarHV thymidine kinase and 54 to 64 of the HSV-1 and HSV-2 enzymes are likewise parts of nucleotide-binding sites.

Nucleotide sequence of the herpes simplex virus type 2 (HSV-2) thymidine kinase gene and predicted amino acid sequence of thymidine kinase polypeptide and its comparison with the HSV-1 thymidine kinase gene

To analyze the boundaries of the functional coding region of the HSV-2(333) thymidine kinase gene (TK gene), deletion mutants of hybrid plasmid pMAR401 H2G, which contains the 17.5 kbp BglII-G fragment of HSV-2 DNA, were prepared and tested for capacity to transform LM(TK-) cells to the thymidine kinase-positive phenotype. These studies showed that hybrid plasmids containing 2.2-2.4 kbp subfragments of HSV-2 BglII-G DNA transformed LM(TK-) cells to the thymidine kinase-positive phenotype and suggested that the region critical for transformation might be less than 2 kbp. That the activity expressed in the transformants was HSV-2 thymidine kinase was shown by experiments with type-specific enzyme-inhibiting rabbit antisera and by disc-polyacrylamide gel electrophoresis analyses. DNA fragments of the HSV-2 TK gene were subcloned in phage M13mp9 and M13mp8. A sequence of 1656 bp containing the entire coding region of the TK gene and the flanking sequences was determined by the dideoxynucleotide chain termination method. Comparisons with the HSV-1(Cl 101) TK gene revealed that PstI, PvuII, and EcoRI cleavage sites had homologous locations as did promoter, translational start and stop, and polyadenylation signals. Extensive homology was observed in the nucleotide sequence preceding the ATG translational start signal and in portions of the coding region of the genes. Comparisons of the predicted amino acid sequences of the HSV-1 and HSV-2 thymidine kinase polypeptides revealed that both were enriched in alanine, arginine, glycine, leucine, and proline residues and that clear, but interrupted homology existed within several regions of the polypeptide chains. Stretches of 15-30 amino acid residues were identical in conserved regions. The possibility is suggested that domains containing some of the conserved amino acid sequences might have a role in substrate binding and as major antigenic determinants.

Gene transfer: DNA microinjection compared with DNA transfection with a very high efficiency

We have developed a procedure that gives a very high efficiency of transfection in mammalian cells with low-molecular-weight DNA (approximately 10(4) base pairs). The procedure uses cells in suspension that are shocked with polyethylene glycol 4 h after replating. We compared this transfection technique to the standard technique involving manual microinjection of DNA into the nuclei of mammalian cells, using recombinant plasmids containing the simian virus 40 A gene or the herpes simplex virus thymidine kinase gene or both. The efficiency of transfection depends on a number of variables, the most important of which is the difference in transfectability of different cell lines. In our laboratory, the cell line that had the highest efficiency of transfection was tk-ts13, which is derived from baby hamster kidney cells that are deficient in thymidine kinase and temperature sensitive for growth. Under the appropriate conditions, as many as 70% of these cells can be transfected so that transient gene expression can be detected. With the manual microinjection technique, gene expression is independent of the cell line used and occurs faster than after transfection. The results suggest that the critical stage in transfection is the delivery of DNA molecules to the nucleus. Our experiments also indicate that an enzymatic function, in our case, thymidine kinase activity, gives a higher percentage of positive transfectants than when proteins are visualized only by indirect immunofluorescence. The transfection procedure described in this paper is simple and reproducible and, although less efficient than microinjection, ought to be useful in phenotypic and genotypic studies in which transfer of genes to a large number of cells is desirable.

Inhibition by interferon of biochemical transformation induced by cloned herpesvirus thymidine kinase genes

To learn whether interferon could prevent the biochemical transformations induced by cloned herpesvirus thymidine kinase (TK) genes, LM(TK-) mouse fibroblast cultures were pretreated for 24 h with 2.4-40 international units (I.U.)/ml mouse alpha + beta interferon, and subsequently transformed to the TK+ phenotype with recombinant plasmids containing the herpes simplex virus type 1 (HSV-1) TK gene (pAGO and pMH110) and the marmoset herpesvirus (MarHV) TK gene (pMAR035). Mouse alpha + beta interferon inhibited transformation and the inhibition was interferon dose-dependent. Transformation was also inhibited when LM(TK-) cells were pretreated for 2-5 h with 40 I.U./ml interferon. Maximal inhibitions of TK+ colony formation were observed following a 9-20 h pretreatment period with interferon. In contrast, 40 I.U./ml interferon treatment for 20 h did not reduce the rate or extent of LM(TK-) cell growth. Experiments in which cultures were first treated with plasmid pAGO and only afterwards treated with interferon also showed that, as the interferon concentration used, interferon did not inhibit the outgrowth of transformated colonies. Enzyme assays showed that pretreatment with interferon inhibited the induction of TK activity in cells that had been transfected with pAGO DNA.

Gene transfer: DNA microinjection compared with DNA transfection with a very high efficiency

We have developed a procedure that gives a very high efficiency of transfection in mammalian cells with low-molecular-weight DNA (approximately 10(4) base pairs). The procedure uses cells in suspension that are shocked with polyethylene glycol 4 h after replating. We compared this transfection technique to the standard technique involving manual microinjection of DNA into the nuclei of mammalian cells, using recombinant plasmids containing the simian virus 40 A gene or the herpes simplex virus thymidine kinase gene or both. The efficiency of transfection depends on a number of variables, the most important of which is the difference in transfectability of different cell lines. In our laboratory, the cell line that had the highest efficiency of transfection was tk-ts13, which is derived from baby hamster kidney cells that are deficient in thymidine kinase and temperature sensitive for growth. Under the appropriate conditions, as many as 70% of these cells can be transfected so that transient gene expression can be detected. With the manual microinjection technique, gene expression is independent of the cell line used and occurs faster than after transfection. The results suggest that the critical stage in transfection is the delivery of DNA molecules to the nucleus. Our experiments also indicate that an enzymatic function, in our case, thymidine kinase activity, gives a higher percentage of positive transfectants than when proteins are visualized only by indirect immunofluorescence. The transfection procedure described in this paper is simple and reproducible and, although less efficient than microinjection, ought to be useful in phenotypic and genotypic studies in which transfer of genes to a large number of cells is desirable.

Partial purification and characterization of the mRNA for human thymidine kinase and hypoxanthine/guanine phosphoribosyltransferase

We used direct microinjection of poly(A)+RNA into individual hypoxanthine/guanine phosphoribosyltransferase-deficient or thymidine kinase-deficient cells and detected the specific in vivo translation products as an assay for human hypoxanthine/guanine phosphoribosyltransferase or thymidine kinase mRNAs. The incorporation of [3H]hypoxanthine or [3H]thymidine into cells in response to injected mRNA was assayed in situ by autoradiography. Methylmercuric hydroxide/agarose gel analysis showed that human hypoxanthine/guanine phosphoribosyltransferase mRNA contains approximately 1,530 nucleotides, which is twice the number required for its protein coding capacity. The mRNA for human cytoplasmic thymidine kinase is estimated to be approximately the same length; thus, the size of the cytosol thymidine kinase subunit can be predicted to be approximately 47,000 daltons, if the full coding capacity of its mRNA is utilized.

Functional expression of the Herpes simplex virus thymidine kinase gene in Escherichia coli K-12

The recombinant plasmid pAGO contains the Herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) gene and consists of a 2-kb PvuII fragment of HSV-1 DNA inserted into the PvuII site of pBR322. A deletion mutant of pAGO, designated pMH110, has been isolated which removes the normal HSV-1 TK gene promoter but places the promoter of the pBR322 tetracycline-resistance (tetr) gene only about 400 bp from the translational start codon of the HSV-1 TK polypeptide. In contrast to pAGO, which transforms mouse LM(TK-) cells to TK+ but is only weakly expressed in TK- bacteria, pMH110 not only efficiently transforms LM(TK-) cells to TK+ but also enables TK- Escherichia coli K-12 cells to form colonies on selective plates containing 5-fluorodeoxyuridine (FdUrd) plus thymidine (dThd) and to exhibit fully restored ability to incorporate [3H]dThd into DNA. The levels of TK activity expressed by bacteria harboring pMH110 were about as high as those expressed by bacteria harboring plasmid pTK3, which contains the wild-type E. coli TK gene. The TK activity expressed in bacteria harboring pMH110 was partially purified and shown to be HSV-1-specific by serological and disc PAGE analyses and by experiments demonstrating that this enzyme phosphorylated [125I]deoxycytidine.

The site of integration of the herpes simplex virus type 1 thymidine kinase gene in human cells transformed by an HSV-1 DNA fragment

To analyze the site of integration of the herpes simplex virus type I (HSV-I) thymidine kinase (TK) gene in biochemically transformed human cells, TK-HeLa-(BU25) cells were transformed to the TK+ phenotype by a cloned, 2 kbp Pvull fragment of HSV-I DNA. The transformed cells [HeLa(BU25)/TF pAGO PP3] were fused with mouse LM(TK-) cells, and human-mouse somatic cell hybrid clones (LH PP3 clones 1, 2, 3, 5 and 6) were isolated in HATG-ouabain selective medium. The HeLa(BU25)/TF pAGO PP3 cells and the LH PP3 hybrid clones expressed HSV-I specific TK activity and a herpesvirus-associated nuclear antigen, and contained herpesvirus nucleotide sequences. Molecular hybridization experiments were carried out to map the HSV-I and flanking cellular nucleotide sequences in the biochemically transformed cells. These experiments demonstrated that the HSV-I nucleotide sequences were integrated at a single site, and that the same cellular nucleotide sequences flanked the viral DNA in transformed HeLa(BU25)/TF pAGO PP3 and LH PP3 clone 5 cells. TK- revertant subclones isolated by growing the LH PP3 clone 5 cells in BrdUrd (and diphtheria toxin) failed to form colonies in HATG medium, but retained HSV-I nucleotide sequences. Isozyme analyses on 21 gene-enzyme systems representing 21 human chromosomes revealed that all of the LH PP3 clonal lines expressed human hexosaminidase B, which has been assigned to chromosome 5, and all were sensitive to diphtheria toxin, which is also a marker for chromosome 5. Chromosome analyses showed that chromosome 5 was the nly human chromosome present in mitoses of LH PP3 clone 5 cells and that human chromosome 5 was present in most of the mitoses of LH PP3 clone 1, 2, 3, and 6 cells. The latter clones also contained 1 or 2 additional human chromosomes in some of the cells. As expected from the molecular hybridization analyses, TK- revertants of LH PP3 clone 5 cells retained portions of chromosome 5 and expressed human hexosaminidase B. The results indicate that HSV-I nucleotide sequences were stably integrated in the biochemically transformed cells, most likely in human chromosome 5.

Expression of SV40 T antigen polypeptides in cells biochemically transformed by plasmids containing the herpes simplex virus thymidine kinase gene and the genome of an SV40tsA mutant

To study the expression of SV40 tsA genomes that had been non-selectively introduced into mouse cells, SV40 tsA207 DNA was cleaved with BamH I and ligated to BamH I-cleaved plasmid pAGO DNA, which contains a functional HSV-1 thymidine kinase (TK) gene in the form of 2 kbp Pvu II fragment inserted at the Pvu II site of pBR322. Recombinant plasmids (11-12 kbp) were isolated and amplified in E. coli K12 strain RRI. Restriction nuclease analyses demonstrated that recombinant plasmids pSB15 and pSB10 contained intact SV40 genomes with the polarity of transcription oriented in the same direction (clockwise) or the opposite direction (counterclockwise), respectively, in relation to that of the HSV-1 TK gene. Cla I-cleaved pSB10 and pSB15 DNAs were used to transform LM(TK-) cells to TK+. Serological and disc PAGE analyses showed that clonal lines transformed by these plasmids all expressed the selected marker, HSV-1 TK. Molecular hybridization experiments showed that transformed clonal lines TF pSB10 C7 and TF pSB15 C10 had integrated intact SV40 genomes at one integration site, TF pSB10 C3 had integrated an SV40 genome with a small deletion near the BamH I site, but TF pSB15 Cl had integrated a plasmid from which most of the SV40 nucleotide sequences had been deleted. IF assays with hamster anti-SV40 tumor sera showed that TF pSB10 C7 and TF pSB15 C10 strongly expressed SV40 T antigens in over 90% of the cells, TF pSB10 C3 expressed SV40 T antigens in a minority of the cells, and TF pSB15 C1 did not express SV40 T antigens at all. [35S]-methionine labelling and immunoprecipitation experiments showed that, at 36.5 degrees C: (1) TF pSB10 C7 and TF pSB15 C10 expressed 92K and 20K mol. wt. species of SV40 T antigens and 50-55K cellular protein; (2) expression of all three was reduced in TF pSB10 C3 cells; and (3) TF pSB15 C1 expressed none of the SV40 T antigens, nor did parental LM(TK-) or TF 8-2 transformed cells (which contained the HSV-1 TK gene but not SV40 DNA). At 40 degrees C, labelling of the 50-55K cellular protein was markedly reduced in TF pSB10 C7 and pSB15 C10 cells. The results suggest that SV40 large T antigen (92K) induces and/or stabilizes the 50-55K cellular protein in these mouse cells.

Mapping functional domains in the promoter region of the herpes thymidine kinase gene

The cloned herpes simplex virus type 1 (HSV-1) thymidine kinase (TK; ATP:thymidine 5'-phosphotransferase, EC 2.7.1.21) gene can be used to transform TK- cells to a TK+ phenotype. Transformants generated in this way express TK at a basal constitutive level that is inducible to a higher level by infection with TK- herpes virus. We have studied the effect of mutations generated in vitro on both the constitutive and virus-induced expression of TK in transformants. Four Xho I linker insertions and two deletions in the 5' untranscribed region of the cloned HSV-1 TK gene were generated in vitro. A deletion that removed all but nine base pairs of the 5' untranscribed region virtually eliminated constitutive expression and completely prevented induction by herpes virus infection. Two of the insertions have particularly interesting properties. One, nine base pairs upstream from the cap site, inactivates constitutive expression without stopping induction. The other, 50 base pairs upstream from the cap site has the opposite effect (i.e., normal constitutive expression but no induction). Analysis of these results leads us to propose that the 5' untranscribed region of the HSV-1 TK gene is quite complex with several functional domains having differential roles in the constitutive and herpes-induced expression of the TK gene.