Induction of cellular thymidine kinase occurs at the mRNA level

Mitotic degradation of human thymidine kinase 1 is dependent on the anaphase-promoting complex/cyclosome-CDH1-mediated pathway

The expression of human thymidine kinase 1 (hTK1) is highly dependent on the growth states and cell cycle stages in mammalian cells. The amount of hTK1 is significantly increased in the cells during progression to the S and M phases, and becomes barely detectable in the early G(1) phase by a proteolytic control during mitotic exit. This tight regulation is important for providing the correct pool of dTTP for DNA synthesis at the right time in the cell cycle. Here, we investigated the mechanism responsible for mitotic degradation of hTK1. We show that hTK1 is degraded via a ubiquitin-proteasome pathway in mammalian cells and that anaphase-promoting complex/cyclosome (APC/C) activator Cdh1 is not only a necessary but also a rate-limiting factor for mitotic degradation of hTK1. Furthermore, a KEN box sequence located in the C-terminal region of hTK1 is required for its mitotic degradation and interaction capability with Cdh1. By in vitro ubiquitinylation assays, we demonstrated that hTK1 is targeted for degradation by the APC/C-Cdh1 ubiquitin ligase dependent on this KEN box motif. Taken together, we concluded that activation of the APC/C-Cdh1 complex during mitotic exit controls timing of hTK1 destruction, thus effectively minimizing dTTP formation from the salvage pathway in the early G(1) phase of the cell cycle in mammalian cells.

Degradation of human thymidine kinase is dependent on serine-13 phosphorylation: involvement of the SCF-mediated pathway

The expression level of human thymidine kinase (hTK) is regulated in a cell-cycle-dependent manner. One of the mechanisms responsible for the fluctuation of TK expression in the cell cycle can be attributed to protein degradation during mitosis. Given the facts that cell-cycle-dependent proteolysis is highly conserved in all eukaryotes and yeast cells are an excellent model system for protein-degradation study, here we report on the use of Saccharomyces cerevisiae and Schizosaccharomyces pombe to investigate the degradation signal and mechanism required for hTK degradation. We found that the stability of hTK is significantly reduced in mitotic yeasts. Previously, we have observed that Ser-13 is the site of mitotic phosphorylation of hTK in HeLa cells [Chang, Huang and Chi (1998) J. Biol. Chem. 273, 12095-12100]. Here, we further provide evidence that the replacement of Ser-13 by Ala (S13A) renders hTK stable in S. pombe and S. cerevisiae. Most interestingly, we demonstrated that degradation of hTK is impaired in S. cerevisiae carrying a temperature-sensitive mutation in the proteasomal gene pre1-1 or the Skp1-Cullin-1/CDC53-F-box (SCF) complex gene cdc34 or cdc53, suggesting the contribution of the SCF-mediated pathway in hTK degradation. As phosphorylation is a prerequisite signal for SCF recognition, our results implied that phosphorylation of Ser-13 probably contributes to the degradation signal for hTK via the SCF-mediated proteolytic pathway.

Mechanisms of transcriptional regulation of cellular genes by SV40 large T- and small T-antigens

During the past decade a number of virus-encoded transcriptional trans-activators that regulate the expression of viral genes have been reported. These trans-activators may also affect the expression or activity of several cellular genes or gene products to create an optimal cellular environment that favors viral replication. Among the better-studied viral trans-activating proteins are the Simian virus 40 large T- and small t-antigens. During the last few years, mechanisms by which these two viral proteins influence cellular gene expression start to emerge. They are grouped provisionally and reflect the methods used to determine the effects of large T-antigen. Large T-antigen may influence cellular gene expression by: i. altering mRNA levels of cellular transcription factors; ii. interacting with and regulating the DNA-binding or transcriptional activity of specific transcription factors; iii. functionally substitution of eukaryotic transcription factors; iv. direct binding to DNA; or v. regulating components of signaling transduction pathways. Small t-ag seems to exert its effect mainly through inhibiting a cellular phosphatase, protein phosphatase 2A, thereby modulating components of signal transduction pathways and preventing dephosphorylation of several transcription factors. However, small t-ag may also control cellular gene expression by regulating mRNA levels of transcription factors or by interacting with other transcription factors.

Activation of the human thymidine kinase (TK) promoter by simian virus 40 large T antigen requires both the T antigen pRb family-binding domain and TK promoter sequences resembling E2F-binding sites

Infection of quiescent cells with the DNA tumor virus simian virus 40 induces expression of the cellular thymidine kinase (TK) gene a minimum of 10- to 20-fold, and this induction depends upon the viral protein large T antigen (T-Ag). To define both human TK promoter elements and T-Ag functional domains required for transcriptional induction, we have established a system in which stable Rat-1 transfectants harboring TK promoter-luciferase hybrid genes are infected with recombinant adenoviruses expressing either wild-type or mutant forms of T-Ag and luciferase expression is measured as an indicator of promoter activity. The results show that (i) a 135-bp TK promoter fragment is activated 10- to 15-fold by viral infection; (ii) this activation is the result of both T-Ag-dependent and -independent mechanisms; (iii) the T-Ag pRb family-binding domain, but not the p53-binding, helicase, or ATPase domain, is required for activation; and (iv) activation is severely diminished with a TK promoter fragment in which E2F-like-binding sites have been removed. These data demonstrate a requirement for both an E2F-related factor and a pRb family member in activation of the TK promoter by T-Ag. This contrasts with the promiscuous activation of many cellular and viral genes by T-Ag, which is independent of its ability to bind pRb.

The immediate-early gene Egr-1 regulates the activity of the thymidine kinase promoter at the G0-to-G1 transition of the cell cycle

Production of thymidine kinase (TK) protein parallels the onset of DNA synthesis during the cell cycle. This process is regulated at transcriptional, posttranscriptional, and translational levels to cause a 40- to 50-fold increase in cytosolic enzymatic activity as cells progress from G1 to S phase. Transcriptional activation of the mouse TK gene through the cell cycle is dependent upon previously characterized cis elements of the proximal promoter, called MT1, MT2, and MT3, that bind at least two different complexes: TKE during the transition of cells from quiescence (G0) to G1, and Yi later at the G1/S boundary. To identify the transcription factors involved in this regulation, we screened a mouse fibroblast cDNA expression library with a labeled MT3 oligonucleotide probe and isolated a clone that encodes Egr-1, an immediate-early transcription factor, whose expression is regulated by serum or growth factors during the G0-to-G1 transition when cells reenter the cell cycle. Electrophoretic mobility shift assays demonstrate that Egr-1 is involved in the TKE complex that binds to the MT3 element and that expression of Egr-1 induces transcription of a mouse TK-chloramphenicol acetyltransferase reporter in transient transfections. These results suggest a role for Egr-1 in regulating expression of the TK gene at the G0-to-G1 transition.

The immediate-early gene Egr-1 regulates the activity of the thymidine kinase promoter at the G0-to-G1 transition of the cell cycle

Production of thymidine kinase (TK) protein parallels the onset of DNA synthesis during the cell cycle. This process is regulated at transcriptional, posttranscriptional, and translational levels to cause a 40- to 50-fold increase in cytosolic enzymatic activity as cells progress from G1 to S phase. Transcriptional activation of the mouse TK gene through the cell cycle is dependent upon previously characterized cis elements of the proximal promoter, called MT1, MT2, and MT3, that bind at least two different complexes: TKE during the transition of cells from quiescence (G0) to G1, and Yi later at the G1/S boundary. To identify the transcription factors involved in this regulation, we screened a mouse fibroblast cDNA expression library with a labeled MT3 oligonucleotide probe and isolated a clone that encodes Egr-1, an immediate-early transcription factor, whose expression is regulated by serum or growth factors during the G0-to-G1 transition when cells reenter the cell cycle. Electrophoretic mobility shift assays demonstrate that Egr-1 is involved in the TKE complex that binds to the MT3 element and that expression of Egr-1 induces transcription of a mouse TK-chloramphenicol acetyltransferase reporter in transient transfections. These results suggest a role for Egr-1 in regulating expression of the TK gene at the G0-to-G1 transition.

Regulation of the cellular thymidine kinase gene promoter in simian virus 40-infected cells

We examined the regulation of the cellular thymidine kinase (TK) gene promoter in simian virus 40 (SV40)-infected simian CV1 cells. Nuclear run-on transcription assays demonstrated a three- to fourfold increase in the rate of transcription of the endogenous gene at 14 to 16 h following viral infection. In addition, hybrid genes containing the human TK promoter linked to the bacterial neomycin resistance gene were induced by SV40 in stably transfected cells, indicating that promoter sequences are sufficient to confer viral regulation. Analysis of human TK promoter deletion mutants indicated that sequences localized between -67 and +30 bp relative to the transcriptional initiation site are sufficient to confer regulation on SV40-infected cells. These sequence elements are distinct from those required for serum induction, which were previously localized to the region between -135 and -67. These results suggest that SV40 activates novel cellular pathways that are not activated by serum stimulation of quiescent cells.

Multiple bidirectional initiations and terminations of transcription in the Marek's disease virus long repeat regions

Marek's disease is an oncogenic disease of chickens caused by a herpesvirus, Marek's disease virus (MDV). Serial in vitro passage of pathogenic MDV results in amplification of a 132-bp direct repeat in the MDV genome's TRL and IRL repeat regions and loss of tumorigenicity. This led to the hypothesis that upon such expansion, one or more tumor-inducing genes fail to be expressed. In this report a group of cDNAs mapping in the expanded regions were isolated from a pathogenic MDV strain in which the 132-bp direct repeat number was found to range between one and seven. Partial cDNA sequencing and S1 nuclease protection analysis revealed that the corresponding transcripts are either initiated or terminated within or near the expanded regions at multiple sites in both rightward and leftward directions. Furthermore, each 132-bp repeat contains one TATA box and two polyadenylation consensus sequences in each direction. These RNAs contain a partial copy or one or more full copies of the 132-bp direct repeat at either their 5' or 3' end. Northern (RNA) blot analysis showed that the majority of transcripts are 1.8 kb in size, while the minor species range in size from 0.67 to 3.1 kb. Together, these data raise the possibility that the 132-bp direct repeat, and indirectly its copy number, may be involved in the regulation of transcriptional initiation and termination and therefore in the generation of four groups of transcripts from the TRL and IRL, although this remains to be demonstrated.

A requirement for Zn2+ for the induction of thymidine kinase but not ornithine decarboxylase in 3T3 cells stimulated from quiescence

In 3T3 cells stimulated from quiescence by serum, impaired thymidine incorporation caused by inadequate supply of Zn2+ was associated with both decreased thymidine kinase activity and a comparable decrease in its mRNA concentration. In contrast, the amount of mRNA for ribosomal protein S6 was not affected, nor was the earlier increase in the activity of ornithine decarboxylase.

Identification of a G1-S-phase-regulated region in the human thymidine kinase gene promoter

We have identified a regulatory region in the human thymidine kinase gene promoter. A set of promoter deletion mutants was constructed, linked to the bacterial neomycin resistance gene, and stably transfected into Rat3 cells. It was shown that the region between 135 and 67 base pairs upstream of the cap site is required for conveying G1-S-phase regulation to the linked neo gene. In addition, primer extension assays demonstrated that the same transcriptional start sites were used in G1- and S-phase cells and in the various deletion mutants tested.

Transcriptional and posttranscriptional regulation of the proliferating cell nuclear antigen gene

The steady-state mRNA levels of the proliferating cell nuclear antigen (PCNA) gene are growth regulated. In a previous paper (L. Ottavio, C.-D. Chang, M. G. Rizzo, S. Travali, C. Casadevall, and R. Baserga, Mol. Cell. Biol. 10:303-309, 1990), we reported that introns (especially intron 4) participate in growth regulation of the PCNA gene. We have now investigated the role of the 5'-flanking sequence of the human PCNA gene stably transfected into BALB/c 3T3 cells. Promoters of different lengths (from -2856 to -45 upstream of the cap site) were tested. All promoters except the AatII promoter (-45), including a short HpaII promoter (-210), were sufficient for a response to serum, platelet-derived growth factor, and to a lesser extent epidermal growth factor. No construct responded to insulin or platelet-poor plasma. The AatII promoter had little detectable activity. Transcriptional activity was also determined in BALB/c 3T3 cells carrying various constructs of the human PCNA gene by two methods: run-on transcription and reverse transcription-polymerase chain reaction (the latter measuring the heterogeneous nuclear RNA [hnRNA] steady-state levels). There was very little difference in the rate of transcription of the PCNA gene between G0 cells and serum-stimulated cells, although the levels of hnRNA were much higher after stimulation. In G0 cells carrying a human PCNA gene without introns 4 and 5, both transcription rate and hnRNA levels were high. Together with data on the mRNA half-life, these results suggest a posttranscriptional component in the regulation of PCNA mRNA levels after serum stimulation but a transcriptional regulation by intron 4.

Transcriptional and posttranscriptional regulation of the proliferating cell nuclear antigen gene

The steady-state mRNA levels of the proliferating cell nuclear antigen (PCNA) gene are growth regulated. In a previous paper (L. Ottavio, C.-D. Chang, M. G. Rizzo, S. Travali, C. Casadevall, and R. Baserga, Mol. Cell. Biol. 10:303-309, 1990), we reported that introns (especially intron 4) participate in growth regulation of the PCNA gene. We have now investigated the role of the 5'-flanking sequence of the human PCNA gene stably transfected into BALB/c 3T3 cells. Promoters of different lengths (from -2856 to -45 upstream of the cap site) were tested. All promoters except the AatII promoter (-45), including a short HpaII promoter (-210), were sufficient for a response to serum, platelet-derived growth factor, and to a lesser extent epidermal growth factor. No construct responded to insulin or platelet-poor plasma. The AatII promoter had little detectable activity. Transcriptional activity was also determined in BALB/c 3T3 cells carrying various constructs of the human PCNA gene by two methods: run-on transcription and reverse transcription-polymerase chain reaction (the latter measuring the heterogeneous nuclear RNA [hnRNA] steady-state levels). There was very little difference in the rate of transcription of the PCNA gene between G0 cells and serum-stimulated cells, although the levels of hnRNA were much higher after stimulation. In G0 cells carrying a human PCNA gene without introns 4 and 5, both transcription rate and hnRNA levels were high. Together with data on the mRNA half-life, these results suggest a posttranscriptional component in the regulation of PCNA mRNA levels after serum stimulation but a transcriptional regulation by intron 4.

Identification of a G1-S-phase-regulated region in the human thymidine kinase gene promoter

We have identified a regulatory region in the human thymidine kinase gene promoter. A set of promoter deletion mutants was constructed, linked to the bacterial neomycin resistance gene, and stably transfected into Rat3 cells. It was shown that the region between 135 and 67 base pairs upstream of the cap site is required for conveying G1-S-phase regulation to the linked neo gene. In addition, primer extension assays demonstrated that the same transcriptional start sites were used in G1- and S-phase cells and in the various deletion mutants tested.

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.

Thymidine kinase synthesis is repressed in nonreplicating muscle cells by a translational mechanism that does not affect the polysomal distribution of thymidine kinase mRNA

The molecular basis for replication-dependent expression of thymidine kinase (TK) activity (EC 2.7.1.21) was investigated in mouse skeletal muscle cells transformed with multiple copies of the chicken TK gene. When shifted to mitogen-depleted medium, proliferating myoblasts irreversibly withdraw from the cell cycle and commit to terminal differentiation. Early after commitment, postreplicative myocytes maintain nearly proliferative levels of TK mRNA but have greatly reduced levels of TK activity. Metabolic labeling studies with [35S]methionine indicated that the decrease in TK activity was associated with a 10-fold reduction in the rate of TK protein synthesis. Commitment had little effect on the stability or catalytic efficiency of TK protein. The decrease in TK synthetic rate in the continued presence of TK mRNA indicated that translation of TK mRNA was repressed in committed cells. The distribution of TK mRNA between ribonucleoprotein particles and polysomes was determined. In both proliferative cells and committed cells, TK mRNA levels were maximal in polysomes containing five to seven ribosomes. Thus, the synthesis of TK protein in nonreplicating muscle cells was inhibited by a translational mechanism that did not alter the average number of ribosomes engaged by TK mRNA.

Transcriptional and posttranscriptional mechanisms regulate murine thymidine kinase gene expression in serum-stimulated cells

We previously isolated and characterized the structure of murine thymidine kinase (tk) genomic and cDNA sequences to begin a study designed to identify regions of the tk gene important for regulated expression during the transition of cells from G0 to a proliferating state. In this report, we describe the stable transfection of the cloned gene into L-M(TK-) cells and show that both thymidine kinase (TK) enzyme activity and DNA synthesis increase in parallel when transfectants in G0 arrest are stimulated by serum. To define promoter and regulatory regions more precisely, we have constructed a series of tk minigenes and have examined their expression in stable transfectants after serum stimulation. We have identified a 291-base-pair DNA fragment at the 5' end of the tk gene that has promoter function, and we have determined its sequence. In addition, we have found that DNA sequences which mediate serum-induced expression of TK are transcribed, since expression of the murine tk cDNA, fused to a promoter from either the murine tk gene, the simian virus 40 early region, or the herpes simplex virus tk gene, is stimulated by serum. Our constructs also reveal that the murine tk polyadenylation signal is not required for regulation, nor is most of the 3' untranslated region. RNA dot blot analysis indicates that murine cytoplasmic tk mRNA levels always parallel TK enzyme activity. Nuclear runon transcription assays show less than a 2-fold increase in transcription from the cloned tk gene in serum-stimulated transfectants, but an 11-fold increase in mouse L929 cells, which are inherently TK+. These results taken together suggest that the murine tk gene is controlled in serum-stimulated cells by a transcriptional mechanism influenced by DNA sequences that flank tk and also by a posttranscriptional system linked to gene sequences that are transcribed.

Mouse thymidine kinase: the promoter sequence and the gene and pseudogene structures in normal cells and in thymidine kinase deficient mutants

The mouse genome carries one gene and two pseudogenes for cytoplasmic thymidine kinase. The overall structure of these genes was determined with the help of cosmids and lambda phage clones and the upstream sequence containing the promoter was determined. The data allow an allocation of bands seen in the complex patterns of genomic Southern blots obtained from the DNA of wild type cells and of thymidine kinase deficient mutants to the gene as well as to the two pseudogenes. The much used LTK cell line was found to lack the entire gene but to retain the pseudogenes. Two other TK cell lines had DNA patterns indistinguishable from the wild type. Whereas the LTK line did not produce any TKmRNA, the two other mutants had normal amounts of TKmRNA but no cytoplasmic TK activity.

Regulation of thymidine kinase protein levels during myogenic withdrawal from the cell cycle is independent of mRNA regulation

Replication-dependent changes in levels of enzymes involved in DNA precursor biosynthesis are accompanied frequently by changes in levels of cognate mRNA. We tested the common assumption that changes in mRNA levels are responsible for growth-dependent expression of these enzymes using a line of mouse muscle cells that irreversibly withdraws from the cell cycle as part of its terminal differentiation program. Thymidine kinase (TK) mRNA, activity, and protein levels were quantitated in cells transformed with multiple copies of the chicken TK gene. The decline in TK mRNA (both whole cell and cytoplasmic) during myogenesis was poor (2-fold average) and variable (1.2 to 8-fold). In contrast, TK activity always was regulated efficiently (20-fold), even in cells which regulated TK mRNA very poorly. Thus, regulation of TK activity was independent of TK mRNA regulation as myoblasts withdrew from the cell cycle. A TK/beta-galactosidase fusion protein was used to derive an antibody against chicken TK. Immunoblot and immunoprecipitation analyses demonstrated TK protein levels, like TK activity levels, declined to a greater extent than TK mRNA levels. Thus, TK activity likely was regulated by a mechanism involving either decreased translation of TK mRNA or increased degradation of TK protein in committed muscle cells.

Transcriptional and posttranscriptional mechanisms regulate murine thymidine kinase gene expression in serum-stimulated cells

We previously isolated and characterized the structure of murine thymidine kinase (tk) genomic and cDNA sequences to begin a study designed to identify regions of the tk gene important for regulated expression during the transition of cells from G0 to a proliferating state. In this report, we describe the stable transfection of the cloned gene into L-M(TK-) cells and show that both thymidine kinase (TK) enzyme activity and DNA synthesis increase in parallel when transfectants in G0 arrest are stimulated by serum. To define promoter and regulatory regions more precisely, we have constructed a series of tk minigenes and have examined their expression in stable transfectants after serum stimulation. We have identified a 291-base-pair DNA fragment at the 5' end of the tk gene that has promoter function, and we have determined its sequence. In addition, we have found that DNA sequences which mediate serum-induced expression of TK are transcribed, since expression of the murine tk cDNA, fused to a promoter from either the murine tk gene, the simian virus 40 early region, or the herpes simplex virus tk gene, is stimulated by serum. Our constructs also reveal that the murine tk polyadenylation signal is not required for regulation, nor is most of the 3' untranslated region. RNA dot blot analysis indicates that murine cytoplasmic tk mRNA levels always parallel TK enzyme activity. Nuclear runon transcription assays show less than a 2-fold increase in transcription from the cloned tk gene in serum-stimulated transfectants, but an 11-fold increase in mouse L929 cells, which are inherently TK+. These results taken together suggest that the murine tk gene is controlled in serum-stimulated cells by a transcriptional mechanism influenced by DNA sequences that flank tk and also by a posttranscriptional system linked to gene sequences that are transcribed.

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.

The lit gene product which blocks bacteriophage T4 late gene expression is a membrane protein encoded by a cryptic DNA element, e14

Escherichia coli lit(Con) mutations cause a severe inhibition of gene expression late in infection by bacteriophage T4 owing to the overproduction of one, and possibly two, proteins (C. Kao, E. Gumbs, and L. Snyder, J. Bacteriol. 169:1232-1238, 1987). One or both of these proteins interact, either directly or indirectly, with a short sequence about one-quarter of the way into the major capsid protein gene of T4, and the inhibition occurs when this late gene of the virus is expressed. In this report we show that lit(Con) mutations are up-promoter mutations in the cryptic DNA element e14 and that only one of the proteins, gplit, of about 34 kilodaltons, is required for the inhibition. We have sequenced the lit gene and the surrounding regions. From the sequence, and from cell fractionation studies, we conclude that gplit is an inner membrane protein. Since the assembly of T4 heads is thought to occur on the inner face of the inner membrane, we propose that gplit interferes with a normal regulation which coordinates the synthesis of proteins and the assembly of T4 heads.

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.

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.

Cell-cycle-specific interaction of nuclear DNA-binding proteins with a CCAAT sequence from the human thymidine kinase gene

Induction of thymidine kinase parallels the onset of DNA synthesis. To investigate the transcriptional regulation of the thymidine kinase gene, we have examined whether specific nuclear factors interact in a cell-cycle-dependent manner with sequences upstream of this gene. Two inverted CCAAT boxes near the transcriptional initiation sites were observed to form complexes with nuclear DNA-binding proteins. The nature of the complexes changes dramatically as the cells approach DNA synthesis and correlates well with the previously reported transcriptional increase of the thymidine kinase gene.

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.

Evidence for transcriptional and post-transcriptional control of the cellular thymidine kinase gene

We have studied the cell cycle-regulated expression of the thymidine kinase (TK) gene in mammalian tissue culture cells. TK mRNA and enzyme levels are low in resting, G0-phase cells, but increase dramatically (10- to 20-fold) during the S phase in both serum-stimulated and simian virus 40-infected cells. To determine whether an increase in the rate of TK gene transcription is responsible for this induction, nuclear run-on transcription assays were performed at various times after serum stimulation or simian virus 40 infection of growth-arrested simian CV1 cells. When assays were performed at 12-h intervals, a small (two- to threefold) but reproducible increase in TK transcription was detected during the S phase. When time points were chosen to span the G1-S interface a larger (six- to sevenfold) increase in transcriptional activity was observed in serum-stimulated cells but not in simian virus 40-infected cells. The large increase in TK mRNA levels and the relatively small increase in transcription rates in growth-stimulated cells suggest that TK gene expression is controlled at both a transcriptional and post-transcriptional level during the mammalian cell cycle. To identify the DNA sequences required for cell cycle-regulated expression, several TK cDNA clones were transfected into Rat-3 TK- cells, and their expression was examined in resting and serum-stimulated cultures. These experiments indicated that the body of the TK cDNA is sufficient to insure cell cycle-regulated expression regardless of the promoter or polyadenylation signal used.

Evidence for transcriptional and post-transcriptional control of the cellular thymidine kinase gene

We have studied the cell cycle-regulated expression of the thymidine kinase (TK) gene in mammalian tissue culture cells. TK mRNA and enzyme levels are low in resting, G0-phase cells, but increase dramatically (10- to 20-fold) during the S phase in both serum-stimulated and simian virus 40-infected cells. To determine whether an increase in the rate of TK gene transcription is responsible for this induction, nuclear run-on transcription assays were performed at various times after serum stimulation or simian virus 40 infection of growth-arrested simian CV1 cells. When assays were performed at 12-h intervals, a small (two- to threefold) but reproducible increase in TK transcription was detected during the S phase. When time points were chosen to span the G1-S interface a larger (six- to sevenfold) increase in transcriptional activity was observed in serum-stimulated cells but not in simian virus 40-infected cells. The large increase in TK mRNA levels and the relatively small increase in transcription rates in growth-stimulated cells suggest that TK gene expression is controlled at both a transcriptional and post-transcriptional level during the mammalian cell cycle. To identify the DNA sequences required for cell cycle-regulated expression, several TK cDNA clones were transfected into Rat-3 TK- cells, and their expression was examined in resting and serum-stimulated cultures. These experiments indicated that the body of the TK cDNA is sufficient to insure cell cycle-regulated expression regardless of the promoter or polyadenylation signal used.

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.

Growth-rate-dependent regulation of the expression and inactivation of thymidylate synthase in Saccharomyces cerevisiae

Thymidylate synthase activity fluctuated dramatically as cultures of Saccharomyces cerevisiae progressed through the different stages of batch culture growth. During logarithmic growth these yeast cultures each contained about 40 microU (1 microU is 1 pmol of 3H released per min) of thymidylate synthase activity per 10(8) haploid cells, but as cultures entered the stationary phase and during the stationary phase, activity dropped dramatically, eventually reaching undetectable levels. Stimulation of stationary-phase cells with fresh medium resulted in rapid reestablishment of log phase levels. Two mechanisms, the regulation of thymidylate synthase-specific transcripts and the irreversible inactivation of thymidylate synthase activity, acted in concert to regulate activity levels. These results suggested that thymidylate synthase represents a special subset of yeast proteins whose levels per cell vary quickly and dramatically in response to changes in proliferation rates.

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.