DNA-mediated transfer of an RNA polymerase II gene: reversion of the temperature-sensitive hamster cell cycle mutant TsAF8 by mammalian DNA

A mutation in the largest (catalytic) subunit of RNA polymerase II and its relation to the arrest of the cell cycle in G(1) phase

Transcriptional activity of RNA polymerase II is modulated during the cell cycle. We previously identified a temperature-sensitive mutation in the largest (catalytic) subunit of RNA polymerase II (RPB1) that causes cell cycle arrest and genome instability. We now characterize a different cell line that has a temperature-sensitive defect in cell cycle progression, and find that it also has a mutation in RPB1. The temperature-sensitive mutant, tsAF8, of the Syrian hamster cell line, BHK21, arrests at the non-permissive temperature in the mid-G(1) phase. We show that RPB1 in tsAF8--which is found exclusively in the nucleus at the permissive temperature--is also found in the cytoplasm at the non-permissive temperature. Comparison of the DNA sequences of the RPB1 gene in the wild-type and mutant shows the mutant phenotype results from a (hemizygous) C-to-A variation at nucleotide 944 in one RPB1 allele; this gives rise to an ala-to-asp substitution at residue 315 in the protein. Aligning the amino acid sequences from various species reveals that ala(315) is highly conserved in eukaryotes.

Effects of PKC inhibitors on suppression of thermotolerance development in tsAF8 cells

Effects of protein kinase C (PKC) inhibitors (H7, staurosporine, calphostin C) on thermotolerance development were investigated in temperature sensitive tsAF8 cells derived from Syrian hamster BHK21 cells. Cells were pre-heated at 45 degrees C for 20 min, incubated at 34 degrees C with PKC inhibitors for varying lengths of time, i.e. 1.25-10.0 h, and then heated at 45 degrees C for 30 min. Increasing survival fractions after the second heat treatment was inhibited by the treatment with H7 (40-160 microM), with staurosporine (0.05-1.0 microM), and with calphostin C (0.8, 1.2 microM) in a concentration dependent manner. When the concentrations of these PKC inhibitors were low, the restraint of increasing survival fractions was temporary, since survival fractions increased 3-7.5 h after pre-heating. However, the survival fractions were almost constant by the treatment with 160 microM H7 and 1.0 microM staurosporine. Induction of HSP72 after heat stress was investigated in tsAF8 and BHK21 cells. Cells were heated at 45 degrees C for 20 min and incubated at 34 or 39.7 degrees C (tsAF8), at 37 degrees C (BHK21). Intensity of intracellular fluorescence from HSP72 was measured by flow cytometry. HSP72 was induced in BHK21 cells, but there was no definite induction of HSP72 in tsAF8 cells at either 39.7 or 34 degrees C. These results suggest that PKC is related with the thermotolerance development in tsAF8 cells; however, HSP72 is not involved in the thermotolerance development in tsAF8 cells.

Temporary complementation of temperature-sensitive mutants of the cell cycle by transfection with a wild-type or a mutant cDNA of ADP/ATP translocase

A number of cell-cycle-specific temperature-sensitive (ts) mutants have been isolated from animal cells, especially Syrian hamster cells. These ts mutants, like cell cycle ts mutants of yeast, can be complemented by specific genes, some of which have been molecularly cloned. We have isolated a cDNA clone that complements TK-ts13 cells, but only temporarily. This clone, called B1, differs from a previously isolated clone (Sekiguchi et al.: EMBO Journal 7:1683-1687, 1988) that specifically complements ts13 cells. In addition, B1 also complemented temporarily three other ts mutants of the cell cycle, tsAF8, ts694, and ts550C cells. These mutants have different mutations since, in cell fusion experiments, they complement each other. Sequencing of the B1 cDNA clone revealed that it was a mutant of human ADP/ATP translocase in which some human sequences at the 5' end have been replaced by SV40 sequences. The wild-type translocase was less effective but could still increase the survival time of cell cycle ts mutants at the restrictive temperature. Using the polymerase chain reaction, it was possible to demonstrate that the B1 plasmid is expressed in TK-ts13 cells undergoing temporary complementation.

Regional localization of CCG1 gene which complements hamster cell cycle mutation BN462 to Xq11-Xq13

The human CCG1 gene, which complements the temperature-sensitive hamster cell cycle mutations BN462 and ts13, has recently been cloned and shown to be located on the X chromosome. We have used somatic cell hybrids segregating portions of multiple X--autosome translocations to localize this gene to the Xq11 to Xq13 region of the human X chromosome.

Steady-state and nuclear run-on analyses of transcription in a temperature-sensitive Chinese hamster cell mutant with a defect in RNA metabolism

We have further characterized a temperature-sensitive mutant of Chinese hamster lung fibroblasts in tissue culture with a defect in RNA metabolism. The mutant phenotype is reflected in transcription in crude extracts or in isolated nuclei, when these are made from cells shifted to the nonpermissive temperature; however, differential heat inactivation between mutant and wild-type extracts cannot be demonstrated with cell-free systems. We tentatively conclude that the mutation may affect initiation of transcription which cannot be observed in our in vitro systems. Partially purified RNA polymerase I, II, and III fractions are indistinguishable from wild type. A temperature shift does not affect transcription by RNA polymerase III measured with intact cells or by nuclear run-on experiments. The nuclear run-on and other experiments suggest that RNA polymerase II-dependent transcription is inhibited before RNA polymerase I-dependent transcription. This conclusion is also supported by Northern analyses of selected mRNAs in nonsynchronized and synchronized cells after a shift to the nonpermissive temperature.

Identification of human gene complementing ts AlS9 mouse L-cell defect in DNA replication following DNA-mediated gene transfer

The temperature-sensitive (ts) mouse L-cell, ts AlS9, is defective in a gene required for nuclear DNA replication early in the S phase of the cell cycle. Human DNA sequences were introduced into ts AlS9 cells together with the plasmid pSV2neo, which can confer resistance to the drug geneticin. Cotransformants, expressing both the plasmid-derived neomycin gene and the transferred human AlS9 gene, were selected for growth in the presence of the drug at the nonpermissive temperature (npt). The resulting transformants retained a common set of human-specific Alu repetitive DNA sequences. These are likely to be accommodated within, or in proximity to, the transferred human AlS9 gene. The results obtained provide the basis for cloning human genes required for DNA replication.

Cloning of a human S-phase cell cycle gene: use of transient expression for screening

We report here the cloning of a human cell cycle gene capable of complementing a temperature-sensitive (ts) S-phase cell cycle mutation in a Chinese hamster cell line. Cloning was performed as follows. A human genomic library in phage lambda containing 600,000 phages was screened with labeled cDNA synthesized from an mRNA fraction enriched for the specific cell cycle gene message. Plaques containing DNA inserts which hybridized to the cDNA were picked, and their DNAs were assayed for transient complementation in DNA transformation experiments. The transient complementation assay we developed is suitable for most cell cycle genes and indeed for many genes whose products are required for cell proliferation. Of 845 phages screened, 1 contained an insert active in transient complementation of the ts cell cycle mutation. Introduction of this phage into the ts cell cycle mutant also gave rise to stable transformants which grew normally at the restrictive temperature for the ts mutant cells.

Regulation of the expression of the SV40 T-antigen coding gene under the control of an rDNA promoter

We have constructed a hybrid gene in which the SV40 T-antigen coding gene is driven by a mouse rDNA promoter and we have compared its expression to that of an SV40 T-antigen coding gene under the control of its own promoter. The comparison has been carried out in microinjected cells, in transfected cells, and in stable cell lines carrying the respective T-antigen coding genes in an integrated form. These cell lines were derived from ts AF8 cells, a mutant which is temperature sensitive for RNA polymerase II activity. The hybrid gene clearly expresses T-antigen, albeit less efficiently than when the T antigen coding gene is under the control of the SV40-promoter. We also show that the expression of T-antigen by the hybrid gene is 50% inhibited by an antibody against RNA polymerase I. In tsAF8 cells carrying the hybrid gene, T-antigen is still expressed at the restrictive temperature (where RNA polymerase II is inactive) at a level again about 50% of controls. However, our findings also confirm those of Smale and Tjian (Mol. Cell. Biol. 5:352, 1985) that such hybrid genes are in part transcribed by RNA polymerase II and generate abnormal transcripts.

Molecular biology of the cell cycle

Genes and cDNA clones have been identified in animal cells that are cell cycle-regulated, i.e. they are preferentially expressed in a phase of the cell cycle. Some of these genes, including four oncogenes, are induced when G0 cells are stimulated to proliferate. Four approaches are described to identify the genes that regulate the transition of cells from a resting to a growing stage. The interrelationship among cell cycle-regulated genes, oncogenes, growth factors and receptors for growth factors points the way to a genetic dissection of cell cycle progression.

Quantitative genetic analysis of tumor progression

Metastasis and resistance to chemotherapy are common features of progressed cancers. With respect to the latter phenotype, it is thought that during tumor growth drug-resistant cells arise spontaneously at rates characteristic of the genetic alterations involved. On application of chemotherapy, such variant tumor cells are more likely to survive, and they may eventually dominate, resulting in a non-responsive malignancy. Aspects of this model have been confirmed in a number of experimental systems and in patients. In contrast to our understanding of drug resistance, steps involved in the progression to metastatic spread of tumor cells are much less well-understood. In this review we describe methodologies of quantitative genetic analysis with reference to development of drug resistance. We then describe attempts by ourselves and others to use a similar approach to investigate metastatic properties. Based on these studies, we have proposed the quantitative 'dynamic heterogeneity' model of tumor metastasis, which is presented here. Using an 'experimental' metastasis assay and Luria-Delbruck fluctuation analysis, we determined that in murine KHT fibrosarcoma and B16 melanoma lines, 'metastatic' variants with a distinct phenotype are generated at high rates. These variants are relatively unstable resulting in a dynamic equilibrium between generation and loss of metastatic variants. The metastatic ability of such a tumor population is thus dependent on the frequency of a subpopulation of metastatic variants which are turning over rapidly. This dynamic heterogeneity model is able to quantitatively provide a unifying explanation for a wide range of observations concerning tumor heterogeneity and clonal instability. Genetic mechanisms involving rapid rates have been characterized in drug-resistant variants. We speculate that similar processes may be involved in different aspects of tumor progression such as those resulting in metastasis.

Eukaryotic RNA polymerases

This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.

Transformation of temperature-sensitive growth mutant of BHK21 cell line to wild-type phenotype with hamster and mouse DNA

A temperature-sensitive (ts) mutant, tsBN2, which was derived from BHK21 and is defective in the regulatory mechanism for chromosome condensation, was transformed to the temperature-resistant (ts+) phenotype by means of DNA-mediated gene transfer with hamster and mouse DNA. Treatment of mouse DNA with the restriction enzymes EcoRI, HindIII, PstI and SalI, but not with XhoI, almost completely abolished the transforming activity. A fluctuation test, originally devised by Luria and Delbrück, was used to estimate the reversion and transformation frequencies of tsBN2 cultures.