Genetic complementation of radiation response by 3' untranslated regions (UTR) of RNA.

The molecular basis of radiosensitivity was studied using a cDNA complementation approach to correct radiosensitivity in cells. Four cDNAs of sizes 1.6, 2.0, 2.2 and 2.5 kb were isolated that corrected several aspects of the phenotype of cells from patients with the human genetic disorder ataxia-telangiectasia, characterized by hypersensitivity to ionizing radiation. The criteria used to assess correction included cell viability, induced chromosome aberrations, G2 phase delay and induction of p53 after exposure to radiation. One cDNA (2.5 kb) was identified as the complete sequence of the RNA helicase p68, which was capable of correcting radiosensitivity based on two of the above four criteria, with p53 induction post irradiation being partially corrected. The 2.2 kb cDNA was shown to correspond to the complete sequence of arginyl tRNA synthetase and the other two cDNAs were identical to the 3' untranslated regions (UTR) of the transcription factor TFIIS (1.6 kb) and phospholipase A2 (2.0 kb) respectively. Additional transfections with the 3'UTR (198 nucleotides) of p68 RNA helicase and its inverse sequence revealed that the 3'UTR had the same complementation capacity as the full-length cDNA, whereas the inverse construct failed to complement radiosensitivity. These data provide additional support for a novel role for 3'UTRs in the regulation of gene expression.

The activation of a specific DNA binding protein by neutron irradiation.

PURPOSE: To determine whether the quality of ionizing radiation is critical for activation of a radiation-specific DNA binding protein. METHODS AND MATERIALS: We have previously shown that after exposing Epstein Barr virus-transformed lymphoblastoid cells to ionizing radiation, a specific DNA binding factor appears in the nucleus apparently as a result of translocation from the cytoplasm. This protein binds to a number of different genomic sequences and a consensus motif has been identified. Because the protein was not activated by UV light, it was of interest whether high linear energy transfer (LET) radiation was capable of activation. RESULTS: We describe here the activation of a specific DNA binding protein by high LET neutron radiation. The protein binds a region adjacent to and overlapping with the distal repeat within a 179 base-pair fragment of the well-characterized Simian Virus (SV40) bidirectional promoter/enhancer element. The appearance of the DNA binding activity was dose dependent and reached a maximum level by 90 min postirradiation. A reduction in DNA binding activity was evident at later times after irradiation. CONCLUSIONS: The specific nature of this response and the rapidity of activation may indicate a pivotal role for this protein in repair or in some other aspect of the cellular response to radiation damage.

Defect in radiation signal transduction in ataxia-telangiectasia.

Exposure of mammalian cells to ionizing radiation causes a delay in progression through the cycle at several checkpoints. Cells from patients with ataxia-telangiectasia (A-T) ignore these checkpoint controls postirradiation. The tumour suppressor gene product p53 plays a key role at the G1/S checkpoint preventing the progression of cells into S phase. The induction of p53 by radiation is reduced and/or delayed in A-T cells, which appears to account for the failure of delay at the G1/S checkpoint. We have investigated further this defect in radiation signal transduction in A-T. While the p53 response was defective after radiation, agents that interfered with cell cycle progression such as mimosine, aphidicolin and deprivation of serum led to a normal p53 response in A-T cells. None of these agents caused breaks in DNA, as determined by pulse-field gel electrophoresis, in order to elicit the response. Since this pathway is mediated by protein kinases, we investigated the activity of several of these enzymes in control and A-T cells. Ca+2-dependent and -independent protein kinase C activities were increased by radiation to the same extent in the two cell types, a variety of serine/threonine protein kinase activities were approximately the same and anti-tyrosine antibodies failed to reveal any differences in protein phosphorylation between A-T and control cells. It is not evident what is the nature of the defect in signal transduction in A-T cells. However, it is clear that the p53 response is normal in these cells after exposure to some agents and it is mediated through protein kinase C or another serine/threonine kinase.

A specific DNA-binding protein activated by ionizing radiation in normal cells and constitutively present in ataxia telangiectasia cells.

Exposure of mammalian cells to ionizing radiation gives rise to a complex series of changes. This response is characterized by the induction of a variety of genes and the activation of pre-existing proteins. We describe here activation of a specific DNA-binding protein by ionizing radiation. The response was dose-dependent and specific for ionizing radiation. The binding factor appears to be normally present in the cytoplasm and responds to radiation by translocation to the nucleus, or is activated within the nucleus by an unknown mechanism. The radiation-induced activation of this protein appears to be mediated through a protein kinase C-associated pathway. A DNA-binding factor recognizing the same binding motif was found to be abnormally distributed in cells from patients with the human genetic disease, ataxia telangiectasia. The protein was constitutively present in the nucleus and the cytoplasm of ataxia telangiectasia cells and did not respond to radiation.

Radiation-activated DNA-binding protein constitutively present in ataxia telangiectasia nuclei.

We have recently described the appearance of a specific DNA-binding protein in nuclei from human cells exposed to ionizing radiation which was not detected in nuclear extracts from unperturbed cells (Singh, S. P., and Lavin, M. F. (1990) Mol. Cell. Biol. 10, 5279-5285). We report here a similar activity which is constitutively present in nuclei of both unirradiated and irradiated cells from patients with the human genetic disorder ataxia telangiectasia (A-T). Activity was present in unirradiated nuclear extracts from 3 A-T cell lines of different complementation groups, but was not detected or was present only at a low level in 4 controls. Active protein was detected in the cytoplasm of both cell types. Exposure to ionizing radiation did not change the amount of DNA binding activity in A-T nuclei but led to an increase in nuclei from 4 control cell lines. Purification of the binding activities from A-T nuclei and control cytoplasm was carried out by affinity chromatography, as described previously for control extracts (Teale, B., Singh, S. P., Khanna, K. K., Findik, D., and Lavin, M. F. (1992) J. Biol. Chem. 267, 10295-10301). Southwestern analysis and UV cross-linking confirmed the presence of a major DNA-binding species at 70 kDa in both cases with a minor binding activity at 47 kDa also evident. It was not possible to distinguish between the binding activities from A-T and control cells under different conditions, and phosphorylation was required for binding activity in both cases. Footprint analysis revealed that the same sequence was being recognized by the control and A-T proteins. The constitutive presence of a specific radiation-responsive DNA-binding protein in A-T cells may be indicative of a continuous state of stress in these cells.

Purification and characterization of a DNA-binding protein activated by ionizing radiation.

Exposure of mammalian cells to a variety of agents leads to the activation of pre-existing proteins and the induction of specific genes. We have recently described the appearance of a specific DNA-binding protein in nuclei from cells exposed to ionizing radiation (Singh, S. P., and Lavin, M. F. (1990) Mol. Cell. Biol. 10, 5279-5285). This protein is present in the cytoplasm of unperturbed cells and is apparently translocated to the nucleus in response to radiation damage. We describe here the purification and characterization of this specific DNA-binding protein. Purification involved the use of affinity chromatography employing a multimeric form of the DNA-binding motif conjugated to cyanogen bromide-activated Sepharose. Three DNA-binding species were recognized by UV-cross-linking and South-Western analysis. The major species or that with the highest affinity was approximately 70 kDa in size. DNase-1 footprint analysis revealed a single binding site in the kappa immunoglobulin gene enhancer and in a putative control sequence upstream from the c-myc gene. At salt concentrations as high as 1 M, up to 40% of the DNA-binding activity was maintained and the Kd was calculated to be 1.205 x 10(-6) M-1. Binding activity was found to be modulated by phosphorylation. Removal of phosphate groups from the protein resulted in a major loss of binding activity. It is not clear at this stage whether the factor(s) described here plays a role in transcription control or a more general DNA-processing role in response to radiation damage.