EFFECT OF IONIZING-RADIATION ON THE EXPRESSION OF CHLORAMPHENICOL ACETYLTRANSFERASE GENE UNDER THE CONTROL OF COMMONLY USED CONSTITUTIVE OR INDUCIBLE PROMOTERS

We examined the effect of ionizing radiation on the expression of the chloramphenicol acetyltransferase (CAT) gene driven by commonly used viral or semi-synthetic promoters in transient transfection assays using COS-7 cells. Specifically, we studied the constitutive early promoters of cytomegalovirus (CMV), Rous sarcoma virus (RSV), and the simian virus 40 (SV40), and the inducible promoters from mouse mammary tumour virus (MMTV, inducible by dexamethasone) and the semi-synthetic SVlacO promoter (SVlacO, inducible by isopropyl beta-D-thiogalactoside). CAT activity was measured in cells exposed to 20 Gy X-rays and in non-irradiated controls 46 h after transient transfection. We observed that CAT expression from the CMV and SV40 promoters remained unchanged in cells exposed to radiation. Similar results were obtained with MMTV and SVlacO promoters when cells were irradiated 16 h after induction. However, expression from the RSV promoter was significantly increased after radiation exposure and similar results were obtained with the MMTV and SVlacO promoters when irradiation immediately preceded induction. The results suggest a classification of promoters as radiation-independent and -dependent. The consitutive CMV and SV40 promoters belong to the first category, whereas RSV belongs to the second category. The inducible MMTV and SVlacO promoters would be classified to the first category when induction precedes irradiation by 16 h, and to the second category when irradiation immediately follows induction. It will be desirable to use promoters under conditions that classify them to the first category to study the effect on cellular response to radiation of genes suspected to influence the intrinsic radiosensitivity of cells.