p53-mediated transcription induces resistance of DNA to UV inactivation

A possible role of p53-dependent transcription in the induction of DNA repair was explored by transfecting a UV-irradiated chloramphenicol acetyl transferase (CAT) reporter plasmid (pRGC.FOS.CAT), containing a minimal FOS promoter driven by a consensus p53 binding site, into a p53 negative-mouse cell line [(10)1]. When a p53-expressing plasmid (pSV.p53) was cotransfected into these cells, CAT expression levels persisted even after prolonged UV irradiation. In comparison, CAT expression from pSV2.CAT, which lacks a p53-responsive element in its SV40 promoter, dropped off much more precipitously after UV irradiation in the absence or presence of WT p53 expression. A similar sharp drop was observed with three other constructs when the reporter gene was under the control of the ras, β-actin or fos promoter. Mouse cells (A1-5) that constitutively express a temperature-sensitive mutant (135 AV) of mouse p53 also generated, at 32°C, higher levels of enzyme expressed from UV-irradiated pRGC.FOS.CAT than from UV-irradiated pSV2.CAT. The frequency of cyclobutane pyrimidine dimers in UV-irradiated pRGC.FOS.CAT was determined with T4 endo V, and the probability of having an undamaged CAT coding strand was calculated by the Poisson distribution for various times of UV-irradiation. The observed relative CAT expression levels from irradiated pSV2.CAT and pRGC.FOS.CAT in the absence of p53 were consistent with those numbers. These results show that WT p53-mediated transcription directs a resistance of the transcribed DNA to UV inactivation and reactivates the reporter gene. Furthermore, some single point substitution mutants of p53 that maintain a near normal ability to activate transcription had lost their ability to extend CAT gene expression after UV irradiation. Conversely, other mutants with reduced transcriptional activity retained this ability. This indicates that although resistance to UV inactivation is transcriptionally-dependent, these two activities are genetically distinct. These data, taken together, suggest that the transcription of UV-damaged DNA by a p53-dependent process promotes its repair.