LACK OF TRANSCRIPTION-COUPLED REPAIR OF ACETYLAMINOFLUORENE DNA-ADDUCTS IN HUMAN FIBROBLASTS CONTRASTS THEIR EFFICIENT INHIBITION OF TRANSCRIPTION

The N-(deoxyguanosine-8-yl)-2-acetylaminofluorene (dG-C8-AAF) lesion i s among the most helix distorting DNA lesions. In normal fibroblasts d G-C8-AAF is repaired rapidly in transcriptionally active genes, but wi thout strand specificity, indicating that repair of dG-C8-AAF by globa l genome repair (GGR) overrules transcription-coupled repair (TCR), Ye t, dG-C8-AAF is a very potent inhibitor of transcription. The target s ize of inhibition (45 kilobases) suggests that transcription inhibitio n by dG-C8-AAF is caused by blockage of initiation rather than elongat ion. Cockayne's syndrome (CS) cells appear to be extremely sensitive t o the cytotoxic effects of dG-C8-AAF and are unable to recover inhibit ed RNA synthesis. However, CS cells exhibit no detectable defect in re pair of dG-C8-AAF in active genes, indicating that impaired TCR is not the cause of the enhanced sensitivity of CS cells. These and data rep orted previously suggest that the degree of DNA helix distortion deter mines the rate of GGR as well as the extent of inhibition of transcrip tion initiation. An interchange of the transcription/repair factor TFI IH from promoter sites to sites of damage might underlie inhibition of transcription initiation. This process is likely to occur more rapidl y and efficiently in the case of strongly DNA helix distorting lesions , resulting in a very efficient GGR, a poor contribution of TCR to rep air of lesions in active genes, and an efficient inhibition of transcr iption.