Repair of UV damage in actively transcribed ribosomal genes.

Repair of UV-induced cyclobutane pyrimidine dimers (CPDs) was measured in the individual strands of transcriptionally active ribosomal genes in mouse Friend erythroleukemia cells. Ribosomal genes (rDNA) are multicopied, but only a fraction is transcriptionally active (or transcriptionally "poised"). Selective psoralen binding was used to separate the active fraction, which has an open chromatin structure, from inactive rDNA. EcoRI digestion was used to selectively release the active fraction from nuclei for DNA repair studies. UV dose response curves indicate there is no significant bias for CPD formation in either strand of both types of rDNA chromatin. More importantly, there was no evidence for transcription repair coupling in the individual strands of active and total rDNA. Indeed, over an 8 h period (one cell-cycle), repair of CPDs was almost nonexistent in either strand of active and total rDNA. Furthermore, the fraction of each chromatin structure remains constant during these repair times, suggesting that chromatin rearrangements observed during excision repair of nonnucleolar chromatin do not occur following UV damage of rDNA. However, CPDs are removed efficiently from the transcribed strand of the constitutively expressed c-abl gene (transcribed by Pol II), demonstrating that these cells are capable of transcription repair coupling.