title: Repair of active and silenced rDNA in yeast: the contributions of photolyase and transcription-couples nucleotide excision repair.
abstract: DNA repair by photolyase (photoreactivation) and nucleotide excision repair (NER) are the major pathways to remove UV-induced cyclobutane-pyrimidine dimers (CPDs). The nucleolus is a nuclear subcompartment containing the ribosomal RNA genes (rDNA) of which a fraction is transcribed by RNA polymerase I (RNAP-I), and the rest is silenced. Here yeast was used to investigate how photoreactivation and NER contribute to repair of active and inactive rDNA. Cells were irradiated with UV light and exposed to different repair conditions. Nuclei were isolated, and the active genes were separated from the inactive genes by restriction endonuclease digestion. CPDs were measured in total rDNA, in both fractions, and in the GAL10 gene. Repair in rDNA was as efficient as in GAL10 indicating that both pathways have unrestricted access to the nucleolus. Photoreactivation was much faster than NER and therefore was the predominant repair pathway. Active genes were faster repaired by photolyase than were silenced genes providing evidence for an open chromatin structure during repair. The transcribed strands of active genes, but not of inactive genes, were slightly faster repaired by NER providing evidence for transcription-coupled repair by RNAP-I. There was no pronounced inhibition of photoreactivation by RNAP-I in the transcribed strand, which is in contrast to genes transcribed by RNAP-II and suggests different stabilities of RNAP-I and RNAP-II stalled at CPDs.
authors.affiliation: Institut für Zellbiologie, Departement Biologie, Eidgenössische Technische Hochschule (ETH), Hönggerberg, CH-8093 Zürich, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Meier
authors.forename: Magdalena
authors.initials: M
authors.lastname: Livingstone-Zatchej
authors.forename: Fritz
authors.initials: F
authors.lastname: Thoma
mesh_terms: D002843:Chromatin / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000494:pharmacology; D053938:DNA Fragmentation; D004260:DNA Repair; D004275:DNA, Ribosomal / Q000737:chemistry* / Q000378:metabolism; D004255:Deoxyribodipyrimidine Photo-Lyase / Q000378:metabolism*; D005363:Ficusin / Q000494:pharmacology; D020868:Gene Silencing; D006801:Humans; D008027:Light; D008957:Models, Genetic; D017319:Photosensitizing Agents / Q000494:pharmacology; D011485:Protein Binding; D012318:RNA Polymerase I / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D013347:Subcellular Fractions; D013997:Time Factors; D014158:Transcription, Genetic; D014466:Ultraviolet Rays
chemical_list: D002843:Chromatin; D003432:Cross-Linking Reagents; D004275:DNA, Ribosomal; D017319:Photosensitizing Agents; D012318:RNA Polymerase I; D004255:Deoxyribodipyrimidine Photo-Lyase; D005363:Ficusin