At DNA damages, RNA polymerases-I fall off and are replaced by nucleosomes, concomitantly DNA repair switches from TC-NER to GG-NER Audrey Paillé1, François Peyresaubes1, José Carlos Zeledon-Orellana1, Antonio Conconi1*, 1Université de Sherbrooke Nucleotide excision repair allows to remove a large variety of DNA damage, like pyrimidine dimers induced by UV radiations. These damages are extremely mutagen and obstruct the elongation of RNA polymerases. They are removed by the two sub-pathways of the nucleotide excision repair (NER): the global genome repair (GG-NER) that repairs most of the genome, and the transcription coupled repair (TC-NER) that only repairs the transcribed strand of active genes. How NER proceeds through non-nucleosomal chromatin and how open chromatin is reestablished after repair are widely unknown. In our lab, we analyzed NER in ribosomal genes (rDNA) which are present in multiple copies. The nucleolus is formed around clusters of rDNA that are transcribed by RNA polymerase I (RNAPI). The amount of rDNA varies among organisms, and there are ~150 genes in yeast. At any given time, only a portion of rDNA is transcribed and has no nucleosomes, whereas non-transcribed rDNA are folded in nucleosomes. In this study we want to understand the interplays between UV induced DNA damages, nucleotide excision repair and chromatin structure. Previously, we showed that RNAPI encountering DNA damage falls off the transcribed strand and is replaced by nucleosomes. Starting from this discovery, we want to know how the transcribed strand newly folded by nucleosomes is repaired. To answer this question, we employed a Primer Extension technique, based on DNA-polymerase extension of DNA primers to measure pyrimidine dimers and repair at the nucleotide level, in both strands of rRNA genes. In agreement with the current knowledge, the rDNA non-transcribed strand is repaired by GG-NER. Remarkably, we found that the transcribed-strand is not only repaired by TC-NER at the 5’-end but also by GG-NER towards the middle and at the 3’-end of the gene, mirroring the replacement of RNAPI by nucleosomes. These findings indicate that the nucleosomes folded on the transcribed strand induce a different NER sub-pathway rather than being a passive impediment. In perspective of this observation, we want to study more specifically the implication of nucleosomes and rDNA chromatin state after UV irradiation in the maintenance of genome stability.