Fob1p recruits DNA topoisomerase I to ribosomal genes locus and contributes to its transcriptional silencing maintenance

S. cerevisiae ribosomal DNA (rDNA) locus hosts a series of highly complex regulatory machineries for RNA polymerase I, II and III transcription, DNA replication and units recombination, all acting in the Non Transcribed Spacers (NTSs) interposed between the repeated units by which it is composed. DNA topoisomerase I (Top1p) contributes, recruiting Sir2p, to the maintenance of transcriptional silencing occurring at the RNA Polymerase II cryptic promoters, located in the NTS region. In this paper we found that Fob1p presence is crucial for Top1p recruitment at NTS, allowing transcriptional silencing to be established and maintained. We also showed the role of Nsr1p in Top1p recruitment to rDNA locus. Our work allows to hypothesize that Nsr1p targets Top1p into the nucleolus while Fob1p is responsible for its preferential distribution at RFB.