Transcriptional silencing of Tyl elements in the RDNl locus of yeast

We demonstrate that in Saccharomyces cerevisiae, the tandem array of ribosomal RNA genes [RDNl) is a target for integration of the Tyl retrotransposon that resuhs in silencing of Tyl transcription and transposition. Tyl elements transpose into random rDNA repeat units and are mitotically stable. In addition, we have found that mutation of several putative modifiers of RDNl chromatin structure abolishes silencing of Tyl elements in the rDNA array. Disruption of SIR2, which elevates recombination in RDNl, or TOPI, which increases psoralen accessibility in rDNA, or HTAl-HTBl, which reduces histone H2A-H2B levels and causes localized chromatin perturbations, abolishes transcriptional silencing of Tyl elements in RDNl. Furthermore, deletion of the gene for the ubiquitin conjugating enzyme Ubc2p, which ubiquitinates histones in vitro, derepresses not only Tyl transcription but also mitotic recombination in RDNl. On the basis of these results, we propose that a specialized chromatin structure exists in RDNl that silences transcription of the Tyl retrotransposon.