title: The enigmatic conservation of a Rap1 binding site in the Saccharomyces cerevisiae HMR-E silencer.
abstract: Silencing at the HMR and HML loci in Saccharomyces cerevisiae requires recruitment of Sir proteins to the HML and HMR silencers. The silencers are regulatory sites flanking both loci and consisting of binding sites for the Rap1, Abf1, and ORC proteins, each of which also functions at hundreds of sites throughout the genome in processes unrelated to silencing. Interestingly, the sequence of the binding site for Rap1 at the silencers is distinct from the genome-wide binding profile of Rap1, being a weaker match to the consensus, and indeed is bound with low affinity relative to the consensus sequence. Remarkably, this low-affinity Rap1 binding site variant was conserved among silencers of the sensu stricto Saccharomyces species, maintained as a poor match to the Rap1 genome-wide consensus sequence in all of them. We tested multiple predictions about the possible role of this binding-site variant in silencing by substituting the native Rap1 binding site at the HMR-E silencer with the genome-wide consensus sequence for Rap1. Contrary to the predictions from the current models of Rap1, we found no influence of the Rap1 binding site version on the kinetics of establishing silencing, nor on the maintenance of silencing, nor the extent of silencing. We further explored implications of these findings with regard to prevention of ectopic silencing, and deduced that the selective pressure for the unprecedented conservation of this binding site variant may not be related to silencing.
authors.affiliation: Department of Molecular and Cell Biology and California Institute for Quantitative Biosciences, University of California, Berkeley, California 94720-3220, USA.
authors.forename: Leonid
authors.initials: L
authors.lastname: Teytelman
authors.forename: Erin A
authors.initials: EA
authors.lastname: Osborne Nishimura
authors.forename: Bilge
authors.initials: B
authors.lastname: Özaydin
authors.forename: Michael B
authors.initials: MB
authors.lastname: Eisen
authors.forename: Jasper
authors.initials: J
authors.lastname: Rine
mesh_terms: D001665:Binding Sites; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D020868:Gene Silencing; D005800:Genes, Fungal; D056426:Genetic Loci; D051738:Origin Recognition Complex / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism; D000089804:Shelterin Complex; D034501:Telomere-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: C061886:ABF1 protein, S cerevisiae; D004268:DNA-Binding Proteins; D051738:Origin Recognition Complex; C440829:RAP1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D000089804:Shelterin Complex; D034501:Telomere-Binding Proteins; D014157:Transcription Factors
keywords: Rap1; genomics; sensu stricto; silencing; transcription factors