RAP1 stimulates single- to double-strand association of yeast telomeric DNA: implications for telomere - telomere interactions

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Abstract
Repressor Activator Protein 1 (RAP1) of Saccharo-myces cerevlslae Is an abundant nuclear protein Implicated In telomere length maintenance, transactiv-atlon, and In the establishment of silent chromatln domains. The RAP1 binding site 5' of the yeast HIS4 gene Is also a region of hyperrecombinatlon In melosls. We report here that as RAP1 binds Its recognition consensus, it appears to untwist double-stranded DNA, which we detect as the Introduction of a negative supercoil in clrcularizatlon assays. Coincident with the RAP1 -dependent untwisting, we observe stimulation of the association of a single-stranded yeast telomeric sequence with its homologous double-stranded sequence In a supercolled plasmld. This unusual distortion of the DNA double helix by RAP1 may contribute to the RAP1-dependent enhancement of recombination rates and promote non-duplex strand Interactions at telomeres.