Specific interactions between nucleosomes and Rap1p protein suggest the involvement of nucleosomes in the dynamics of telosomes in yeast.

Saccharomyces cerevisiae Rap1p is a multifunctional protein; it binds telomeric DNA every 18 bp, and recognizes many internal sites of yeast chromosomes acting both as an activator and a repressor of transcription. The cristallografic structure of the complex between the DNA binding domain (DBD) of Rap1p and its DNA binding site showed that the protein contains two similar Myb domains. Yeast telomeres seem packaged in a non-nucleosomal structure named telosome, similarly to other lower eukaryotes with short telomeres. However, recent results show that nucleosomes are present in a small percentage of Tetrahymena telomeres, suggesting that telomeric chromatin structure could derive from a dynamic competition between telomeric proteins and the histone octamer. We studied the formation of a ternary complex between Rap1p and nucleosomes reconstituted in vitro on yeast telomeric sequences. Band shift assays, and DNaseI and KMnO4 footprintings indicate that Rap1p recognizes specifically its binding site even when located on the nucleosome. Rap1p binding decreases as the binding site is shifted toward the dyad axis of the nucleosome. These results, together with the high energy and consequent high mobility of nucleosomes formed on telomeric DNA, may explain the dynamic structure of yeast telosome.