Saccharomyces telomeres assume a non-nucleosomal chromatin structure

The chromatin structures of the telomeric and subtelomeric regions on chromosomal DNA molecules in Saccharomyces cerevisiae were analyzed using micrococcal nuclease and DNase I. The subtelomeric repeats X and Y' were assembled in nucleosomes. However, the terminal tracts of C_1.3A repeats were protein protected in a particle larger than a nucleosome herein called a telosome. The proximal boundary of the telosome was a DNase I hypersensitive site. This boundary between the telosome and adjacent nucleosomes was completely accessible to Escherichia coli dam methylase when this enzyme was expressed in yeast, whereas a site 250 bp internal to the telomeric repeats was relatively inaccessible. Telosomes could be cleaved from chromosome ends with nuclease and solubilized as protein-DNA complexes. Immunoprecipitation of chromosomal telosomes with antiserum to the RAP1 protein indicated that RAP1 was one component of isolated telosomes. Thus, the termini of chromosomal DNA molecules in yeast are assembled in a non-nucleosomal structure encompassing the entire terminal C_1-3A tract. This structure is separated from adjacent nucleosomes by a region of DNA that is highly accessible to enzymes.