Model of repressive and non-repressive chromatin at Saccharomyces cerevisiae telomeres. Schematic shows a URA3 marked telomere with the positions of the URA3 coding sequence, upstream activating sequence (UAS), and TATA box (T) indicated. (A) The non-repressive state is characterized by an open promoter conformation and the presence of unpositioned nucleosomes (light grey circles) upstream of the URA3 promoter region. This structure permits transcription (grey arrow) of the URA3 gene. (B) The repressive state is formed in two steps: (1) Phased nucleosomes are positioned upstream of the URA3 gene by a mechanism dependent on the Sir proteins (2 - 4). Interaction of the Sir proteins with this upstream region could occur by looping or folding-back of the telomere (not depicted). (2) Full repression is established by an additional step, such as further Sir recruitment or histone modification, which results in the URA3 promoter assuming a closed chromatin structure.