Telomeric nucleosomes: forgotten players at chromosome ends.

Telomeres are the special nucleoprotein structures that protect chromosome ends from both recombination and degradation. In most organisms, telomeric DNA consists of short sequences repeated in tandem ending in single-stranded G-rich overhangs. In higher eukaryotes, about 80% of telomeric DNA is organized in tightly packed nucleosomes separated by 10-20 bp of linker DNA. Several specific proteins contribute to telomeric structure. At the moment, a satisfactory description of telomere organization is still lacking. Whereas the role played by telomeric proteins in telomere function and regulation has been widely investigated, little is known about the contribution of nucleosomes to the protection of chromosome ends. In this review we present an overview on the chromatin organization in lower and higher eukaryotes, and discuss the recent results on the peculiar features of telomeric nucleosomes and on the epigenetic status of mammalian telomeres.