Sequence-dependent mobility of telomeric nucleosomes

Nucleosome mobilization plays a relevant role in regulating the accessibility of DNA in chromatin. A class of enzymes known as ATP-dependent chromatin remodeling complexes has been isolated that actively alter nucleosome positioning. Nucleosomes possess also a certain degree of inherent mobility that is likely to depend on DNA sequence. Both processes contribute to facilitate the access of regulatory factors to their binding sites. The structure of human telomeres is not yet completely characterized. Several proteins involved in the formation of the human telomeric complex have been identified, such as hTRF1 and hTRF2 which bind to GGGTAA duplex telomeric repeats. Nevertheless, most human telomeric DNA is organized into tightly spaced nucleosomes. Due to the peculiar features of telomeric repeated sequences, telomeric nucleosomes are less stable than bulk nucleosomes and occupy multiple isoenergetic positions spaced every telomere repeat. The emerging view is that telomeres are extremely dynamic structures whose organization and function change throughout the cell cycle. Since hTRF proteins and the histone octamer compete for telomeric DNA, to understand telomere dynamics it is clearly important to understand the interphase between regular chromatin and telomere structure, or more specifically the ability of nucleosomes to move along DNA and how proteins recognize their binding sites on nucleosomes. Using an in vitro model system, we have analyzed telomeric nucleosome mobility as a function of temperature and ionic strength. We found that nucleosomes formed on telomeric sequences are significatively more mobile than bulk nucleosomes. Moreover, we found that hTRF1 and hTRF2 are able to specifically recognize their binding sites in a nucleosomal context. Experiments are in progress to examine if binding of hTRF proteins could induce nucleosome mobilization or dissociation.