In vivo chromatin organization on native yeast telomeric regions is independent of a cis-telomere loopback conformation.

BACKGROUND

DNA packaging into chromatin regulates all DNA-related processes and at chromosomal ends could affect both essential functions of telomeres: protection against DNA damage response and telomere replication. Despite this primordial role of chromatin, little is known about chromatin organization, and in particular about nucleosome positioning on unmodified subtelomere-telomere junctions in Saccharomyces cerevisiae.

RESULTS

By ChEC experiments and indirect end-labeling, we characterized nucleosome positioning as well as specialized protein-DNA associations on most subtelomere-telomere junctions present in budding yeast. The results show that there is a relatively large nucleosome-free region at chromosome ends. Despite the absence of sequence homologies between the two major classes of subtelomere-telomere junctions (i.e.: Y'-telomeres and X-telomeres), all analyzed subtelomere-telomere junctions show a terminal nucleosome-free region just distally from the known Rap1-covered telomeric repeats. Moreover, previous evidence suggested a telomeric chromatin fold-back structure onto subtelomeric areas that supposedly was implicated in chromosome end protection. The in vivo ChEC method used herein in conjunction with several proteins in a natural context revealed no evidence for such structures in bulk chromatin.

CONCLUSIONS

Our study allows a structural definition of the chromatin found at chromosome ends in budding yeast. This definition, derived with direct in vivo approaches, includes a terminal area that is free of nucleosomes, certain positioned nucleosomes and conserved DNA-bound protein complexes. This organization of subtelomeric and telomeric areas however does not include a telomeric cis-loopback conformation. We propose that the observations on such fold-back structures may report rare and/or transient associations and not stable or constitutive structures.