Yeast telomeric sequences function as chromosomal anchorage points in vivo

Site‐specific recombination in Saccharomyces cerevisiae was used to generate non‐replicative DNA rings containing yeast telomeric sequences. In topoisomerase mutants expressing Escherichia coli topoisomerase I, the rings adopted a novel DNA topology consistent with the ability of yeast telomeric DNA to block or retard the axial rotation of DNA. DNA fragments bearing portions of the terminal repeat sequence C 1–3 A/TG 1–3 were both necessary and sufficient to create a barrier to DNA rotation. Synthetic oligonucleotide sequences containing Rap1p binding sites, a well represented motif in naturally occurring C 1–3 A arrays, also conferred immobilization; mutant Rap1p binding sites and telomeric sequences from other organisms were not sufficient. DNA anchoring was diminished by addition of competing telomeric sequences, implicating a role for an as yet unidentified limiting trans ‐acting factor. Though Rap1p is a likely protein constituent of the DNA anchor, deletion of the non‐essential C‐terminal domain did not affect the topology of telomeric DNA rings. Similarly, disruption of SIR2 , SIR3 and SIR4 , genes which influence a variety of telomere functions in yeast, also had no effect. We propose that telomeric DNA supports the formation of a SIR ‐independent macromolecular protein–DNA assembly that hinders the motion of DNA because of its linkage to an insoluble nuclear structure. Potential roles for DNA anchoring in telomere biology are discussed.