Saccharomyces telomeres acquire single-strand TG_1-3 tails late in S phase

Saccharomyces telomeres consist of ∼ 300 bp of C_1-3A TG_1-3 DNA. Nondenaturing Southern hybridization, capable of detecting ∼60 to ∼300 bases of TG_1-3 DNA, revealed that yeast telomeres acquired and lost TG_1-3 tails, the predicted intermediate in telomere replication, in a cell cycle-dependent manner. TG_1-3 tails were also detected on the ends of a linear plasmid isolated from late S phase cells. In addition, a nonlinear form of this plasmid was detected: this structure migrated in two-dimensional agarose gels like a nicked circle of the same size as the linear plasmid, but had considerably more single-stranded character than a conventional nicked circle. The evidence indicates that these circles were formed by telomere-telomere interactions involving the TG_1-3 tails. These data provide evidence for a cell cycle-dependent change in telomere structure and demonstrate that TG_1-3 tails, generated during replication of a linear plasmid in vivo, are capable of mediating telomere-telomere interactions.