The chromosome end in yeast: its mosaic nature and influence on recombinational dynamics

Yeast chromosome ends are composed of several different repeated elements. Among six clones of chromosome ends from two strains of Saccharomyces cerevisiae, at least seven different repeated sequence families were found. These included the previously identified Y' and X elements. Some families are highly variable in copy number and location between strains of S. cerevisiae, while other elements appear constant in copy number and location. Three repeated sequence elements are specific to S. cerevisiae and are not found in its evolutionarily close relative, Saccharomyces paradoxus. Two other repeated sequences are found in both S. cerevisiae and S. paradoxus. None of those described here is found (by low stringency DNA hybridization) in the next closest species, Saccharomyces bayanus. The loosely characterized X element is now more precisely defined. X is a composite of at least four small (ca. 45-140 bp) sequences found at some, but not all, ends. There is also a potential "core" X element of approximately 560 bp which may be found at all ends. Distal to X, only one of six clones had (TG1-3)n telomere sequence at the junction between X and Y'. The presence of these internal (TG1-3)n sequences correlates with the ability of a single Y' to expand into a tandem array of Y's by unequal sister chromatid exchange. The presence of shared repeated elements proximal to the X region can override the strong preference of Y's to recombine ectopically with other Y's of the same size class. The chromosome ends in yeast are evolutionarily dynamic in terms of subtelomeric repeat structure and variability.