Is there left-handed DNA at the ends of yeast chromosomes?

Tracts of the alternating copolymer poly(dGdT . dCdA) have been observed in a variety of eukaryotes. Such tracts are of particular interest since homopolymers of this sequence can exist in vitro as left-handed Z form DNA. We have found that the yeast Saccharomyces cerevisiae contains at least 30 poly(GT) tracts at dispersed genomic locations. We show here that one subset of these tracts is located at the ends (telomeres) of the yeast chromosome. In addition, we show that poly(dGdT . dCdA) tracts are added to the ends of the extrachromosomal ribosomal DNA molecules of Tetrahymena when cloned in yeast. These data represent the first reported association between a homopolymeric sequence and a chromosome structure.