Cloning a fragment from the telomere of the long arm of human chromosome 9 in a YAC vector

Characterization of Arabidopsis thaliana telomeres isolated in yeast

In an effort to learn more about the genomic organization of chromosomal termini in plants we employed a functional complementation strategy to isolate Arabidopsis thaliana telomeres in the yeast, Saccharomyces cerevisiae. Eight yeast episomes carrying A. thaliana telomeric sequences were obtained. The plant sequences carried on two episomes, YpAtT1 and YpAtT7, were characterized in detail. The telomeric origins of YpAtT1 and YpAtT7 insert DNAs were confirmed by demonstrating that corresponding genomic sequences are preferentially degraded during exonucleolytic digestion. The isolated telomeric restriction fragments contain G-rich repeat arrays characteristic of A. thaliana telomeres, as well as subterminal telomere-associated sequences (TASs). DNA sequence analysis revealed the presence of variant telomeric repeats at the centromere-proximal border of the terminal block of telomere repeats. The TAS flanking the telomeric G-rich repeat in YpAtT7 corresponds to a repetitive element present at other A. thaliana telomeres, while more proximal sequences are unique to one telomere. The YpAtT1 TAS is unique in the Landsberg strain of A. thaliana from which the clone originated; however, the Landsberg TAS cross-hybridizes weakly to a second telomere in the strain Columbia. Restriction analysis with cytosine methylation-sensitive endonucleases indicated that both TASs are highly methylated in the genome.

A human DNA telomeric fragment contains yeast ARS and mitotic stabilizing sequences

A minilibrary of human DNA fragments was prepared in the vector YIP5 from a DNA preparation enriched for telomeric sequences. Screening of the library produced one clone that hybridized to the TTAGGG sequence. The cloned DNA fragment was shown to be telomeric by a number of criteria. In situ hybridization to metaphase human chromosomes showed that the fragment hybridized to the tips of all human chromosomes. The fragment contained at least two yeast autonomously replicating sequences (ARS) and stabilizing sequences, since it transformed Saccharomyces cerevisiae with high efficiency, giving rise to clones which were mitotically stable under non-selective growth.