Identification of autonomously replicating circular subtelomeric Y' elements in Saccharomyces cerevisiae.

Abstract

We marked a large number of yeast telomeres within their Y' regions by transforming strains with a fragment of Y' DNA into which the URA3 gene had been inserted. A few of the Ura+ transformants obtained were very unstable and were found to contain autonomously replicating URA3-marked circular Y' elements in high copy number. These marked extrachromosomal circles were capable of reintegrating into the chromosome at other telomeric locations. In contrast, most of the Ura+ transformants obtained were quite stable mitotically and were marked at bona fide chromosomal ends. These stable transformants gave rise to mitotically unstable URA3-marked circular Y' elements at a low frequency (up to 2.5%). The likelihood that such excisions and integrations represent a natural process in Saccharomyces cerevisiae is supported by our identification of putative Y' circles in untransformed strains. The transfer of Y' information among telomeres via a circular intermediate may be important for homogenizing the sequences at the ends of yeast chromosomes and for generating the frequent telomeric rearrangements that have been observed in S. cerevisiae.

Telomere maintenance without telomerase

“…In most wild type strains of S. cerevisiae, approximately 2/3 of the telomeres contain tandem arrays of one to four Y' elements, separated by short stretches of *50 to 100 bp of G-rich telomeric repeats (Figure 2a). In a telomerase-pro®cient strain, exchanges of Y' elements between telomeres had been shown to occur at low frequency, resulting in the occasional dispersal of these repeats among chromosome ends (Horowitz and Haber, 1985). However, in one class of survivors, Y' copy number increased dramatically (5200-fold in some survivor strains), forming extended tandem arrays at many telomeres (Figure 2a).…”

“…For the class of survivors that displayed Y' ampli®cation, Y' elements were proposed to be acquired by either a nonreciprocal translocation event (Figure 3a), or by integration of extrachromosomal Y' circles into a critically short telomere ( Figure 3b). The latter model was based on the demonstration that Y' elements, which contained a weak origin of replication (Chan and Tye, 1983), could be propagated as extrachromosomal plasmids (Horowitz and Haber, 1985). Expansion of sub-telomeric Y' elements to multiple telomeres in this class of est1 7 survivors would have two consequences.…”

Genome-wide Purification of Extrachromosomal Circular DNA from Eukaryotic Cells

“…These methods provide little or no information about the sequence of the circular DNA. Targeted techniques such as Southern blotting 6,7 , inverse PCR 8 , or fluorescence in situ hybridization 9 provide evidence only about specific eccDNA elements. None of these methods provide the sequence of all existing eccDNA types in a cell population.…”

The polymorphic subtelomeric regions of Plasmodium falciparum chromosomes contain arrays of repetitive sequence elements.

“…thereby preventing the silencing of transcriptional activity (21)(22)(23)(24). While the effect of P. falciparum subtelomeric sequences on either chromosome maintenance or transcriptional activity remains unclear, we propose a model whereby these sequences are preferential sites of chromosome pairing and thus help promote recombination and gene conversion events.…”