Transcriptional silencing at Saccharomyces telomeres: implications for other organisms.

Telomeres are the natural ends of eukaryotic chromosomes. In most organisms, telomeres consist of simple, repeated DNA with the strand running 5' to 3' towards the end of the chromosome being rich in G residues. In cases where the very end of the chromosome has been examined, the G-strand is extended to form a short, single stranded tail. The chromatin structure of telomeric regions often has features that distinguish them from other parts of the genome. Because telomeres protect chromosome ends from degradation and end-to-end fusions and prevent the loss of terminal DNA by serving as a substrate for telomerase, they are essential for the stable maintenance of eukaryotic chromosomes. In addition to their essential functions, telomeres in diverse organisms are specialized sites for gene expression. Transcription of genes located next to telomeres is repressed, a phenomenon termed telomere position effect (TPE). TPE is best characterized in the yeast Saccharomyces cerevisiae. This article will focus on the silencing properties of Saccharomyces telomeres and end with speculation on the role of TPE in yeasts and other organisms.