[The telomere position effect: silence in the back row!].

Abstract

Heterochromatin displays repressive histone marks that down-regulate transcription. In the absence of specialized barriers, these repressive marks spread onto nearby nucleosomes and induce transcriptional silencing of these regions. Accordingly, in various species, transgenes that are experimentally inserted directly next to telomeric repeats are silenced. Transcriptional repression induced by the spreading of telomeric heterochromatin is known as the "telomere position effect". Although it is attenuated by the presence of natural subtelomeric barriers acting against the spreading of telomeric heterochromatin, telomere-induced silencing is also observed at the level of endogenous loci where it was initially proposed to provide a mean to regulate gene expression during senescence. This, however, remains to be formally demonstrated. Here, I review the current evidences for a telomere position effect, from yeast to human.