Drosophila telomeric transgenes provide insights on mechanisms of gene silencing

A significant fraction of most eukaryotic genomes is packaged into chromati n that is not permissive for gene expression. This silent chromatin is typi cally located near centromeres and telomeres and has fascinated scientists for more than 70 years, yet many questions remain unanswered. Part of the d ifficulties in studying silent chromatin at the molecular level is the repe titive nature of the DNA sequences in these regions. To overcome this probl em, Drosophila stocks carrying in vitro designed transgenes inserted within silent chromatin have been generated. Molecular analysis of these transgen es has shed light on the nature of the chromatin structure within these reg ions and provided insights on the mechanisms of gene silencing. This review will focus on recent studies using telomeric transgenes. The results from these studies suggest that nuclear organization plays a role in gene silenc ing and that silencing is the result of a block early in the process of tra nscription initiation.