TELOMERES, NOT THE END OF THE STORY

Transcription in organisms as diverse as yeast and mammals is subject to chromosomal position effects that result in heritable and variegate d patterns of gene expression. Two recent studies have employed a reve rsible protein-DNA crosslinking method to identify the structural comp onents of heterochromatin in budding yeast((1,2)). The results show th at a complex containing the proteins Rap1, Sir2p, Sir3p and Sir4p is p hysically associated with nucleosomes at telomere proximal regions, bu t that the repressive chromatin structure extended by Sir3p overexpres sion has a different composition.