When an active eukaryotic gene is moved into a heterochromatic region of the genome, heterochromatin proteins `spread' over the rearranged gene and silence it. One model for this spreading of silencing proposes that heterochromatin protein 1 (HP1) binds to a methylated histone near the gene and recruits Su(VAR)3-9, which methylates adjacent histones to form new HP1-binding sites. On p. 3571, Danzer and Wallrath challenge this model by investigating HP1-mediated gene silencing in vivo in Drosophila. They describe a tethering system in which HP1 fused to the DNA-binding domain of the E coli lacI repressor is expressed in Drosophila that carry lac operator repeats, which lie 1.9 and 3.7 kb upstream of two euchromatic reporter genes. Expression of the HP1 fusion protein silences both reporters and alters the euchromatin to a heterochromatin-like structure. Importantly, silencing of the nearby, but not the distant, reporter gene occurs in a Su(var)3-9 mutant,...

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