Maintenance of a Vertebrate Constitutive Heterochromatin Domain by a Dicer-dependent Mechanism

The 16 kb heterochromatin domain between the chicken β-globin locus and the folate receptor gene is used here to study the roles of RNA dependent mechanisms and histone modifications in maintenance of a constitutively heterochromatic structure. Inhibition of HDAC activity is shown to both increase intergenic transcription and render the heterochromatin more accessible to MspI digestion. We show that siRNA-mediated down regulation of the enzyme dicer has similar effects: histone acetylation is increased, transcript levels rise, and the compact chromatin structure becomes more accessible to restriction endonucleases. We also show that the chicken argonaute 2 homolog binds the 16 kb region in a dicer dependent manner and is necessary for a condensed chromatin structure. Heterochromatic domains of this kind, widely distributed in vertebrate genomes, thus appear to be maintained in their condensed form by previously identified and highly conserved mechanisms.