RNAs templating chromatin structure for dosage compensation in animals.

The role of RNA as a messenger in the expression of the genome has been long appreciated, but its functions in regulating chromatin and chromosome structure are no less interesting. Recent results have shown that small RNAs guide chromatin-modifying complexes to chromosomal regions in a sequence-specific manner to elicit transcriptional repression. However, sequence-specific targeting by means of base pairing seems to be only one mechanism by which RNA is employed for epigenetic regulation. The focus of this review is on large RNAs that act in the dosage-compensation pathways of flies and mammals. These RNAs associate with chromatin over the length of whole chromosomes and are crucial for spreading epigenetic changes in chromatin structure. They do not appear to act in a sequence-specific manner but might provide scaffolds for co-operative binding of chromatin-associated complexes that enable spreading of chromatin modifications.