Noncoding RNAs and the borders of heterochromatin

Eukaryotic genomes contain long stretches of repetitive DNA sequences, which are the preferred sites for the assembly of heterochromatin structures. The formation of heterochromatin results in highly condensed chromosomal domains that limit the accessibility of DNA to the transcription and recombination machinery to maintain genome stability. Heterochromatin has the tendency to spread, and the formation of boundaries that block heterochromatin spreading is required to maintain stable gene expression patterns. Recent work has suggested that noncoding RNAs (ncRNAs) are involved in regulating boundary formation in addition to their well-established roles in chromatin regulation. Here, we present a review of our current understanding of the involvement of ncRNA at the boundaries of heterochromatin, highlighting their mechanisms of action in different settings.