Transcription requires chromatin to be in the unwound, open-access state. Boundary elements create a roadblock to remodeling machinery to prevent euchromatin from improperly converting back to tightly wound heterochromatin, and vice versa. Now, Victoria Lunyak, Geoff Rosenfeld (University of California, San Diego, CA), and colleagues reveal that boundary elements can be created by the transcription of noncoding repeat sequences derived from retrotransposons.
Lunyak et al. were dissecting the unwinding and regulation of the mouse growth hormone gene (GH). They mapped histone modifications that mark the transition to unwound chromatin to a region far upstream of the gene's enhancer—a spot containing a short interspersed nuclear element (SINE) B2 repeat. SINE B2 repeats are best known as retrotransposon-derived pseudogenes of the tRNA gene that are uniquely transcribed on opposite strands by RNA polymerases II and III.
Enhancer-blocking assays confirmed the sequence's role as a boundary...
