Constricting restricted transcription: the (actively?) shrinking web.

Acidic transcription factors position the genome at nuclear speckles through transcription-dependent and -independent mechanisms.

A small fraction of the genome reproducibly positions near nuclear speckles (NSs), increasing the expression and/or splicing efficiency of NS-associated genes. How specific genomic regions in mammalian cells are targeted to NSs remains unclear. Here, we demonstrate the establishment of genome-wide NS association without active transcription. We show that DNA sequences derived from NS-associated regions, when integrated as transgenes, are autonomously targeted to NSs. By systematically dissecting one such genomic locus, the COL1A1-SGCA locus, we identified redundant NS-targeting cis-regulatory elements, including an ∼600-bp fragment with 17 binding motifs for 8 transcription factors (TFs). Four NS-targeting TFs within this fragment contain acidic activation domains (AADs) that provide both chromatin-context and transcription-dependent NS targeting, properties that appear to be common among several other tested AADs. A subset of acidic activator TFs contains an additional, transcription-independent NS-targeting activity. Our findings establish diverse and partially redundant NS-targeting activities, which may facilitate dynamic gene positioning at the NS periphery for context-specific transcriptional responses.