Estrogen-responsive enhancers occur throughout the genome, often long distances from the genes whose transcription they enhance. To determine how these enhancers and their bound estrogen receptors are brought to their gene targets, the authors used FISH to follow their movements. Under the influence of estrogen, two genes on chromosomes 2 and 21 formed 2–21 pairs or tetrads of all four alleles. The team never observed 2–2 or 21–21 pairings. The factors that cause these specific pairings, and prevent others, are unknown.
Chromosomes usually came together within five minutes of estrogen exposure. The movements required cytoskeletal elements and their motors; blocking myosin or the polymerization of nuclear actin abolished the interactions and reduced gene expression. So did blocking dynein light chain 1, which is known to bind to the estrogen receptor. The authors suggest that dynein links the actin to the DNA-bound estrogen receptor.
Other estrogen-regulated gene sets converged in other nuclear locations, each the site of an RNA-processing nuclear speckle. When the authors knocked down a demethylase that is required for estrogen-dependent gene activation, genes came together but did not bind to the speckle. Gene expression from interacting alleles was much higher than for noninteracting ones, indicating that their linkages increase transcription rates.