Nongenic transcription, gene regulation and action at a distance.

In eukaryotes, motifs such as silencers, enhancers and locus control regions act over thousands of base pairs to regulate adjacent genes; insulators limit such effects, and barriers confine repressive heterochromatin to particular chromosomal segments. Recent results show that many of these motifs are nongenic transcription units, and two of them directly contact their targets lying further down the chromosome to loop the intervening DNA: the barriers (scs and scs') flanking the 87A7 heat-shock locus in the fly contact each other, and a locus control region touches the beta-globin gene in the mouse. I hypothesize that the act of transcription underlies the function of these regulators; active polymerizing complexes tend to cluster into 'factories' and this facilitates molecular contact between the transcribed regulator and its distant (and transcribed) target.