Local Chromatin Motion and Transcription

Eukaryotic chromatin is a complex of nucleic acids and proteins that is central to interpreting the information coded in the genome. Chromatin is rather irregularly folded inside the nucleus in a fluid-like state that exhibits dynamic local movement. The highly dynamic nature of chromatin has become increasingly appreciated, particularly in DNA-templated processes including transcription, because this dynamic property ensures a degree of DNA accessibility, even in compacted chromatin. Many proteins globally constrain local chromatin movements, which seem to be driven essentially by thermal fluctuation in living cells. For instance, loss of the cohesin complex, which can capture chromatin fibers, leads to an increase in chromatin motion. Another constraining factor of chromatin motion is the transcription machinery. Although the previously held view is that transcription requires open and highly dynamic chromatin, a number of studies are now pointing to a more nuanced role of transcription in constraining chromatin movement: dynamic clustering of active RNA polymerase II and other transcription factors can serve as a hub that transiently bridges active DNA regions to be transcribed, thereby loosely networking chromatin and constraining chromatin motion. In contrast, outside heterochromatin, the transcriptionally less active regions might be less constrained, more dynamic and accessible, implying a high competency state for rapid and efficient recruitment of protein factors. This new view on the interplay of local chromatin motion and transcription reflects traditional models of the transcription factories and, more recently, liquid droplets of transcription factors, providing new insight into chromatin function.