eLife Assessment
The Twin Domain model proposed by Lui and Wang proposing that twin supercoiling domains of DNA emerge during transcription were first described decades ago, but direct experimental evidence has been challenging to obtain. Here, the authors make a fundamental contribution by directly measuring DNA torsion in cells using a photoactivatable intrastrand cross-linker compared to controls. They gather compelling data using this clever method, which provides direct evidence in support of the twin-supercoiled domain model, for torsional effects at transcription start and end sites, and thereby uncover novel features of higher order structure of chromatin in yeast. These data are exciting, and the tools will be of interest to anyone studying chromosome structure and gene regulation.