DNA topoisomerase IIa is required for RNA polymerase II transcription on chromatin templates

In the nucleus of the cell, core RNA polymerase II (pol II) is associated w ith a large complex called the pol II holoenzyme (holo-pol)(1,2). Transcrip tion by core pol II in vitro on nucleosomal templates is repressed compared with that on templates of histone-free naked DNA(3-5). We found that the t ranscriptional activity of holo-pol, in contrast to that of core pol II, is not markedly repressed on chromatin templates. We refer to this property o f holo-pol as chromatin-dependent coactivation (CDC). Here we show that DNA topoisomerase II alpha is associated with the holo-pol and is a required c omponent of CDC. Etoposide and ICRF-193, specific inhibitors of topoisomera se II, blocked transcription on chromatin templates, but did not affect tra nscription on naked templates. Addition of purified topoisomerase IIa recon stituted CDC activity in reactions with core pol II. These findings suggest that transcription on chromatin templates results in the accumulation of s uperhelical tension, making the relaxation activity of topoisomerase II ess ential for productive RNA synthesis on nucleosomal DNA.