Template topology and transcription: Chromatin templates relaxed by localized linearization are transcriptionally active in yeast

To address the role of transient torsional stress in transcription, we have utilized the regulated expression of HO endonuclease in yeast to create double-strand breaks in DNA templates in vivo at preselected sites. Linearization of circular minichromosomes, either 2 kb upstream or immediately downstream of a lacZ reporter gene controlled by the yeast metallothionein gene (CUP1) promoter, did not alter the copper induction profile of lacZ RNA transcripts compared to that of nonlinearized controls. Constructs site-specifically integrated into yeast chromosome II gave similar results. In vivo cross-linking with psoralen as a probe for negative DNA supercoiling demonstrated that template linearization efficiently dissipated DNA supercoiling induced by transcription. Therefore, the efficient transcription of linearized, relaxed templates found here demonstrates that transient torsional tension is not required for transcription of chromatin templates in yeast.