Potent stimulation of transcription-coupled DNA supercoiling by sequence-specific DNA-binding proteins.

Abstract

Transcription by RNA polymerase can stimulate localized DNA supercoiling in Escherichia coli. In vivo, there is extensive experimental support for a "twin-domain" model in which positive DNA supercoils are generated ahead of a translocating RNA polymerase complex and negative supercoils are formed behind it. Negative supercoils accumulate in the template DNA because the positive supercoils are preferentially removed by cellular topoisomerase action. Yet, in vitro, clear and convincing support for the twin-domain mechanism has been lacking. In this article, we reconcile this inconsistency by showing that, in a defined in vitro system with plasmid DNA templates, a variety of sequence-specific DNA-binding proteins, such as the bacteriophage lambda O replication initiator or the E. coli lactose or galactose repressors, strikingly stimulate transcription-coupled DNA supercoiling. We demonstrate further that this stimulation requires the presence in the DNA template of a recognition sequence for the relevant DNA-binding protein and depends on the production of long RNA chains by an RNA polymerase. Our data are most consistent with a model in which specific DNA-binding proteins facilitate a twin-domain mechanism to enhance DNA supercoiling during transcription. More precisely, we suggest that some nucleoprotein complexes, perhaps those that contain sharply bent DNA, can form barriers that impede the diffusion and merger of independent chromosomal supercoil domains. Localization of DNA supercoils by nucleoprotein complexes may serve as a general mechanism for modulating DNA transactions that are sensitive to DNA superhelicity.