Transitions in topological organization of supercoiled DNA domains as a potential regulatory mechanism

We present an analysis of the influence of DNA superhelicity on the topology of chimeric plasmids. A correlation is found between topological variations and in vivo/in vitro functions. Data refer to the topological transitions observed in function of variations of the superhelical density in the three closed DNA domains that represent the three extreme examples, from the point of view of the topological organization, among many chimeric systems analyzed. The plasmids studied are ADR2-BS-pBR322 (the vector pBR322 + the yeast gene ADR2). ADR3-5(c)-yRp7 (containing yeast TRP1, a constitutive mutant of yeast ADR2, a fragment of 5 kilobases of unanalyzed chromosome DNA and the vector pBR322), and p31 (pBR322 + a yeast DNA fragment encompassing the right moiety of the Ty1 element and its in vivo promoter). In the model systems analyzed, the topological transitions observed in the eukaryotic sequences (relevant in one case for activation of selective in vitro transcription (Ty), in another (ADR2) for the in vivo expression) take place in the range of superhelical densities predicted on theoretical ground (Vologodskii, A.V., Lukashin, A.V. Anshelevich, V.V. & Frank-Kamenetskii, M.D. (1979) Nucleic Acids Res. 6, 967-982).