Effect of low temperature on the activities of TopR1 and TopR2. Standard positive supercoiling reactions were performed in the presence of a limiting amount of TopR1 (A) or TopR2 (B and C). The reaction products were analyzed by one-dimensional gel electrophoresis (A and B) and for TopR2 also by two-dimensional gel electrophoresis (C). In panels A, B and C, the incubation temperature is indicated above the gels. Pictures in panel C below the 2D gels schematize the position of the various forms of DNA in 2D gel electrophoresis. SC + and SC ++ indicate the positions of positively supercoiled and highly positively supercoiled DNA, respectively, Rel the position of the relaxed DNA, oc the position of nicked DNA, and SC- the position of negatively supercoiled DNA substrate.