Abstract
Superhelical simian virus 40 DNA migrates more rapidly during electrophoresis in agarose gels than covalently closed DNA free of superhelical turns (relaxed DNA). The difference in electrophoretic mobility between superhelical and relaxed DNA was used to monitor the activity of a protein from human tissue culture cells which converts superhelical DNA into relaxed DNA. Purified DNA-relaxing protein removes both negative and positive superhelical turns and acts in a catalytic manner. The relaxation of DNA proceeds in a stepwise fashion and DNA intermediates with decreasing numbers of superhelical turns are seen during the course of the reaction.