Abstract
A topoisomerase I activity has been partially purified from crude extracts of spinach chloroplasts. This activity relaxes the supercoiled covalently closed circular DNA of pBR322. The enzyme requires Mg++, but not ATP, and has an apparent molecular weight of about 115,000. It catalyzes a unit change in the linkage number of supercoiled DNA but cannot relax positive supercoiled DNA. These characteristics of the topoisomerase suggest it is of the prokaryotic type and would tend to support the endosymbiotic theory of plastid origin and evolution.