[DNA supercoiling and topoisomerases in Escherichia coli]

The chromosomal DNA of all cells is under helical tension or supercoiling. There are two classes of DNA supercoiling: plectonemic and toroidal. Plectonemic supercoiling is generated by the action of DNA topoisomerases, while toroidal supercoiling is generated by DNA-protein interactions and by topoisomerase activitities. DNA supercoiling plays an important role in replication, repair, recombination, transposition and transcription. DNA topoisomerases type I are ATP-independent enzymes that cut one DNA strand and relax supercoiled molecules. DNA topoisomerases type II requiere ATP, cut both DNA strands and supercoil relaxed molecules. All organisms have more than one topoisomerase of each, type I and type II. Escherichia coli has two topoisomerases type I: topoisomerase I and topoisomerase III and two topoisomerases type II: topoisomerase II or gyrase and topoisomerase IV. In this review we discuss the concept of DNA supercoiling and present current knowledge on E. coli DNA topoisomerases.