Figure 1.

Cellular functions of bacterial topoisomerases. Gyrase removes positively supercoiled DNA ahead of transcription (top) and replication (bottom) complexes and in conjunction with topoisomerase I, also maintains the negative superhelicity of the genome.^1,3,19−25 Topoisomerase IV can remove positive supercoils, but primarily acts behind the replication fork to resolve precatenanes and unlink daughter chromosomes. Topoisomerase I acts to remove negative supercoils that are generated behind transcription complexes. In species that encode gyrase as the only type II topoisomerase, such as M. tuberculosis, the enzyme likely carries out the functions of both gyrase and topoisomerase IV and acts ahead of transcription complexes and ahead of and behind replication forks.²⁵