Beginning with the Escherichia coli co protein, or bacterial DNA topoisomerase I, an ever-increasing number of enzymes has been identified that catalyze changes in the linkage of DNA strands. DNA topoisomerases are ubiquitous in nature and have been shown to play critical roles in most p- cesses involving DNA, including DNA replication, transcription, and rec- bination. These enzymes further constitute the cellular targets of a number of clinically important antibacterial and anticancer agents. Thus, further studies of DNA topology and DNA topoisomerases are