Relaxation of transcription-induced negative supercoiling is an essential function of Escherichia coli DNA topoisomerase I

It has been suggested that the essential function of DNA topoisomerase I in Escherichia coli is to prevent chromosomal DNA from reaching an unacceptably high level of global negative supercoliing. However, other in vivo studies have shown that DNA topoisomerase I is very effective in removing local negative supercoliing generated during transcription elongation. To determine whether topoisomerase I is essential for controlling global or local DNA supercoliing, we have prepared a set of topA null mutant strains in combination with different plasmid DNAs. Although we found a correlation between the severity of the growth defect with both transcription-induced and global supercoliing, near to complete growth inhibition correlated only with transcription-induced supercoliing. This result strongly suggests that the major function of DNA topoisomerase I is to relax local negative supercoliing generated during transcription elongation.