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J Biol Chem. 1999 Jun 4;274(23):16654-8.

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

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Département de Microbiologie et immunologie, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, Québec, Canada H3C 3J7.


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 supercoiling. However, other in vivo studies have shown that DNA topoisomerase I is very effective in removing local negative supercoiling generated during transcription elongation. To determine whether topoisomerase I is essential for controlling global or local DNA supercoiling, 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 supercoiling, near to complete growth inhibition correlated only with transcription-induced supercoiling. This result strongly suggests that the major function of DNA topoisomerase I is to relax local negative supercoiling generated during transcription elongation.

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