The superhelicity of the chromosome, which is controlled by DNA topoisomerases, modulates global gene expression. Investigations of transcriptional responses to the modulation of gyrase function have identified two types of topoisomerase-mediated transcriptional responses: (i) steady-state changes elicited by a mutation in gyrase, such as the D82G mutation in GyrA, and (ii) dynamic changes elicited by the inhibition of gyrase. We hypothesize that the steady-state effects are due to the changes in biochemical properties of gyrase, whereas the dynamic effects are due to an imbalance between supercoiling and relaxation activities, which appears to be influenced by the RecA activity. Herein, we present biochemical evidence for hypothesized mechanisms. GyrA D82G gyrase exhibits a reduced supercoiling activity. The RecA protein can influence the balance between supercoiling and relaxation activities either by interfering with the activity of DNA gyrase or by facilitating the relaxation reaction. RecA has no effect on the supercoiling activity of gyrase but stimulates the relaxation activity of topoisomerase I. This stimulation is specific and requires formation of an active RecA filament. These results suggest that the functional interaction between RecA and topoisomerase I is responsible for RecA-mediated modulation of the relaxation-dependent transcriptional activity of the Escherichia coli chromosome.
Bibliographical noteFunding Information:
We thank Drs Imogen Wildin and Michael Gwynn for their gift of S.aureus Topo I, Drs Zhiyu Li and Russell DiGate for their gift of B.cereus Topo I, Dr Kenneth Marians for his gifts of replication proteins and comments on these studies, and Dr Lisa Oppegard for her critical reading of the manuscript. This work was supported in part by National Institutes of Health grants GM59465 (to H.H.) and GM66098 (to A.B.K.). Funding to pay the Open Access publication charges for this article was provided by the University of Minnesota Medical School.