Bacterial chromosome segregation: evidence for DNA gyrase involvement in decatenation.

Nucleoids isolated from a temperature-sensitive gyrB mutant of E. coli, incubated at restrictive temperatures, exhibit increased sedimentation rates and an abnormal doublet or dumbbell-shaped morphology. Shifting cells from restrictive to permissive temperature prior to nucleoid isolation leads to decreases in the percentage of doublet nucleoids and in nucleoid sedimentation rates. When nucleoids isolated from mutant cells exposed to restrictive temperature are incubated with purified gyrase, the percentage of doublet nucleoids decreases as the total number of nucleoids increases. These results, together with the demonstrated ability of gyrase to decatenate small circular DNA molecules in vitro, suggest that gyrase participates in bacterial chromosome segregation through its decatenating activity.