Repair of thermal damage to the Escherichia coli nucleoid

The folded chromosome or nucleoid of Escherichia coli was analyzed by low-speed sedimentation in neutral sucrose gradients after heat treatment (30 min at 50 degrees C) and subsequent incubation of cells at 37 degrees C for various times. Heat treatment resulted in in vivo association of the nucleoids with cellular protein and in an increase in sedimentation coefficient. During incubation at 37 degrees C, a fraction of the nucleoids, from heated cells, because dissociated from cellular protein and regained their characteristic sedimentation coefficients. The percentage of nucleoids which returned to their control sedimentation position in the sucrose gradients corresponded to the percentage of cells able to repair thermal damage as assayed by enumeration on agar plates.